'Nother xfmr question: 480 Wye 3-Phase to 240

[Richardh247]

I think I am just over-thinking this but I am at wit's end here. I recently bid (and won) a job that calls for a 3-phase, 400A, 480/277 Wye service. Connected to that was to be a 3-phase, 200A, 208/120 Wye to service the general receptacle and exhaust fan loads. Simple stuff.

I visited the site to finalize some measurements for the lateral primary run and, while there, I noticed that the exhaust fans were in and I took a peek. They are 1/2hp 240V with an FLA of 3.3 or 6.6 if connected to 120V. While there, I was also informed that I needed to add a "typical" (his words) receptacle for a welder - standard 240V., 50A supply.

This throws all of my calcs. If those fans were 230V. listed I might, and I do mean might, take a chance on a 208V supply. But with a plate rating of 240V I am not comfortable chancing it.



All of this swimming around in my head for the last couple of days has done nothing but confuse me. I know the answer is drop dead simple, but it isn't coming to me.

So...

That the MDP is 480, 3-phase in a Wye configuration from the CTs, I have two options that I can see:

1. 208V 3-phase Wye secondary off a 45KVA xfmr and hook up the exhaust fans to single phase 120 and make him buy a welder to accommodate my service;

2. A Wye/Delta 240V 3-phase xfmr, also 45KVA, center tapped for some 2-pole 240 and a 208V stinger that will never be used anywhere for anything and will unbalance the secondary panel.

One of the overriding issues I have, and I'm hoping for some input on this, is that the bids are in. Obviously I will have to recalculate everything and resubmit my price, but I want to make sure his expansion needs are covered to the best of my ability without soaking the material budget with needless excess. This job has no engineer and no prints and, me not being an engineer and just a lowly electrical contractor, I'm in a bit of a learning curve balancing materials costs versus anticipated future need.

The loads on the low volt panel are minuscule: the two 1/2 hp fans, the unknown welder and 12 convenience receps @ 1,800VA/strap.

Anyone want to suggest a cost-efficient, expedient means of navigating this without cutting corners? What would happen if I just went with a Delta-Delta configuration off the Wye primary? Is there a way to avoid a stinger on a 3-p 240V service I am not aware of?

 

[petersonra]

the fans are easy. hook them up to 120V.

the welder is no big deal either. thats what buck boost transformers are for. it is an adder because he never told you about it up front.

it's also possible that the welder will work Ok on 208 instead of 240 V.

 

[kingpb]

First, you say the bids are in. Have you signed a contract? Did they issue an addendum, or design revision for this change prior to you winning the job?

If not, the original price stands, and any change needs to be in writing to you. Do nothing until they accept your official change order.

As far as the technical design, just install a 2-pole breaker in the 480V panel, drop a 480-240V transformer with a small 120/240V panel.

 

[Smart $]

Who spec'd the 208/120 transformer?

If it was not you, then you need to RFI your concerns to that entity.

Seems you could get away with a 120/240 1? 3W secondary system for fans and receptacles without putting too much strain on service load balance. So the question of possible future expansion includes whether 3? loads on the secondary system comes into play.

 

[Smart $]

...

2. A Wye/Delta 240V 3-phase xfmr, also 45KVA...

If you go this route, why not just a delta/delta?

 

[Richardh247]

the fans are easy. hook them up to 120V.

I thought of that. Just hate to reduce the efficiency if not necessary.

the welder is no big deal either. thats what buck boost transformers are for. it is an adder because he never told you about it up front.

And that is a suggestion I hadn't thought of. Thank you. Still, considering that all three pieces of equipment he has added are 2-pole, 240V, I can only assume that any plans he may have for the future would continue along that same schedule. I'd hate to go back for Phase II and have BB Xfmrs all over the place. I know they are safe and everything, but it seems like it would be a design flaw on my end were that to end up happening.

it's also possible that the welder will work Ok on 208 instead of 240 V.

Possible, sure, but I'm really anal about my installations and code. This is the biggest job I have ever tried to design on my own and, not being anything remotely resembling an engineer, I am just trying to cover all the bases without going nuts on material overhead or look like I took no time engineering the job.

