Temporary power station

[Fallriverbryan]

What is a Good way to set up a temporary power system? I want to plug into a 480 3ph welder outlet and have 2 480 3ph plugs and 8 120v outlets with 20 amp breakers. I've seen it done many different ways.

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[zbang]

Just to understand, you have a 480v 3-ph source and you want eight 120v 20a circuits from that?

To start, you'll need a step-down transformer . That, a disconnect/overcurrent device for the primary and a small main breaker 208/120v panel. You could even buy the latter as an assembly from Lex or Trystar.

Is the welder source up to the load? Remember that welder circuits are specifically allowed to use smaller wire than their rating would call for (based on the duty cycle, see art 630). You'll have to check that.

 

[Fallriverbryan]

Just to understand, you have a 480v 3-ph source and you want eight 120v 20a circuits from that?

To start, you'll need a step-down transformer . That, a disconnect/overcurrent device for the primary and a small main breaker 208/120v panel. You could even buy the latter as an assembly from Lex or Trystar.

Is the welder source up to the load? Remember that welder circuits are specifically allowed to use smaller wire than their rating would call for (based on the duty cycle, see art 630). You'll have to check that.

So, I didn't know where the common comes from for the 120v tap. Welder outlet is 3 legs and ground generally.

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[jumper]

So, I didn't know where the common comes from for the 120v tap. Welder outlet is 3 legs and ground generally.

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You derive your neutral(I despise calling it a common) from the secondary of the step down tranny.

 

[drcampbell]

I didn't know where the common comes from for the 120v tap. ...

If that's the case, you shouldn't be attempting to hook this up with only advice from the Internet.

 

[growler]

If that's the case, you shouldn't be attempting to hook this up with only advice from the Internet.

I once saw a guy that was trying to do a basement finish to include electrical and plumbing with advice he had gotten from the internet. He was trying to do the job as a professional handyman at a customer's house. That wasn't near as complicated as this. It was a real mess.

 

[zbang]

If that's the case, you shouldn't be attempting to hook this up with only advice from the Internet.

I have to agree.

 

[ActionDave]

If that's the case, you shouldn't be attempting to hook this up with only advice from the Internet.

I have to agree.

Nobody is giving step by step instructions on hooking up gear.

This forum exists for people in the electrical trades to ask questions and get advice and is the best place on the net to do so. It also helps elevate the trade overall when knowledgeable folks share information and wisdom.

 

[zbang]

This forum exists for people in the electrical trades to ask questions and get advice and is the best place on the net to do so. It also helps elevate the trade overall when knowledgeable folks share information and wisdom.

Sure, but I think it's rational to expect that someone who will be connecting a transformer to know the basics of what they do and how they're wired. I don't mean to trash Fallriverbryan, but connecting a 480-208/120 step-down is pretty basic, and includes making a neutral in the process.

Fallriverbryan- take a look online for step-down transformer wiring diagrams ("480v to 208v transformer wiring diagram") and read up on separately derived systems in the code, they should give you the theory of what you want to accomplish.

Also, you did not mention whether you need 3-phases at 120v. You can also do this job with a single-phase 480-240(center-tapped) transformer if the supply can give you enough current. You may need to add up all the actual loads instead of just 20amp times 8 circuits.

 

[jumper]

Nobody is giving step by step instructions on hooking up gear.

This forum exists for people in the electrical trades to ask questions and get advice and is the best place on the net to do so. It also helps elevate the trade overall when knowledgeable folks share information and wisdom.

I agree.:thumbsup:


Now about that three way in my hall that doesn't work.....er,,,never mind.

 

[Fallriverbryan]

So, a step down transformer can give 120v potential. If you want 120 volts in a 480 3ph panel you grab one leg of 480 and mate it with the common in the panel. That's why I ask....where is the neutral/common for the 480v welder plug setup?

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[zbang]

You grab two legs of the 480 and step that down, which creates the neutral on the 120 side. (And if your 480 3-phase is wye-connected it'll be 277V from phase to neutral.) Did you look at any step-down transformer diagrams? That should clear up some of the confusion.

 

[Fallriverbryan]

So all the draw will be on 2 phases of the 480. Is there not a way to draw from all 3 phases?

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[drcampbell]

It's possible, using either a 3-core, 3-phase 480:120 transformer or three individual single-phase 480:120 transformers.
But unless you need 3-phase 120/208-volt service, or the 120-volt power draw is so large that it must be balanced on all three incoming phases, it's unnecessary.

 

[GoldDigger]

Of academic interest, or if you have just one large single phase load that you need to power in a balanced way from a three phase supply, check out the Leyton 3-2 circuit here.

 

[kwired]

So, a step down transformer can give 120v potential. If you want 120 volts in a 480 3ph panel you grab one leg of 480 and mate it with the common in the panel. That's why I ask....where is the neutral/common for the 480v welder plug setup?

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Good chance your 480 volt supply system is 480/277 wye, which means the neutral or common wire of that system is 277 volts to each of the other "phase" conductors, there is no direct connection to any part of that supply system that gives you 120 volts, you need a transformer to derive 120 volts. There are several ways of doing it - most commonly chosen method would be a single phase transformer - depending on load needs and supply circuit capacity. A three phase transformer can give you more capacity as it will use all three 480 volt supply conductors, but your load will need to be balanced across all three phases or you still can have overload issues on portions of the equipment. Secondary of that system if you need 120 volts on all three phases must be a 208/120 wye connection, any other possible configuration will not have 120 volts on all three phases.

Primary side of your transformer is likely going to be 480 volts line to line (whether single or three phase). Not that something with 277 volts primary side is impossible, but is rare enough that it will be special order equipment vs standard stock equipment. The other issue with this is your welder outlet may not even have a neutral conductor present, or your 480 volt supply system may not even be 480/277 wye system in which case there is no 277 volt neutral. Could be a 480 volt corner grounded system, a high leg delta system, or even an ungrounded system - high leg delta will have two phases that are 240 volts to the system neutral, the others all you have is 480 volts between any two conductors of the system.

That is the basic options, other more complex options will involve electronic conversion equipment