Is this engineer correct?

Status
Not open for further replies.
I have a job at an Air National Guard base job in Scotia NY in a a bit of a jamb. The Engineers specified 460V 3ph. HVAC RTUs They missed the fact that the existing units were 208V. :blink:

So the facility electrician and the idea to put transformers on the roof. Sounded great, except the engineer was coming up with crazy pipe and wire upgrades i did not feel was necessary. can you guys just review my math in my emails to them and let me know who is right here. Wouldn't the nameplate max breaker size be the safe calculation?

[FONT=&quot]Please review my math with me[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]RTU1 Load calculations ( all KVA was based off MAX CB as i did not have actual Full load current[/FONT])

[FONT=&quot] [/FONT]
[FONT=&quot]Secondary side (Unit load)[/FONT]
[FONT=&quot]480v* x 90a x 1.732[/FONT][FONT=&quot] =74.83 KVA [/FONT][FONT=&quot]Need 75 KVA trans[/FONT]
[FONT=&quot] 1000[/FONT]
[FONT=&quot] [/FONT][FONT=&quot](*Need to check w. Mfr for tolerance of Unit as it?s rated for 460V) **This was my comment as I wanted to be sure that applying 480v to a 460 unit wouldn't damage the equipment[/FONT].



[FONT=&quot] [/FONT]
[FONT=&quot]Primary side[/FONT]
[FONT=&quot]75 KVA x 1000[/FONT][FONT=&quot] = 208 A (Primary breaker size) Actually it is the load size, now multiply by 1.25 to get the breaker size. [/FONT]
[FONT=&quot](SQ RT3) X 208v [/FONT]
[FONT=&quot]208*1.25 = 260amps (engineer's comment)[/FONT]

[FONT=&quot]If I remember the existing wiring for RTU1 was 3/0 with existing breaker 225. As per your sketch, why would we need a new 300A 3p breaker and run new 350 mcm wire? [/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]RTU2[/FONT]
[FONT=&quot]Secondary [/FONT]
[FONT=&quot]480x80x1.732 [/FONT][FONT=&quot]= 66.52 KVA Use same 75 KVA trans. Can use same primary load calcs. at 208 Amps. [/FONT]
1000
[FONT=&quot] [/FONT]
[FONT=&quot]Existing primary wiring and breaker is the same as above. Why would we need to tap into that other panel, run 350 mcm and a new 400 amp disc w/ 300 amp fuse[/FONT]
**the engineer made a sketch to upsize the wire to 350 MCM and 300 amps???? wouldn't 208 amps be correct?

[FONT=&quot]
RTU3[/FONT]

[FONT=&quot]Secondary[/FONT]
[FONT=&quot]480x50x1.732[/FONT][FONT=&quot] = 41.56 KVA use 45 kva trans.[/FONT]
[FONT=&quot] 1000[/FONT]

[FONT=&quot]Primary[/FONT]
[FONT=&quot]45kva x 1000[/FONT][FONT=&quot] = 124.9A Has 100amp existing breaker and I believe #1 AWG (155A) wire . Will need 125A CB upsizing but why would we need to install 175 breaker and 3/0 wiring?[/FONT]
[FONT=&quot]1.732 x 208[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]RTU 4[/FONT]
[FONT=&quot]Secondary [/FONT]
[FONT=&quot]480x30x1.732[/FONT][FONT=&quot] = 24.94 [/FONT]
[FONT=&quot] 1000 Use 30 KVA trans[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]Primary[/FONT]
[FONT=&quot]30 KVa x 1000 [/FONT][FONT=&quot]= 83 amps [/FONT]
[FONT=&quot]1.732 x 208[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]The existing unit had a 50 amp breaker but ihave to guess that the wiring is rated for the same. This wiring will need to be upgraded. [/FONT]

End of my first email

Her response

Hi Michael. Your math is good but remember you have to size per inrush current too. According to the equipment documents, you have to size it at an additional 1.2 * the load. We also sized per the actual demand on the units not the circuit breaker rating. Kevin has that if you would like. Also you need to multiply the load by 1.25 to get the breaker sizes. Once you do that you find that the breaker sizes go up and in order to protect the wire correctly by the breaker you have to upsize. Otherwise you won?t have any protection on your wire. If you have any other questions please feel free to let me know. Thank you!!

And my reply back


Yes Michelle, I see your point using the actual KVA, I did not have that actual figure. However I figured that by using the MAX breaker size in the equipment, it would be the safe calculation knowing the equip engineers figure for start up currents. Also Transformers can basically able to be ?overused? by 125% so any inrush currents would probably not affect the transformer. No?

