Three phase service questions

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
A small woodshop does not use 400 amp 3 phase. Unless he has many people working there this shop may only have one machine on at a time.

Is this a shop for a home owner? If so he probably added up all the machines and figured he need 400 amps
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
Most poco’s use CT metering on 400 amp and above, though a lot of 400 amp services use actually a 320 amp meter base. If three phase is available, it will depend on existing transformers. Older complexes may have 120/240 if it’s an industrial area, but if it’s office/warehouse type complex, it will most likely be 120/208 or 277/480. If a new transformer bank is required, as others have said, the utility will want a guaranteed minimum KW usage, or a upfront cost of installing the bank. I had a customer that the utility wanted a minimum kw, and a finance charge of a couple hundred a month, I believe. I think it was for a term of 10 years. Anyway, if the equipment is rated 240 volt only, and only 208 is available, it may be cheaper to use buck/boost transformers on the equipment.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
Please excuse my ignorance. I may be designing a three phase service and it's my first one. Client says they want a 400 amp three phase 240v service for a small workshop. I'll be looking at it tomorrow. I haven't contacted the POCO yet to see if they can supply this voltage directly, or I need to install a transformer. (480 to 240 delta/delta?)

Q1: Does 400 amps mean the total of the phase to phase connections (AB + BC + CA) is 400 amps, or each phase to phase connection is 400 amps?
Q2: The delta transformers I have looked at have a 120 v tap, but it's limited to 5% of the KVA. If I wanted more 120V, would I have to install a single phase transformer connected to two phases of my 240V output?
Q3: If the POCO can supply 240V three phase directly, I assume this would be four wires. Is there the same 5% of KVA limit on 120V?
Q4: If I supply a 480 to 240 transformer, how many KVA would I need for 400 amps?
Q3 is no, the power company uses a larger third transformer in the bank, so it is not as limited.
That is an easy way to spot a delta system on the poco side, one transformer is larger than the other two.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Having been down this road a few times, my thoughts pretty much are repetitive of others::
1. Client will be shocked when he see POCO's charge.
2. Doubtful he needs 400 amp but 200 or 400 won't change overall costs that much and if he's willing to write the check........
3. As noted by Jraef and Hillbilly, much of his equipment is likely compatible with 208. I'd look at individual buck n boost transformers where 240 was absolutely necessary rather than 1 large unit.
4. Unless the client has deep pockets, the plans willm downscale considerably when expense is factored in.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Having been down this road a few times, my thoughts pretty much are repetitive of others::
1. Client will be shocked when he see POCO's charge.
2. Doubtful he needs 400 amp but 200 or 400 won't change overall costs that much and if he's willing to write the check........
3. As noted by Jraef and Hillbilly, much of his equipment is likely compatible with 208. I'd look at individual buck n boost transformers where 240 was absolutely necessary rather than 1 large unit.
4. Unless the client has deep pockets, the plans willm downscale considerably when expense is factored in.
My thoughts exactly. And if this is an investor own POCO (IOU) as opposed to a coop, the price is likely to be staggering especially when they calculate how little the revenue there will be from this service.
 

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
A small woodshop does not use 400 amp 3 phase. Unless he has many people working there this shop may only have one machine on at a time.

Is this a shop for a home owner? If so he probably added up all the machines and figured he need 400 amps
It's a small commercial shop. 3000 square feet. Metal fabrication. It looks like they are in the process of opening. The owner is all starry-eyed and excited. I expect there will be one or more employees. I fully expect plans to change when he hears how much POCO wants to charge for extending lines. I'm just doing my job advocating for him and trying to get some business if possible. Likely this will turn into a single phase service upgrade but since I have never spec'ed a three phase service before, I'm enjoying learning some new stuff.
 
Sounds like one of the times where the client selected the location without considering the power available; maybe the rent was cheaper because no 3-phase on-site. Unfortunately, they're going to pay for either of bringing 3-phase on-site, swapping motors, or VFDs/phase-converters.
 

dkarst

Senior Member
Location
Minnesota
Two points reinforcing above... 1. It's easy to want 400 Amps of 240V service until the estimate comes in...

..... It looks like they are in the process of opening. The owner is all starry-eyed and excited. I expect there will be one or more employees......
and 2. if this turns into billable work, make sure you have a path to getting paid!
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
It's a small commercial shop. 3000 square feet. Metal fabrication. It looks like they are in the process of opening. The owner is all starry-eyed and excited. I expect there will be one or more employees. I fully expect plans to change when he hears how much POCO wants to charge for extending lines. I'm just doing my job advocating for him and trying to get some business if possible. Likely this will turn into a single phase service upgrade but since I have never spec'ed a three phase service before, I'm enjoying learning some new stuff.

