15kVA Single Phase Secondary OCPD size

Sparky2791

Sparky2791

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Per the National Electric Code, can a 15kVA stepdown transfomer feed a 100A panel (208V single phase secondary)

15000/208 = 90.14423A which is over 90A and the next standard OCPD is 100A. While I would recommend the use of a 20kVA would a 15 be acceptable?

Thanks for the replies!
 
A transformer can feed any load you want. The NEC is worried about too much overcurrent protection, not too little. The requirement is that the secondary protection does not exceed 125% of the transformer output only if the primary side protection is also above 125%.
So what is your primary OCPD size?
 
Sparky2791 said:
Per the National Electric Code, can a 15kVA stepdown transfomer feed a 100A panel (208V single phase secondary)

15000/208 = 90.14423A which is over 90A and the next standard OCPD is 100A. While I would recommend the use of a 20kVA would a 15 be acceptable?

Thanks for the replies!
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Pretty sure your math is incorrect. 15,000 / 208 = 72 amps (not ~90 amps). Im assuming you mean (15,000 / 208) x 1.25 = ~90A.

Keep in mind that transformers come in standard sizes. It may be difficult to find a readily available single-phase dry-type transformer in a 480v primary / 208v secondary arrangement. Similarly, a 20kVA single phase transformer isn't a standard size. It goes 15kVA to 25kVA (most with 240v secondary).
 
Let me clear this up, sorry I typed it quickly.

Primary is 480V single phase
Secondary is 120/208V single phase

Yes (Npstewart) my OP forgot to show the 125% in my math and the next standard size single phase transformer size would be a 25kVA.

Since the following calculation gets me over 90A (90.14423A) can maximum secondary OCPD be 100A (next standard size).

15000/208 = 72.115384A x 125% = 90.14423A (100A OCPD permitted?)
 
Sparky2791 said:
Let me clear this up, sorry I typed it quickly.

Primary is 480V single phase
Secondary is 120/208V single phase
Click to expand...

That is an impossible specification.

120/208V 'single phase' is actually 2 legs of a 3 phase system (I propose that we call it 'psingle phase'...) You _can't_ get it from a single phase transformer. (You could in theory have a 208V single phase coil with a 120V tap, but that would put 88V on the other leg and it wouldn't be close to standard 120/208V.)

I suspect that you are looking for a 480: 120/240V single phase transformer, or a 480 : 208/120V three phase transformer.

Sparky2791 said:
Yes (Npstewart) my OP forgot to show the 125% in my math and the next standard size single phase transformer size would be a 25kVA.

Since the following calculation gets me over 90A (90.14423A) can maximum secondary OCPD be 100A (next standard size).

15000/208 = 72.115384A x 125% = 90.14423A (100A OCPD permitted?)
Click to expand...

If you actually have a 15 kVA 208V transformer, then you can use the 'next size up' rule for the secondary _transformer_ protection. But the 'next size up' rule doesn't apply to the secondary conductors. (I think....) So you would need 100A _conductors_ if you use a 100A OCPD.
 
winnie said:
You _can't_ get it from a single phase transformer. (You could in theory have a 208V single phase coil with a 120V tap, but that would put 88V on the other leg and it wouldn't be close to standard 120/208V.)
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In theory I suppose one could make a two core open delta primary / open wye secondary that used two primary legs and a neutral, then you could get 120/208 open wye.
 
tortuga said:
In theory I suppose one could make a two core open delta primary / open wye secondary that used two primary legs and a neutral, then you could get 120/208 open wye.
Click to expand...

Agreed, but calling a transformer bank with two separate cores connected to the three primary phases a single phase transformer is even more of a stretch than calling '120/208V 3 wire' single phase :)
 
Yikes! You are all correct! Tunnel vision on this one and so sorry to confuse. 120/240V is secondary voltage, so my math is all off.

Will need a 25kVA to feed a 120/240V single phase 100A MCB panel.

Sorry folks mostly deal with 120/208V three phase WYE. So the 208V over road the 240V reference!
Apologies for wasting everyone's time!
 
winnie said:
Agreed, but calling a transformer bank with two separate cores connected to the three primary phases a single phase transformer is even more of a stretch than calling '120/208V 3 wire' single phase :)
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Yeah agreed the terminology used to describe open wye needs improvement, the NEC refers to is as single phase in at least one place Article 310.12 formerly 310.15(B)(7) in the 2017:
"single-phase feeder conductors consisting of 2 ungrounded conductors and the neutral conductor from a 208Y/120 volt system"
I personally think adding a definition of 'open wye' to the NEC would be an improvement as none of the current NEC descriptions even cover a true open wye system doing a quick search I see it described a few different ways;
"a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system"
In a least three places; Article 310.15 (E)(2) formerly 310.15(B)(5)(b) in the 2017, 400.5(A) and 520.44
then described as
"a 3-wire circuit consisting of 2 ungrounded conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system"
In 220.61(C).
if the code defined open wye (and open delta for that matter) it would clear up some confusion and add some consistency.

I would calculate open wye like 'single phase' and its typically only used to power single phase residential loads but since it gets special treatment in several articles it should have a definition.

Sparky2791 said:
Apologies for wasting everyone's time!
Click to expand...
Well it was a good thought experiment.
 
Sparky2791 said:
Will need a 25kVA to feed a 120/240V single phase 100A MCB panel.
Click to expand...


Well if you require secondary protection and if you are determined to use a 100A MB panel, then yes. There are other ways to skin the cat, however, like using an MLO and back feeding an 80 amp breaker. I wouldnt be buying the next up transformer size just so i could use a factory 100A MB panel.
 
electrofelon said:
I wouldnt be buying the next up transformer size just so i could use a factory 100A MB panel.
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As long as you keep the primary OCPD to 125% maximum, the secondary protection could any size, like the factory 100A breaker, even if the secondary output is lower.
 
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jim dungar said:
As long as you keep the primary OCPD to 125% maximum, the secondary protection could any size, like the factory 100A breaker, even if the secondary output is lower.
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Correct (note I did start out that response with "if you require secondary protection"). OP didn't state primary OCPD size. I have found those smaller single phase dry type transformers to have quite a bit of inrush. Recently i metered a 25 KVA and it overscaled my 1000A meter. If this is going to be turned on and off frequently and/or there are not knowledgeable people on site, he may want to go bigger than 125% on the primary.
 
electrofelon said:
Sorry, 240V, 25kva single phase. I dont know duration or recall off hand what the parameters of my meter's inrush function are.
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Your 1000A onrush was only about 7.5 times a typical 125A primary side breaker. It likely fits in the short time portion of the trip curve and could probably be held for 2-3 seconds.
 
jim dungar said:
Your 1000A onrush was only about 7.5 times a typical 125A primary side breaker. It likely fits in the short time portion of the trip curve and could probably be held for 2-3 seconds.
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Remember it wasnt 1000A, it overscaled me 1000A meter. A 125 would trip about 1 out of 4-5 times. Im usually a 125% is fine guy, but you do have to be careful sometimes (if it is important that it never drips upon startup, IMO it doesn't always matter)
 
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