3-phase service capacity 480V -> 208Y/120

[JuliaTruchsess]

I have a service rated 480V 200A. I have a piece of 3-phase equipment that draws 400A of 3-phase power at 208V. I want to put in a 3-phase transformer to get 208 from the 480. My calculation is as follows (ignoring transformer losses and assuming a balanced load):

480V 3P @200A = 166kVA

208V 3P @400A = 144kVA

therefore the 200A 480V service has enough power.

I consulted with an electrician, and his response was "Not enough power. 208 X 400 =83,200 / 360=231".

It sort of looks like he's converting the 400A load to kVA as if it were single-phase, and then converting that back to 3-phase current, but it just doesn't seem right to me. Maybe I'm missing something.

Would appreciate any clarification/edification, thanks.

 

[Jraef]

He screwed up.

360 is a number that you use for 208V 3 phase, because it incorporates the square root of 3 into the formula for you already; 208 x 1.732 = 360.

But the way to use that would have been 360 x 400 = 144,000, the number that you came up with (most likely by going 208 x 400 x 1.732).

He applied the 360 value AFTER having already multiplied the 208 x 400, so he in effect doubled the voltage...

 

[JuliaTruchsess]

Thank you, I will start looking for another electrician

 

[electrofelon]

I have a service rated 480V 200A. I have a piece of 3-phase equipment that draws 400A of 3-phase power at 208V. I want to put in a 3-phase transformer to get 208 from the 480. My calculation is as follows (ignoring transformer losses and assuming a balanced load):

480V 3P @200A = 166kVA

208V 3P @400A = 144kVA

therefore the 200A 480V service has enough power.

I consulted with an electrician, and his response was "Not enough power. 208 X 400 =83,200 / 360=231".

It sort of looks like he's converting the 400A load to kVA as if it were single-phase, and then converting that back to 3-phase current, but it just doesn't seem right to me. Maybe I'm missing something.

Would appreciate any clarification/edification, thanks.

I recently had a similar situation. Although there is theoretically enough capacity on the 480 service (you didn't mention existing loads, but let's just assume there is) I would advise to investigate getting a new 208 service. Here's why: your utility is very likely not serving you with 400 amps of transformer capacity, so if this is the case, you are already looking at upgrade charges from the utility. Add to that transformer cost and losses, and you may be better off bringing in a new 208 service.

 

[Jraef]

I recently had a similar situation. Although there is theoretically enough capacity on the 480 service (you didn't mention existing loads, but let's just assume there is) I would advise to investigate getting a new 208 service. Here's why: your utility is very likely not serving you with 400 amps of transformer capacity, so if this is the case, you are already looking at upgrade charges from the utility. Add to that transformer cost and losses, and you may be better off bringing in a new 208 service.

I think that’s a good point. To add to that, just in case there are no 480V loads, when you buy a service from a utility, they own the losses in the transformer (because your meter is down stream of it). When you add a transformer down stream of your existing service, YOU own the losses. It’s not a lot, but at 24/7/365 it adds up.