Transformer Size?

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KSM8565

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I'm trying to calculate the size of a transformer. I would appreciate any and all input. I have 2#8's (THHN) available to use from existing panel (120/208V) to outside hand vault (will be in separate building). I need to feed a 3.3KW heater (208V 1PH), 6-120V fluorescent fixtures (1.5 amps), 1 general purpose receptacle and 1 exhaust fan (non-coincidental load to the heater). I figure a total of no more than 24 FLA at any given time. What's the smallest transformer I can install given that we could install a 2P/50 amp breaker (with 75 degree terminals) in the existing panel?
 
Your loads are all 208V and 120V, not sure why you need a transformer, unless; are you saying you do not have a neutral available?

If no neutral is possible, than you need a 208V - 120/240V transformer, probably 7.5KVA would get you by.

The accuracy of the answer you receive is totally dependent on the accuracy of the information you provide.
 
First, I agree with kingpb's question: why do you need a transformer?

If the limiting factor is that you have 2 wires available, then you have a problem:

With only 2 wires, you have single phase. This means that you could get a 120/240V secondary, but not a 120/208V secondary.

So if you use a 120/240V transformer, then you won't have 208V for your heater. Is it rated for 240V?( I probably is, but at 4.4KVA, which means you need to adjust your load calculations).

You could install a subpanel, and supply 208V to the heater bypassing the transformer, and then use the transformer for 120V loads. This might let you get away with a smaller transformer, if the exhaust fan is a significantly smaller load than the heater.

Finally, since your supply is 208/120V, and the limiting factor is the number of wires already in place, do you have enough wires to install the transformer? In particular, do you have an EGC?

If you need to install more wires, then you might consider simply pulling a neutral along with your hots and avoiding the transformer

-Jon
 
I don't think you will find a single phase transformer with a 208 primary and a 120/208 secondary. Is the electric heat suitable for use at 240 volts?
 
kingpb said:
Your loads are all 208V and 120V, not sure why you need a transformer, unless; are you saying you do not have a neutral available?

If no neutral is possible, than you need a 208V - 120/240V transformer, probably 7.5KVA would get you by.

The accuracy of the answer you receive is totally dependent on the accuracy of the information you provide.
Click to expand...

The conduit and wire was existing. There are 2#8's, 1#10 green, 1#12 Black, 1#12 red, 1#12 blue, 1#12 White & 1#12 green. We tried to pull the old wires out. They are stuck and will not budge. The inspector is requiring disconnects for each load in the separate building. I am planning on installing a sub-panel and feeding it from the transformer.
 
Your calculation of 24A is presumably based on having 208V, implying 5KVA of loading. Reading between the lines, this means 3.3KVA of heater and 1.7KVA of other stuff.

If you use a transformer, you will be forced to 240V, which will increase the loading of the heater from 3.3KVA to 4.4KVA, giving a total load of 6.1KVA; so you are back to using a 7.5KVA transformer.

Pity about the size of the other conductors; the 12ga white wire is sufficiently sized to carry your maximum unbalanced load...but it also needs to be sized as large as the EGC.

The other option is to set a panel (to provide the local disconnect), feed the heater at 208V, and then use a 2 or 3KVA transformer for the other loads.

-Jon
 
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