Transformer size i need

[Willmo]

I am trying to find out what transformer i need to do a job.I am trying to set a 120/240 sub panel off a 277/480 Y feed from a unused a/c unit.The breaker feeding this circuit is a 3 pole 50amp.I have 3 #8awg and 1 #10 grey Nu.only need 5 lighting circuits,but a larger oven may be installed later.Dont know what transformer to order.And yes i plan on fusing before and after transformer.Been researching this for a while,getting too many different answers:? Thanks for any help.

 

[Smart $]

I am trying to find out what transformer i need to do a job.I am trying to set a 120/240 sub panel off a 277/480 Y feed from a unused a/c unit.The breaker feeding this circuit is a 3 pole 50amp.I have 3 #8awg and 1 #10 grey Nu.only need 5 lighting circuits,but a larger oven may be installed later.Dont know what transformer to order.And yes i plan on fusing before and after transformer.Been researching this for a while,getting too many different answers:? Thanks for any help.

Please provide calculated load, separated into continuous and noncontinuous... that is, if you want to use smaller than 50A primary rated current transformer.

 

[petersonra]

This would end up being about a 10 kV single phase transformer if you used the 50 Amp CB to protect the primary. Not all that big.

 

[baskervor]

Your intended application will determine the size of the transformer. If you just want to install the largest transformer you can safely install off of a 50A 3ph/3w circuit breaker, see below.

The largest commercial off-the-shelf transformer you can place is a 25kVA 240x480V-120/240V.

[50A * 80% (most CB's are 80% rated) = 40A
40A * 480V * ?3 = 33,254 VA = 33.254 kVA
standard available configurations 240x480 V Primary, 120/240V Secondary: 15, 25, 37.5, 50, 75, and 100kVA]

 

[kingpb]

Your intended application will determine the size of the transformer. If you just want to install the largest transformer you can safely install off of a 50A 3ph/3w circuit breaker, see below.

The largest commercial off-the-shelf transformer you can place is a 25kVA 240x480V-120/240V.

[50A * 80% (most CB's are 80% rated) = 40A
40A * 480V * ?3 = 33,254 VA = 33.254 kVA
standard available configurations 240x480 V Primary, 120/240V Secondary: 15, 25, 37.5, 50, 75, and 100kVA]

Good answer for single phase. But I have to ask the OP, why not utilize the 3-phase and go with 480V - 208Y/120V. You already have the 3P breaker and the number of conductors you need. You could go all the way up to 30KVA.

 

[baskervor]

The largest commercial off-the-shelf transformer you can place is a 25kVA 240x480V-120/240V.

:slaphead:This is incorrect, a 240x480V-120/240V transformer is a single-phase primary, single-phase secondary transformer, I goofed when I read the catalog for standard sizes thinking the primary was a three-phase primary, single phase secondary. The largest single phase transformer you can place is about 19.2kVA (40A * 480V).

If you must absolutely have 120/240V you have a couple of options, an OCB limited 25kVA (really should go with the 15kVA) single-phase transformer or, as kingpb mentioned, a 30kVA three-phase transformer. However, if you go the three-phase route, you must manage the load such that the 120V loads are limited to 1.5kVA, it is possible to install a 480V delta - 240V delta w/ 120V center taps. If you do not need the 120V capability, you can install a much simpler 480V-240V three-phase transformer.

That said, 480V-208Y/120V transformers are significantly less expensive, and there are few loads that will not operate well on a 208V service since a lot of equipment have a hefty tolerance on input voltage.

 

[augie47]

As far as the lighting goers, I'd go along with Bob in Post #3.
A 10 kva 1ph with proper secondary protection (125%) could be operated on your 50 amp breaker
with no additional protection and provide you with enough power for (5) 20 amp 120v circuits loaded at 16 amps.
If you want to allow for the oven, add the appropriate oven kw.
As noted, a 3 phase transformer would provide you more power but you would need to take the 208 voltage into consideration.