Lastly you are correct about the wire in a HVAC installation we have to over rate the wire by 125% But I figured that the nameplate Max breaker size was already figuring for this over-sizing requirement.
Take for example HV-23 on Bldg 7 has a 30 amp load with a max breaker set for 60a , this which is already set at 200% (max of 225% by NEC). Wire sizing was set at 125% of the 30 amps (=37 amps) allowing us to run #8 AWG rated at 55 amps at 90 deg C.

So basically, final calculated KVA is KVA whether Primary or Secondary and I figured for the max?. or as I thought???


Am i not correct in using the nameplate max CB size to use when calculating the KVA and transfomer sizing? Or is the Engineer with all her schooling correct? If we used actual KVA of the units then i could see adding 125% for wire and breaker sizing. But I thought the name plate is the law... Finally, KVA is KVA is it not?

Your responses will be much appreciated. Sorry for the long post:ashamed:
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Would it not be cheaper to intercept the conduit and run it to the 480V panel or does none exist. Transformer is very expensive plus it needs grounding to the GES since it is a SDS. Also transformer can be loaded 100%- I am not sure that info is very clear in the NEC. You are correct in that KVA is KVA no matter what side of the trany it is on.
 
It turns out they may opt to get new units ... but its getting cold outside. Could take 6-8 weeks.
I could fix it in a week wit the transformers and cheaper than getting new units. I think the Engineer will be chewing on that expense for sure....
 

RobbieR

Member
Location
Atlanta
I always calculate my designs based on what the nameplate rating of the MCA/MOCB is on the mechanical equipment. Anything more than that is just fluff and not needed.

Remember, the MCA rating already has the 125% factor built in.
 

barclayd

Senior Member
Location
Colorado
I agree with Dennis - feed from a 480 panel.
By the way - how did 4 RTU's pass through the review/submittal process and show up on site without this being discovered?
db
 
I believe that was the answer I was looking for. So the Engineer was over rating the primary 208v side of the transformer unnecessarily going to cost the customer more money for upgrading wire and breakers while my calculations were correct, were they not?
Proof you have to go to college to get a PE stamp and still not know what you are talking about. :dunce:

Ok I'm a bit harsh...

Thanks for all your replies guys! Love this forum
 
I agree with Dennis - feed from a 480 panel.
By the way - how did 4 RTU's pass through the review/submittal process and show up on site without this being discovered?
db

Probably because the same Engineer that missed the voltage discrepancy in the design... also reviewed the submittals. :?

We ordered the units but probably should have checked the panels... never double check ourselves. Still the onus is on the Engineering....but that doesn't happen much now does it guys?
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
I believe that was the answer I was looking for. ...
Glad you got the answer you were looking for. All of us are more comfortable when we get the answer we want. Wrong or not we are more comfortable.

I... So the Engineer was over rating the primary 208v side of the transformer unnecessarily going to cost the customer more money for upgrading wire and breakers while my calculations were correct, were they not?
No, you are wrong - but you could get luckly and get away with it. The feeder CBs might not trip. But if one out of four trips -- well, you decide how stupid you will look. My customers would have a freeking fit.

Forget the MCAs and any other irrelevant data. You are sizing a transformer feeder. It would be really good if the feeder CB did not trip on inrush. Normal practice (at least for those of us with a PE stamp and experience - forget the college) is to:
1. Size the feeder at 125% of the xfm FLA. (Unless the xfm is somthing screwie and has a higher than normal inrush - this happens rarely, but often enough that I check)

2. Upsize the CB to the next standard size.

3. Size the wire that the ampacity is protected by the CB. (Note wire is not sized by the load MCA, or xfm FLA)

So, the 208 primary, wire size and feeder CB are set by the xfm FLA. I don't feel any pressing need to supply the code references. You will have to look them up your self.

Can you get away with sizing the feeder CB at 100% xfm FLA? Sometimes - depends on the feeder conductor impedance (size, length, raceway). Definitely not always. In fact, definitely not regularly.

... Proof you have to go to college to get a PE stamp and still not know what you are talking about. :dunce:

Ok I'm a bit harsh...
I wouldn't say harsh - arrogant, demeaning, donkey like - maybe.

But I like this forum too. And I feel an obligation to help out when I see someone is headed wrong.


ice
 

iwire

Moderator
Staff member
Location
Massachusetts
Just curious, this would be a change order correct?

You would charge for this work correct?

You make profit installing electrical equipment correct?

Assuming the above is true why in the world would you argue about it doing it?

Install it, it will work, you will make money.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Just curious, this would be a change order correct?

You would charge for this work correct?

You make profit installing electrical equipment correct?

Assuming the above is true why in the world would you argue about it doing it?

