# Thread: Transformer size question

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## Transformer size question

I haven't messed with transformers in years. I gotta replace one but can't read the nameplate. It's 3 phase 480 to 240. It currently fed with #6 and fused at 60 on primary. Can we assume by the primary fusing it is a 15kva?

2. I am more inclined to suspect it is a 30 KVA. The full load current of a 15 KVA primary is 18 amps. I would fuse that at 25 amps, not 60. The more important consideration, I should think, would be the amount of load. What is known about that? Is the transformer presently operating, and if so can you measure the load?

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Originally Posted by electricblue
I haven't messed with transformers in years. I gotta replace one but can't read the nameplate. It's 3 phase 480 to 240. It currently fed with #6 and fused at 60 on primary. Can we assume by the primary fusing it is a 15kva?
480D primary to 240D secondary?

Primary feeder (480D - nothing screwie)
15kva at 125% = 23A ==> select 30A, #10 (just because a 25A CB is often hard to find
15kva at 250% = 45A ==> select 40A, #8 (often see 50A, #8)

30kva at 125% = 45A ==> select 50A, #8
30kva at 250% = 90A ==> select 90A, #3 (often see 100A, #2)

45kva at 125% = 67A ==> select 70A, #4
45kva at 250% = 135A ==> select 125A, #1

If the xfm is nothing odd, I tend to select 125% primary feeders. So, that's a resounding, "I wouldn't" (assume 15kva).

Best I can suggest is to:
Look at the secondary conductors and protection.
Measure the existing xfm and compare to catalogs.
Do a load calc.

ice

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Its barely operating. It has lost 2 coils. There is only 1 double pole breaker, the rest are singles. It's a half way building at the golf course. It has 2 bathrooms an ice machine and a little vending area with a hotdog warmer. Oh and a window ac unit.

5. Since you know all the loads, add them up, add a little for load growth, and install a new transformer at whatever size you think works best. Do you think a 3-phase system is needed here? It sounds like you could go with a 480-120/240 single phase. That would mean a new panel, but it sounds like this is all old equipment anyway.

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Thanks Ice. I see it now 30kva. Secondary fuse would be 100. Right?

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Charlie I was looking at platt.com and didn't see that option

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Originally Posted by electricblue
Thanks Ice. I see it now 30kva. Secondary fuse would be 100. Right?
For 240D secondary? Secondary FLA = 72A, Secondary fuse - 72 x 1.25 = 90.2A. 100A would likely be fine.

Curiousity Q: Your panel is fused? not a CB? That seems odd.

As I suggested. Do the load calc - don't guess. As charlie suggested - size the transformer to fit. You already have the 3ph panel - I wouldn't change it.

If the two pole load can handle 208V, go with a 208/120 transformer.

In any case - Stop guessing. Do the calc. Check the voltage spec for the two pole load.

ice

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I said fuse. I has a breaker. Thanks for kicking this around guys.

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