ECN Forum Forums Code Related Discussion NEC & other Code issues Transformer Installations

I figured we could talk about Separately Derived Systems, (SDS) specifically SDS derived from transformers.

Here is a great picture of an transformer installation.



I am not interested in 'slamming' the installer I would like to discuss the NEC requirements regarding this installation.

Personally I think this is a nice job and only have one minor code issue with it.

What do the rest of you think of this?

For those of you who have not installed transformers join in, ask questions. No one is going to get flamed for trying to learn new things.

Bob

Bob, I can't tell for sure, but is the bonding jumper undersized on the secondary side?

Ryan just call me Homer.

I don't know what I was thinking. I used a picture that I have no info on.

DOH!

For the sake of things lets say we have a 75 KVA transformer 480-208Y/120.

Where would we get started figuring out conductor sizes assuming we want the maximum capacity from this 75 KVA unit?

Bob

OK, I'll take a stab at this--

Input: 75,000VA / 1.73 / 480V = 90A. Call it #3 or #4, depending on whether you play by the "next larger standard breaker size" rule.

Output: 75,000VA / 3 / 120V = 208A. If you call this 200A, it will take 3/0 cable. I don't know if the rules would allow fusing this at 225A; if so, you'd want to use 4/0.

The above assuming 75 degree wiring.

I would imagine that either the input or the output fusing is supposed to limit the TX to 75 KVA; I don't know what the rule is for this. In a vacuum, I'd say a 90A breaker on primary feeder would do the job.

Well, on a SQ D slide-rule my teacher gave us last semester, it states that a 75 KVA can hold a maximum of 208A at 208V, 180A at 240V, 90A at 480V, and 72A at 600V.

Then, going from 310.15, 4/0 for 208V, 3/0 for 240V, #3 for 480V, and #4 for 600V.

I barely finished my 1-year program, I'm barely going to start my Apprenticeship. Someone correct me if I'm wrong.

SolarPowered, which formula are you using to calculate that?

VoltAmps = Volts * Amps

I suppose that answer was rather sparse. I also used the relationship that the VA of a three-phase circuit is sqrt(3) * the VA of a single phase circuit, at the same number of amperes per conductor.

Thanks guys.

At this point I think we have ampacity of the transformer worked out.

On the primary side we have a full load current of 90 amps

On the secondary side we have a full load current of 208 amps.

Now we need to figure conductor and breaker sizes keeping in mind that we want the maximum capacity from this transformer.

The connected load to this transformer is 75 KVA continuous.

Bob

My problem here is protection of the transformer. Using normal breakers, with a continuous, 100% load, you need to oversize by 25% because of the 80% rule. That would suggest 90*125% = 112.5A on the primary, and 208*125% = 260A on the secondary.

Also, there is some loss in the TX. Supposing 5% loss, you need 94.5A in to get 75kVA out.

So, my thought would be to use 100% rated breakers, with #3 fused at 100A on the input, and with 4/0 fused at 225A on the secondary.

This doesn't really limit the TX to 75 kVA. A 90A, 100% breaker would limit the input to 75kVA, but you won't get 75kVA out of it. Assuming this isn't a trick question where the premise is false, you said that the load is 75kVA; therefore, I have to assume that the TX is designed and listed to have an input large enough to supply that load. Given that, I'd feel comfortable with an adjustable, 100% breaker set to 95A supplying the primary. I have no idea if that would meet Code. And I'll stay with 4/0 at 225A, 100%, on the secondary.