ECN Forum Forums General Topics of the Trade General Discussion Area Sizing a Transformer

Greetings All,

I have a question about sizing a transformer. I've tried looking online and using my understanding of ohms law but I keep getting different answers.

Here's the deal. I need to supply 400amps @ 120V. It will be fairly simple to balance the loads so is there any benefit to setting up the panel as 230/3 phase? (125amp per leg) (Yes I know we'll need a Neutral to get 120V)

If my understanding is correct, using this method will allow me to get away with a 75KVA transformer.

Bottom line, there's plenty of capacity at the 460V main. The guy putting in the transformer is licensed but there's a bit of a language barrier and I'm not certain he understands my question.

Thanks!

Well Mort, I would expect that you might see 120/208 Wye 3 phase Vs 120/240 single phase. The obvious advantage would be that you have the ability to power 3 phase motor loads. I don't see many 400A requirements that don't include motor loads.

That will be 208 3 phase and using my power formula the actual load would be about 45 kva (with 125a per phase) but you need to use 125% of that, so at least a 57 kva transformer. If you want to plan on harmonics and additional load you are still OK with the 75 ... but I am not the engineer here. Maybe Scott will stop by.
If you have a reactive load you also want to make that neutral a couple sizes bigger. (Triplin harmonics)

A 150kVA will give you 416 Amps of 208Y/120, but Mort is looking for 230/3 phase, which is a different animal altogether than the Wye connection we're all familiar with.

He's also going to have a separate zig-zag transformer in order to get a working Neutral, since a Delta connection on a 230/3 phase system is limited to not more than 5% of the transformer rating available for phase-to-neutral loads.

The loads are going to be several computers... no motors and by several, I mean a couple hundred.

What I was getting at was, if I treat the panel like a 208/3ph and run a separate neutral to break each leg down to a 120v circuit, would that allow me to get away with a smaller transformer?

In the end, I need 400amps @ 120v and I want to know if a 75KVA will suffice.

Thanks guys!

Assuming that you what really need is a lot of 120V for the computer loads the answer to your question is no.

A 75kVA transformer will change 90 Amps of 480V-3phase into 208.3 Amps of 208Y/120V-3 phase. There's just no way to draw 400Amps out of a 75kVA transformer...the math just won't allow it.

The next larger standard size transformer is 150kVA, which will change 180 Amps of 480V-3phase into 416 Amps of 208Y/120V-3 phase. You'll probably need to break that down by using a couple of 225A panelboards so you can end up with the plethora of 20A-1p circuits that you will need.
Remember to either run separate neutrals for each circuit or to increase the neutral size to 200% if you wire the branch circuits in a network configuration.

This reminds me of a conversation with a PHDd Electrical Engineer.

He said he had 120 amp service at his home, further questions resulted in 'two (2) 60 amp fuses'. His logic was 120 available amps at 120 volts. I wound up having my guys upgrade his service to 200 amp. He later was bragging that he had a '400 amp service in his home'.

Logic??

I think the OP here is going down the same path....
75 KVA, wye, 120 208 panel with the above logic is 600 amps at 120 volts.

That's a good point, John;
75(a phase)+75(b phase)+75(c phase)+75(Neutral) = 600

I'm having a similar situation here with our IT Department.
They apparently believe that if they plead long enough and hard enough I'll somehow 'find' a way to increase their 100A feed to 200A.

They keep implying that I'm just being mean by telling them that they have to abandon the existing 100A feeder so that they can get a new 200A feeder.

They also (naturally) want to do it for as close to 'free' as possible. Maybe I'll ask them why they want the Electricians to work for free but don't balk about having to pay real money for new computer equipment.

Sometimes folks take the "electricity is just like water" analogy too literally.

Mort,

Where did you get your 400 amps from? Power supply wattage times the number of computers?

Are the computers located in the same room?

Do they all have separate monitors?

What else is going to be powered by the 120V? PC speakers, desk fans, desk lights, copiers and printers, etc. Not to mention the network hardware needed to support 400 computers. Do you have a telephone system too.

Most computers and monitors can run off of 208V or 240V, as well as 120V. It won't reduce the transformer size but it could allow you to run more machines off of a single circuit.

You might want to get a more experienced engineer to look over your assumptions and calculations.

Good luck with the project.

I really wish Scott would stop by because the square root or 3 comes in here.