First, you say the bids are in. Have you signed a contract? Did they issue an addendum, or design revision for this change prior to you winning the job?

No contract yet - as I said, I will have to resubmit.

If not, the original price stands, and any change needs to be in writing to you. Do nothing until they accept your official change order.

Oh, I don't plan on doing anything that isn't in writing and signed. What I am trying to do is ensure that I'm giving him the most bang for the buck, if you'll excuse the idiom. He will have to accept that the price is going to change: the feeder length has increased by one hundred feet, so just the wire cost for parallel 3/0 is going to double.

As far as the technical design, just install a 2-pole breaker in the 480V panel, drop a 480-240V transformer with a small 120/240V panel.

This is where I am getting confused. Are you suggesting forgoing the 3-phase altogether and making the low-volt panel single phase via a 2-pole CB? That is what I originally wanted to do, but how then do I balance the xfmr load? Since the xfmr is a separately derived system, would I just eliminate a primary neutral and pull a pair of conductors with a grounding conductor, tap the xfmr on the XO lug on the secondary side, bond the secondary neutral and ground to the xfmr ground bar and then to building steel and not bond the neutral and ground at the low volt panel?

Who spec'd the 208/120 transformer?

If it was not you, then you need to RFI your concerns to that entity.

It was actually both of us. That is, the business owner communicated his needs, or what he thought were his needs, to me. The equipment and lights were to be on the 480 panel and the low volt panel was to carry 12 receps. He said, and this is as a direct a quote as I can remember, "Just do it the way you would any other building." K. Well, in most commercial projects where I am from, Phoenix, we use a Wye-Wye for a facility like this (automated bagging plant) unless we know we have a need for a single phase 208 load.

This changed when I saw the equipment. The fans were actually 240V, plus he wanted a welder circuit, and that got me to this place of confusion trying to re-figure everything. Oh, and the 600A went to a 400A, but the transformer station moved away from the building 100', and the "small chipper" he was placing by the xfmr station is actually a 300HP unit. :/

This is why I usually refuse to bid a job without cut sheets. But I really, really want this job because the owner is a great guy and this will take my small company to a new level with him as both a customer and a reference.

Seems you could get away with a 120/240 1? 3W secondary system for fans and receptacles without putting too much strain on service load balance. So the question of possible future expansion includes whether 3? loads on the secondary system comes into play.

What am I missing here? The other gentleman suggested a similar scenario: a 2-pole 480 OCPD to a single phase xfmr with a 240V single phase secondary. Maybe I am inexperienced, or maybe I just don't remember, but I cannot recall ever seeing such a thing. The last I knew, there was no way to funnel 3-phase to single phase without a converter without really unbalancing the phases. Is it just because the secondaries have such a small load? Is it common practice to do this and I have just never been exposed to it because I've always done heavy commercial?

I feel like an apprentice asking all these questions but I'd rather look a bit of a fool than give a customer something that won't punch their ticket.

If you go this route, why not just a delta/delta?

The primary from the utility company is Wye. Odd, I know.

 

[ActionDave]

Just a wire pulling grunt here but I am thinking you are over thinking a bit.

The fans will be no less efficient running at 120V, a watt is a watt.
The welder will in all likelihood operate on 208V, best to check.
If there is a need for real 240V power a small transformer is a easy add on.

Unless this job goes easier than any other on the planet there will be other problems that need solved down the road, save your energy.

 

[Smart $]

...

What am I missing here? The other gentleman suggested a similar scenario: a 2-pole 480 OCPD to a single phase xfmr with a 240V single phase secondary. Maybe I am inexperienced, or maybe I just don't remember, but I cannot recall ever seeing such a thing. The last I knew, there was no way to funnel 3-phase to single phase without a converter without really unbalancing the phases. Is it just because the secondaries have such a small load? Is it common practice to do this and I have just never been exposed to it because I've always done heavy commercial?

