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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?

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    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?
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Quote Originally Posted by electricblue View Post
    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
    Without data you’re just another person with an opinion – Edwards Deming

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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.

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    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.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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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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    Quote Originally Posted by electricblue View Post
    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
    Without data you’re just another person with an opinion – Edwards Deming

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

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