The reason I asked is because I had a Home owner insisted he needed a 600 amp service because he had a woodshop that needed 400 amps. I politely told him he was crazy and that 200 amps was more than he could ever use in a shop. He didn't except more than one or two machines on at the same time. LOL
 

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
I read that and said to myself, "yea, that's spot on!". Then I thought "how would you do that?".

Suggestions? :)
If you sense you might have trouble getting paid:
1) Make sure the client understands that payment is due when the job is completed.
2) Collect as much upfront as they will pay.
3) Be prepared to file a lien if they don't pay. (Only works in limited circumstances.)
4) Decide whether you really need the job. Decline it if you think it's going to be too much trouble.

There are jobs where I will only do the job if payment in full is given before starting.
 
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Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
If you sense you might have trouble getting paid:
1) Make sure the client understands that payment is due when the job is completed.
2) Collect as much upfront as they will pay.
3) Be prepared to file a lien if they don't pay. (Only works in limited circumstances.)
4) Decide whether you really need the job. Decline it if you think it's going to be too much trouble.

There are jobs where I will only do the job if payment in full is given before starting.
5) Don't call for final inspection until you're paid. Well maybe leave 10% for after passing final.
 

sw_ross

Senior Member
Location
NoDak
I had a customer that has a metal fab shop with an assortment of 3p tools who wanted 3p supply to his shop but the poco wouldn’t run it unless he wanted to pay. It was about a mile away from his location so not an option cost wise.

He had a single phase 200 amp panel/service at the time. He knew he was only going to be running the equipment with no employees.
What we ended up doing was getting a 20-HP phase converter to feed a 3p panel and he feed the 3p equipment out of there.
All of his 120/240 loads came out of his existing panel.
It was the least cost method for him to have 3p in his shop. He’s very happy about how it worked out. That was 3-years ago.
 
I did a shop for a woodworker. It was a renovation, not built new from the ground up. It had an existing 200 A single phase panel. I don't even want to know what size transformer was feeding it, it was in more of a residential area. it was all single phase equipment at first, some larger stuff probably a few motors that were five horsepower. Later he acquired a three phase 30 HP belt sander. He got a rotary phase converter. we had to put one of those 150 amp breakers that takes up 4 spaces to feed it. It's been fine, no problems whatsoever. I would doubt a 320 / 400 amp service would be necessary for the OP unless maybe he had multiple guys working there running a bunch of machines at the same time
 

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
I would doubt a 320 / 400 amp service would be necessary for the OP unless maybe he had multiple guys working there running a bunch of machines at the same time
I don't know how many machines will be on simultaneously. I don't know how many employees he might end up with. He wasn't very forthcoming with answers. Normally I will advise potential clients against getting more than they need, however, if they insist, I'm not going to dissuade them from giving me a big check.
 

Coppersmith

Senior Member
Location
Tampa, FL, USA
Occupation
Electrical Contractor
He had a single phase 200 amp panel/service at the time. He knew he was only going to be running the equipment with no employees.
What we ended up doing was getting a 20-HP phase converter to feed a 3p panel and he feed the 3p equipment out of there.
All of his 120/240 loads came out of his existing panel.
It was the least cost method for him to have 3p in his shop. He’s very happy about how it worked out. That was 3-years ago.
This might work. I need to find out how much actual 3p equipment he has. I was unable to look at the equipment when I was there. This client seemed more concerned about the savings from using three phase equipment than anything else. I assume a phase converter doesn't save any amps and probably uses a few extra.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
This might work. I need to find out how much actual 3p equipment he has. I was unable to look at the equipment when I was there. This client seemed more concerned about the savings from using three phase equipment than anything else. I assume a phase converter doesn't save any amps and probably uses a few extra.
It does not save any amps except that the same motor will draw less current on each of three wires than it does from each of two wires to get the same power. And, yes, that reduced load amperage will be reversed by the phase converter to get the same two wire draw plus some losses.
The total kW will definitely be slightly greater.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
I’ve got a woodworking shop that wants a three phase vacuum pump installed for a CNC router. The router is single phase 220, but the vacuum is 10 hp three phase. Only a 200 amp single phase service, but not many machines being run at one time. They are having me hold off for now, as they are contemplating a move to a larger suite in the same complex, but everything in the complex is single phase for some reason. The complex across the road is three phase.
 
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