Here's what I get for your secondary system load:

(2) 1/2 HP fans 240V 4.9FLA 2,352VA
240V 50A Welder recept. 12,000VA
12 GP 120V 20A Recepts. 2,160VA
25% largest motor 294VA
Total 16,806VA
Amperes@240V 1? 70
Amperes@480V 1? 35


With service 480/277 3? 4W, and running single-phase lighting on this system, just run the lighting load a little light on the two lines the secondary system is connected to... perhaps padding it a little for future loads on secondary system.
...

The primary from the utility company is Wye. Odd, I know.

480Y/277 3? 4W is commonplace, even in commercial. So I have no idea why you would think it is odd.

Regardless, if you use a 3? secondary transformer it doesn't matter. The primary should still be delta IMO. Even if your primary was wye, it is recommended by most EE's to only run three line conductors, no neutral, and leave the H0 floating.

 

[kingpb]

I wouldn't think twice about dropping a 2-pole breaker in for 480-120/240V, but then I've only been doing engineering for +20yrs :blink:

If your concerned about balancing, than load up the other phase with all the 277V lighting, but as long as the main is based on the highest loaded phase; i.e. 3 x KVA_high, then its gonna work just fine. If you need 3ph 208V, than set a transformer for that too.
Better yet, just install 480V 3ph, no neutral, and put the lighting on the 240V or 208V.

Word of caution - taking your business to the next level because of a job with this Owner can lead to a lot of false hopes. :weeping: Think of carrot and the stick. Be the carrot, not the donkey! Just saying...

 

[augie47]

To me, you are in a no-win situation as the correct route won't be apparent until the next few unknown pieces of equipment roll in
Personally I'd probably go with my initial plan, supply the fans with 120..no big load anyway, no real power loss.
Address the welder when necessary.. 208 may be fine, if not a buck-boost.
Before the job is over chances are you will have an opportunity to add a 480/240 transformer for that "other equipment".
One word of caution based on local history.Since you may one day end up with a 240/120 hi-leg system, keep 210.5 in mind. If you do end up with a high-leg you are going to be required to have an "orange" leg (unless you choose some other method of ID). With the 210.5 requirement this may be a stumbling block if you use B-O-Y for your 480 colors. Locally we still have a lot of 240 hi-leg systems so the ECs have been using Brown-Purple- Yellow on the 480 to sidestep the potential ID problem.

 

[Smart $]

To me, you are in a no-win situation as the correct route won't be apparent until the next few unknown pieces of equipment roll in
Personally I'd probably go with my initial plan, supply the fans with 120..no big load anyway, no real power loss.
Address the welder when necessary.. 208 may be fine, if not a buck-boost.
Before the job is over chances are you will have an opportunity to add a 480/240 transformer for that "other equipment".

The issue I see with the original plan is with the addition of the welder receptacle (1?) to the original secondary system load makes impossible to balance any 3? scheme. Ends up costing extra for the higher-than-necessary rated transformer, and using a buck-boost, if 240 for welder is necessary, just adds to the unnecessary cost.

One word of caution based on local history.Since you may one day end up with a 240/120 hi-leg system, keep 210.5 in mind. If you do end up with a high-leg you are going to be required to have an "orange" leg (unless you choose some other method of ID). With the 210.5 requirement this may be a stumbling block if you use B-O-Y for your 480 colors. Locally we still have a lot of 240 hi-leg systems so the ECs have been using Brown-Purple- Yellow on the 480 to sidestep the potential ID problem.

Great advice!

Your wisdom is showing... :happyyes:

 

[iceworm]

... The fans were actually 240V, plus he wanted a welder circuit, and that got me to this place of confusion trying to re-figure everything. Oh, and the 600A went to a 400A, but the transformer station moved away from the building 100', and the "small chipper" he was placing by the xfmr station is actually a 300HP unit. :/ ...

Rich -

What is your plan to feed this chipper? A 300hp motor on a 400A, 480V, 3ph service? That doesn't sound like a good idea.

