277/480 to 120/208

[mstrlucky74]

I have 800A 120/208 switchboard I'm trying to feed but since the distance from the source to this 800a board is far I'd like to run 277/480v feeders from the source to a xfrmr by the 800A board. How do I size those 277/480v feeders? Based on the transformer size I'll need? if so how do I size the xfrmr? Thanks.

 

[lielec11]

Take a look at 450.3 and 240.21

 

[electrofelon]

Note that you only need to run 480 without a neutral for a delta-wye transformer. As far as sizing, you've gotta know what the load is, unless you by default want the full 800 amps. Then size your tansformer to the load, then size your conductors. How far are we talking?

 

[lielec11]

I would think the largest transformer you'd like to use would be 225kVA in order to utilize the existing 800A, 208V board. You can then size the 480V feeders based on that size transformer.

 

[Smart $]

In simple terms, the minimum transformer rating:

P_IN = P_OUT

480V x ___A x sqrt(3) = 208V x ___A x sqrt(3)

 

[electrofelon]

Generally, its got to be pretty damn far to make it worth going step up step down when you look at the cost of the transformers and losses for eternity. The wire for 800 amps 208 is about $13 per foot. Figure 1/3 of that for 480. I dont know off hand transformer numbers, but last I checked they did not give them away. 208 will cost you more for pipe and labor, and maybe VD depending on what you want/need.

 

[Jraef]

I have 800A 120/208 switchboard I'm trying to feed but since the distance from the source to this 800a board is far I'd like to run 277/480v feeders from the source to a xfrmr by the 800A board. How do I size those 277/480v feeders? Based on the transformer size I'll need? if so how do I size the xfrmr? Thanks.

So you didn't make this very clear. Do you ALREADY HAVE 480V at your service, and you are wanting to ADD an 800A 208V panel down stream? Or do you only have a 208V service, and you want to add ANOTHER 800A sub panel off my of it? Because it seems odd that you would have something big enough, such as a 1600A existing 208V service to where you could easily just drop in another 800A. Why don't you expand on your description some more if you want clear help rather than make us play 20 questions?

 

[mstrlucky74]

So you didn't make this very clear. Do you ALREADY HAVE 480V at your service, and you are wanting to ADD an 800A 208V panel down stream? Or do you only have a 208V service, and you want to add ANOTHER 800A sub panel off my of it? Because it seems odd that you would have something big enough, such as a 1600A existing 208V service to where you could easily just drop in another 800A. Why don't you expand on your description some more if you want clear help rather than make us play 20 questions?

Service is 277/480v.

 

[mstrlucky74]

Thanks everyone.

 

[Jraef]

I have 800A 120/208 switchboard I'm trying to feed but since the distance from the source to this 800a board is far I'd like to run 277/480v feeders from the source to a xfrmr by the 800A board. How do I size those 277/480v feeders? Based on the transformer size I'll need? if so how do I size the xfrmr? Thanks.

Service is 277/480v.

OK, good. Just wanted to avoid the XFMR-Up / XFMR-Down scenarios that were playing out, that's not what you have.

Yours is simpler then, you are going to run 480V out to your transformer some distance away, at which point it will feed an 800A distribution board at that remote site.
800A at 208V 3 phase is a 300kVA transformer, assuming no spare capacity for starting large motors, etc.
480V primary current is 360A, so fed with a 400A feeder breaker in your 480V distribution board.
480V primary conductor size based on 360A load, 3 phase conductors + ground (no need for Neutral), size adjusted for distance / voltage drop and conduit fill, etc. I'd probably run it as 2 x 3/0 THHN per phase, but I don't know your distance, routing, etc.

 

[lielec11]

800A at 208V 3 phase is a 300kVA transformer, assuming no spare capacity for starting large motors, etc.

300kVA at 208V is 833A before overcurrent protection. Overizing by 125% puts us over 1000A. Shouldn't you stick to a 225kVA or am I missing something?

 

[Danelectron]

300kVA at 208V is 833A before overcurrent protection. Overizing by 125% puts us over 1000A. Shouldn't you stick to a 225kVA or am I missing something?

Jraef's calcs look good. The transformer was selected (from standard available) to provide the capacity for the 800A load @ 208. The smaller (225k) would only be able to supply 625A to the load ( if I'm not mistaken). The desired output and supply are the limiting factors to size the transformer and feeders.

 

[mstrlucky74]

OK, good. Just wanted to avoid the XFMR-Up / XFMR-Down scenarios that were playing out, that's not what you have.

Yours is simpler then, you are going to run 480V out to your transformer some distance away, at which point it will feed an 800A distribution board at that remote site.
800A at 208V 3 phase is a 300kVA transformer, assuming no spare capacity for starting large motors, etc.
480V primary current is 360A, so fed with a 400A feeder breaker in your 480V distribution board.
480V primary conductor size based on 360A load, 3 phase conductors + ground (no need for Neutral), size adjusted for distance / voltage drop and conduit fill, etc. I'd probably run it as 2 x 3/0 THHN per phase, but I don't know your distance, routing, etc.

Thanks

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