480 volt to 120/208 New Commercial Service Design Help

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I have a new 65'X65' Commercial repair shop that is to be feed with 480 Volt 3 Phase transformer. I don't know how to build the service. Do I go from the transformer to a 480 volt disconnect then feed a meter then feed a step down transformer that feeds the 200 amp 3 phase 120/208 panel? How big should the transformer be?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The arrangement varies from area to area. Your first two steps would be to determine the load (480 and 208) and then contact your POCO and see how they plan to serve it and meter it. Depending on the local method and the load POCO may CT meter it or may use a self contained meter and may ot may not want a disconnect ahead of the meter.
A 75 kva transformer would likely be the one needed for your 200 amp 208v panel, but again, you need to know the load,.
There are a lot of unique NEC requirements for buildings with two voltages and transformers. If you have never wired one, you might come out ahead to find some experienced help.
 
More info

More info

Ok, I got more info on from the serving utility. They will let me install a 480 volt meter as long as my load is under 200 amps. The loads are as follows: 3 lighing circuits. 4 Plug circuits (16 total plugs) 1 30 amp welder and 8 50 amp welders all single phase. I'm going to go look at the name plates to see what the duty cycle is on the welders and if they are OK with 208 volts. Is the 75 KVA transformer still OK?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
This job, to me, begs for some engineering.
The loads need to be calculated and presented to POCO to see if the 480/277 transformer is the only option.
If the welders are going to run on a transformer that you install, you need some engineering input there, IMHO.
I see no way you are going to get by with a 100 amp 480v service.
With that, I will politely bow out.
 

ramsy

Owner/Operator
Location
LA basin, CA
Occupation
Service Electrician 2017 NEC
I see no way you are going to get by with a 100 amp 480v service. With that, I will politely bow out.
Customer is trying to skate without paying for approved plans, as normally required during the permit process.

"Bid with material list", or "Design my job & you can build it", is tasty bait for the uninitiated rookie.

Giving away material lists or specifications, is rewarded by clients running off to pay anyone to install it. Especially, if anyone exploitable is available.
 
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