Once you solve this, I don't see a 240V, 50A, 1ph circuit being a problem.

ice

 

[hurk27]

Where I work we have quite a few welders on 208, but I have yet to see any welder of any capacity that would not also be able to be changed to run on 480, maybe a cheap big box store bought one but in my experience most all industrial grade welders can be changed from 240 to 480 by just moving some taps, Lincolns Hobart's, as well as Millers all are like this.

 

[iceworm]

... I recently bid (and won) a job that calls for a 3-phase, 400A, 480/277 Wye service.

... the bids are in.

... This job has no engineer and no prints

I'm not much help, but I do have a curiousity question:

It sounds as though there were more than one bidder. As noted there are no plans, no engineer. So what documents did you get to base your bid on? Who provided the documents?

Just curious

ice

 

[iwire]

As noted there are no plans, no engineer. So what documents did you get to base your bid on? Who provided the documents?

Documents?

We get this stuff all the time, customers call and want stuff wired. They don't know what they need, they don't wish to spend money on engineering.




Its more like this.

Them: Hi, we just bought machines / equipment X, y and Z and need them wired. The people we bought them from say they will work and will not cost much to wire

Can you give us a price?

Me: What voltage is it?

Them: 220-221 whatever it takes

Me: Do you have a cut-sheet you can email me?

Them: Yes (when it arrives in my email it is not a cut sheet, it is the purchase and sales agreement for the equipment with no electrical details)

After site visits, phone calls to manufacturers for details etc I provide a price.

Them: We received your price and it is very high, you want almost $3K to wire a machine we already paid $40K for. Are you sure about the price.

Me: Well its a 60 amp 3 phase feeder 200' from the panel, you asked for cord and plug connection and it needs to run trough a contactor for remote control. I am actually cutting it tight now.

Them: That cant be right the salesman told us just a couple hundred dollars to wire it.





When I was sitting in the office the above was pretty much a weekly deal.


So when I look at this below ....

I recently bid (and won) a job that calls for a 3-phase, 400A, 480/277 Wye service. Connected to that was to be a 3-phase, 200A, 208/120 Wye to service the general receptacle and exhaust fan loads. Simple stuff.

... I am willing to bet the documents were an email or phone call.

 

[iceworm]

... We get this stuff all the time, customers call and want stuff wired. They don't know what they need, they don't wish to spend money on engineering. ....

Absolutely. And that is why you make the big bucks. Really - I'm good with that.

...Can you give us a price?
.
.
.
When I was sitting in the office the above was pretty much a weekly deal.

So when I look at this below ....

... I am willing to bet the documents were an email or phone call.

Yes, your scenerio looks like what I would expect for a no docs job. However, rich says there were "bids". That makes it sound like multiple bidders.

Does your scenerio fit for multiple bidders? Is each bid based on a separate phone call? I supose it's possible. What does the owner do? Pick the lowest? Interview each and pick the most cost effective? But I'd be out of my element - so I wouldn't guess.

And that 300hp chipper does not sound like "simple stuff". A 300hp motor on 480V is not easy on a good day. Toss in a 400A service and it gets downright brutal. Even if the 300hp is by others, having a 300hp tied on to the same 400A service as the rest of the building sounds brutal.

Like I said, my questions were curiousity only. I've got no idea how to put this job together with the available information.

ice

 

[hurk27]

I just saw the 300hp chipper motor and the 600 amp service that got reduced to a 400 amp service part, not sure how I missed it but you have a really big problem.

that motor is going to have a FLA of over 336 amps fla and that is if it is 80% efficient, this is not even including PF and start up current to which I doubt very much that your 400 amp main will even hold????

to top that your looking at a long start up because a chipper is a large flywheel and it takes time for the motor to spin it up to speed.

then you have other 480 volt equipment and 120/240 volt to add to this or as you said 45kva which is about another 54 amps?

