Dino9
Member
- Location
- Pomfret, maryland USA
For a 2400 amp 120/208 service what size Xfrmr would you deem correct?
Transformer size
- Thread starter Dino9
- Start date
- Status
- Location
- Massachusetts
Whatever size the power company provides.
On the other hand if this is under the NEC at least 870 kva.
On the other hand if this is under the NEC at least 870 kva.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
were in the NEC does it specify anything about xfmr sizing?
- Location
- Massachusetts
I don't think it does.
But I work in the real world with real inspectors and there is no way they would allow a calculated greater than the rating of the transformer.
At the least they could cite 110.3(A)(7).
Dino9
Member
- Location
- Pomfret, maryland USA
I have the answer buts its easy to screw up depending on how you calculate it. I=kvax1000 divided by 208x1.73. I made the mistake of multiplying 208x1.73=359.84. 1000kva =1,000,000 divided by 359.84=2779.012 but this is actually incorrect. You have to do the formula in order. 300kva is the correct answer because if you do the formula in order it comes out quite different. 300,000 divided by 208=1442.30x1.73=2495.19. By doing it in my head in 2 parts I managed to look like an idiot yesterday. Thanks to all who chimed in though. Remember kids, do your formulas in order.
JDBrown
Senior Member
- Location
- California
- Occupation
- Electrical Engineer
Um... I'm not sure I understood all that, but the way to calculate this is:
(208V x 1.73 x 2400A) / 1000 = 864 kVA
Which is why Mr. iwire said your transformer should be at least 870 kVA. So you did it right yesterday. You only get 300 kVA if you divide by 1.73 instead of multiplying by it, which is not the way you calculate kVA.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
For some people, the calculation is easier to remember as just 120x2400x3, as if the setup were wye, because the power going through the three phase wires is exactly the same whether it is going to a balanced delta or balanced wye load at 2400A per line.
You can demonstrate that the formula is exactly the same because 208 = 1.73 x 120 and 1.73 x 1.73 = 3.
So 2400 x (120 x 3) is exactly equal to 2400 x (208 x 1.73).
If you are given that the supply is 2400A 208 delta instead of 2400A 208Y/120, then the other way is simpler to compute but harder to remember for me.
wallypiper
Member
- Location
- Atlanta, GA, USA
I thought transformer sizing could take diversity and such into account so that they don't have to be sized to handle 100% of the connected load.
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