Install it, it will work, you will make money.

Some of us are actually concerned for the cost to their client. That is part of good business, IMO. Shave them bucks and they are eternally grateful. Usually, anyhow.
 
Glad you got the answer you were looking for. All of us are more comfortable when we get the answer we want. Wrong or not we are more comfortable.


No, you are wrong - but you could get luckly and get away with it. The feeder CBs might not trip. But if one out of four trips -- well, you decide how stupid you will look. My customers would have a freeking fit.

Forget the MCAs and any other irrelevant data. You are sizing a transformer feeder. It would be really good if the feeder CB did not trip on inrush. Normal practice (at least for those of us with a PE stamp and experience - forget the college) is to:
1. Size the feeder at 125% of the xfm FLA. (Unless the xfm is somthing screwie and has a higher than normal inrush - this happens rarely, but often enough that I check)

2. Upsize the CB to the next standard size.

3. Size the wire that the ampacity is protected by the CB. (Note wire is not sized by the load MCA, or xfm FLA)

So, the 208 primary, wire size and feeder CB are set by the xfm FLA. I don't feel any pressing need to supply the code references. You will have to look them up your self.

Can you get away with sizing the feeder CB at 100% xfm FLA? Sometimes - depends on the feeder conductor impedance (size, length, raceway). Definitely not always. In fact, definitely not regularly.


I wouldn't say harsh - arrogant, demeaning, donkey like - maybe.

But I like this forum too. And I feel an obligation to help out when I see someone is headed wrong.


ice

Thanks ICE I guess i should have waited for more replies from the senior members before i patted myself on the back.. Now i do feel like and ASS
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Proof you have to go to college to get a PE stamp and still not know what you are talking about. :dunce:

Everyone makes errors and no one knows everything about everything. It's why we have internal peer review; there is nearly always something amiss in the first draft. Bashing one another over mistakes is counterproductive; the priority should be to pass more than one set of eyes over a design and collectively ferret out the blips.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Are you buying "step up" transformers or are you going to run "step down" units in reverse?

For the past few years (since the Energy Act of 2007), I usually assume a 14x transformer inrush current (for sizes up to 150kVA) then I work backwards to pick a breaker (or fuse) that will handle this current. Most breakers have a maximum current pick up of about 10X so this means I usually start looking for breakers that can handle 150% of the primary FLA.
 

iwire

Moderator
Staff member
Location
Massachusetts
Some of us are actually concerned for the cost to their client.

I am as well, ....... when I am hired to be the 'designer'.

In this case the customer chose to hire an engineer for that part of the job, often the engineers have very good reasons for things I may see as over done. Perhaps future expansion, perhaps just good stout design.

That is part of good business, IMO.


Lets look at this the other direction, lets say the EC gets the owner to go along with less and it does not work out.

Has that helped anyone?
 
Well in this case it looks like the Commanding Officer has opted to exchange the 4 units for ones that will work with the supplied voltage. So no electrical change order thats is, other than the tens of thousands $ to replace the 4 units.... Uh, would have been cheeper to us ethe tansformers and done sooner....
 

JoeStillman

Senior Member
Location
West Chester, PA
If there is a change order for transformers and wire, the engineer who specified the wrong voltage on the AC units should be paying it.

An inexperienced electrical engineer will think it's up to the HVAC engineer to decide what voltage to use. It isn't. With few limitations, AC units come in whatever voltage you need. The electrical engineer MUST let her own mechanical department know what's in the building.
 

barclayd

Senior Member
Location
Colorado
I'm still just a little bit confused as to why the Electrical Contractor would purchase the RTU's rather than the Mechanical Contractor. (There IS a Mechanical Contractor on the job, right?)
I've been associated with some pretty goofy government jobs, but .... is this some kind of hybrid, design-build thing?
db
 

bullheimer

Senior Member
Location
WA
did i miss that you already have all the equipment? if not, boy do you ever need an RFI quick. they screwed up big time.
 
Probably because the same Engineer that missed the voltage discrepancy in the design... also reviewed the submittals. :?

We ordered the units but probably should have checked the panels... never double check ourselves. Still the onus is on the Engineering....but that doesn't happen much now does it guys?

Becasue the Mechanical Engineer should have specified the units and they would check with the EE on the available power.

If anything your EE was lenient in recommending the transformer sizes. There is just insufficient data in what you supplied to be able to determine the correct transformer size. Would need to know the startup characteristics of the largest compressor, assurance that only one of the compressors can start at any given time and knowing what is the load that may allready be running, knowing the SC and impedance characteristic of the supply side of the transformer and of course the transformer impedance.
 
Status
Not open for further replies.
Top