I would not even start this with out a 800 amp service and that may be too small

This why without cut sheets on this chipper and any other equipment is a quick way to under bid a project of this size, do you even have the motor name plate info such as the locked rotor, FLA, PF, efficiency, to even start calculating the service size, trust me these plant operators will try to get the lowest price they can and they will leave out a lot of info knowing that you Will miss allot and underbid.

and once you have signed a contract your hooked and you take the loss, because you should have the knowledge to not make a bid until you have all the info you need to size everything correctly.

that 300hp chipper is a red flag and the fact that they left this info out makes me think they are trying to get over on you, so I would be very careful and not sign anything untill you have all the info you need, I would not even have bid a job like this without it.

A statement like "Just do it the way you would any other building." is a red flag because you should know that this is impossable because simple does any other building have the same loads? nope.

 

[Richardh247]

Just now getting back to this - apologies for not having time to be as attentive to this thread as I'd originally planned to be. It wasn't and isn't a lack of appreciation. You that are self-employed know the feast/famine side of things and I've been trying to be four places at the same time for a couple of weeks now.

As I am rereading this I realize that I missed some rather critical information. It wasn't important at the time because a couple of change orders eliminated some equipment like the 300HP chipper, and mentioning it was more an expression of the frustration that comes from bidding a job several times. That said, I'm going to back up a bit and, having read all of the input, try and construct the scope of this from the get-go.

I met the gentleman that owns this bark and soil plant (Tim) through a general contractor who regularly does his work. The GC had called me in for a total house renovation which, unbeknownst to me at the time, belonged to Tim and was to be a gift to his nephew - yeah, I know, must be nice to have the kind of money that spends $80,000 on a home remodel for an extended family member. Tim, the owner of the plant that is the subject of this thread, was impressed by my work and the detail I had provided in my original bid: my company (I personally) take(s) an inordinate amount of time when planning even small jobs to provide customers with what they need and want even when they don't know what that is. He noted that I struck him as highly professional and detail orientated, that our site was clean and organized, that our proposed additions made sense and would have been a change order after occupancy and that his previous contractors had made a habit of only doing the minimum and then charging him out the butt to add things they should have known would have been needed later.

At that time, I had no idea who Tim was. To me, he was just a gentleman that had stopped by the site out of curiosity to have a look around; I silently speculated that he was probably a friend of the home's owner or something. It wasn't until the end of our half-hour conversation that he properly introduced himself as the man writing the checks and asked me if I would be interested in bidding another job for him - this plant.

Our initial look at the plant didn't yield much. He told me that what he wanted in lighting and convenience receptacles, showed me the break room and mezzanine areas (that weren't yet built), where the supplier was mounting their xfmr bank and such. I was told at that time that he wanted a bid only for Phase I: a 600A main service, a 200A secondary, the lights and receps and a couple of 1/3HP exhaust fans. That was it. And that is what I bid on and won. There were three other bids, and from Tim's language I inferred that accepting those were a matter of propriety over any real consideration. None put together a complete proposal as I did, none broke down the material and labor estimates, none asked about Phase II, and all of them had a one-page Word version of an estimate compared to my 13-page contract.

Since then there have been numerous changes. I poked and prodded him and finally convinced him that I would not be able to adequately size any potential future needs if I didn't know what Phase II was going to encompass. That's when I learned about that 300HP chipper, finally got the phone number for the POC at the utility side of things and was put in contact with the equipment manufacturer for the production line equipment (who were everything but helpful).

That chipper changed everything, but not on the service side of things where the building was concerned. It was to be located about 150' from the plant itself, out by the utility xfmr bank. I changed my primaries so I would have the underground part done and we could just pull a dedicated set of 500's to a 400A disconnect on an H-frame straight from the utility's CTs; the building would be fed from another 600A disconnect on that H-frame to the MDP mounted inside. I bid it again. Then he eliminated the chipper completely. Then he decided against moving the utility poles because it was going to cost $28,000 to do so.

See where things get a bit difficult to adequately convey on a computer screen? Since my original bid, there have been six changes to the primaries. Complicating things from there was finding out that he didn't even need 3-phase service on the secondaries, much less 200 available amps at 208. Not a big change, but it did change my layout, wire colors, etc. Back to the supply house, bid it again please, sorry about all of this...

Things finalized the other day, thank God. While the line equipment manufacturer still hasn't provided me any cut sheets, I at least know the drops: two 60A discos and a single 30A disco per line, one line being installed now and POSSIBLY a second line later (on down the line, if you will). Those come off the 480 service along with the lights (10 wall packs and 12 F-5 hi-bays). That's it on the primary.

Instead of CTs, we are now hitting a 400A meter to feed a 400A, 480V 3-phase MDP. I'm going to provide a single phase (indoor, dry-type) 50KvA xfmr feeding a 150A MBO single phase can.

[Was kind of funny: I had included a slip meter riser in my request for material bid sheets and one supplier called me and asked me what that was. Needless to say, I won't be using them for anything other than a bag of wing nuts...]

I have to draw a line somewhere: the amount of man hours I have bidding this is ridiculous. I can't, and should not be expected to, keep up with every change he thinks sounds good only to revert to previous configurations later. I try, guys, I really do. I put an enormous amount of effort into making absolutely certain that any job I build has little need for major upgrades for the life of the service. But there comes a time when I have to say that enough is enough and that my very best has to be a settling point. I'm at that point.

My little itty bitty company is growing and I'm exited to be extended the trust to design and build my first commercial project (back in Phoenix I was a commercial foreman for 10 years and ran crews up to 27 men, but on my own where there isn't any estimator, project manager, senior foreman and investor is a whole different, and scary, ball of wax). I'm trying to give my best while protecting myself - Tim is a nice guy and a Christian gentleman, but business is business and I have a bottom line to account for and profitability ratios to hold to. My time is worth something, and I feel that I have gone way, way out of my way to accommodate this job.

I'm an old-school sparky. I applied for my first electrical job straight out of the Infantry in 1993 and was handed a shovel and told to dig. I did. I spent a summer in the Phoenix summer digging - the foreman of that company told me that he didn't care who anyone was, everyone digs, even the foreman if needed. Heck, I like to dig. I still dig. Crank the radio, fill the water bottle and sleep great when the day is done. My helper is only now getting that I enjoy the manual work and I'm not there trying to make up for an inadequacy of his work.

I was never offered school and never went - back then, in Phoenix anyway, only union guys went to school. They didn't dig, so they didn't get a job and sat on the books instead (that isn't meant as any slight to any union sparky - how I wish I had gone to school!). Not my cup of tea. And school would have interfered with my drinking career. Couldn't have anything getting in the way of my nightly drunk. I bought a code book and studied it every night. When I began bending pipe, I would take home sticks to practice bending on. I took home copies of the prints and studied those. I would show up a half-hour before work and follow the foreman around while he unlocked things and set out the water jugs and bug him with the dumbest questions... but that is how I learned.

I ran my first job after three years in the trade and never stopped from there. I didn't, and for the most part still don't, have a clue about what makes an xfmr work. The only theory I know is what I have picked up in personal inquisitiveness. I never even knew that "real" electricians carried apprentice and JW cards - I honestly thought that was a union only thing.

Not having any school behind me makes things super tough on jobs like this. For every hour I spend tools on in this job I'll have five or six hours of research. So I am extremely thorough when I bid a job. I've made friends with some of my competition in the area (it's a small area where people are generally friendly and there is plenty of work to go around) and I bounce questions and ideas off of them and check their input against other sources.

I'm 43 now and, hindsight being what it is... well, I don't regret the past - it made me who I am today. Still, I would like to have the education side of things and have been seriously considering online courses like a JATC program. Many of you are going to have something negative to say--guy with no school doing work on his own? Preposterous!--but this trade and the pointy end of an issued rifle going thataway is all I have done my life and I need to make a living. Not much call for shooting things as a civilian, and I sure can't afford to take a $10/hr job just so I can go to school. Somehow I doubt I am the only guy here that learned the way I did.

I want to extend my most sincere appreciation for those that have opined. I know time is valuable and that you took some of yours to benefit some of mine. Thanks. Hopefully I will be able to pay the favor forward and help someone else that has some seemingly ridiculous set of questions.

I always take before, during and after pictures of any installation and I'll be sure to post some of them here for anyone interested. Thank you again.

 

[Richardh247]

Documents?

We get this stuff all the time, customers call and want stuff wired. They don't know what they need, they don't wish to spend money on engineering.




Its more like this.

Them: Hi, we just bought machines / equipment X, y and Z and need them wired. The people we bought them from say they will work and will not cost much to wire

Can you give us a price?

Me: What voltage is it?

Them: 220-221 whatever it takes

Me: Do you have a cut-sheet you can email me?

Them: Yes (when it arrives in my email it is not a cut sheet, it is the purchase and sales agreement for the equipment with no electrical details)

After site visits, phone calls to manufacturers for details etc I provide a price.

Them: We received your price and it is very high, you want almost $3K to wire a machine we already paid $40K for. Are you sure about the price.

Me: Well its a 60 amp 3 phase feeder 200' from the panel, you asked for cord and plug connection and it needs to run trough a contactor for remote control. I am actually cutting it tight now.

Them: That cant be right the salesman told us just a couple hundred dollars to wire it.





When I was sitting in the office the above was pretty much a weekly deal.


So when I look at this below ....



... I am willing to bet the documents were an email or phone call.

This gentleman hit the nail on the head.

Initial conversation, this is what I want, blah, blah... give me a price.

It only takes about a week in this trade to know that most customers have little idea as to what they actually want and no idea at all what they really need. There are the exceptions, but they are rare. That's why we make the big money :lol:

Everything on this job has been document free and primarily conducted via phone and office visits scribbling on whatever piece of paper was hanging around at the time. Design/build is a different animal than working with an approved set of prints and simply transforming a 2-d image into a 3-d product.

A perfect example of this was that chipper. What he thought was "something we are thinking about later on" is a fairly massive piece of equipment as far as electrical demand. It changed the manner in which we were hitting the xfmr bank completely. Nobody accounted for the fact that CTs didn't provide any means for the building disconnect. The footers and pad have no ufer. And the CTs themselves weren't even large enough to pass a set of 500's and parallel 300's through. The H-structure supporting the primary disco would not have been large enough to set a 400A disco.

I have learned a LOT on this job. Some things I will not do again, some things I now know I have to do. I don't mind working sans prints, but I made a mistake in assuming that the plant owner and the utility company were on the same page and I could go with a simple set of future provisions. That and all the changes have had my head spinning.

I'm glad I didn't just stop before now and think that it would all work itself out. Now only would I have cheated myself out of some valuable learning experience, the customer would have had a service undersized for his needs. Whether that's what he wanted or not, I am the one that pays that ticket. At the end of the day, regardless of how well the installation itself is, regardless of whose fault it is, if the customer is disappointed then it is the contractor that pays the price.

In this case I finally had to tell him, "Look, I have done the best I can up to this point but the continual changes are warning me that things are bound to change again. As such, consider my proposal and estimate only and not a quote. It's a baseline and your actual costs will go up and down from here on an as-build basis at $____ an hour plus $____ an hour for office time." He was perfectly fine with that; I knew I should have done so earlier but I was so afraid of losing the job if I wasn't perfect. Another lesson in the books.

 

[Richardh247]

So will someone check my math on this for me, please? I'm feeding a single phase 240V xfmr from a 3-phase 480V service.

A 50 KVA looks like too much - after everything, it would require a 250A OCPD on the secondary side, which is a bit big for what he needs. So I worked this up with a 37.5:

Primary:

37,500/480 = 78.125A primary current

78.125*.125 = 87.51A, so a 100A OCPD

Two #3, #8 EG

Secondary:

37,500/240 = 156.25A secondary current

156.25*.125 = 175.78A, so a 200A OCPD

Two 3/0, #6 EG

Transformer to building steel is based on the largest current carrying conductor, right? That would be the 3/0, so the xfmr bond would be a #4 solid.

And just to make sure I am up to speed on this, if I was using a 3-phase I would multiply the voltage by 1.73 before dividing, the secondary feeders would be derated to 80% because the neutral is current conducting and the bond would again be based on the largest current carrying conductor.