Adding 110V Outlet On 3 Phase Machine

Do you need a transformer when adding a 110v outlet? Or can you just take a 110v leg to one side of the outlet & white/common to ground to the other side?

Need a transformer.

I have several 3 phase machines running off of a 240 volt rotary phase converter. All of them that have 120 volt outlets have transformers. They can be set for 240 or 480 input.

I should add that on most of my manual machines I have mounted a 4" box with 2 outlets and a cord with a plug. That way if I want to use the light or power feed I don't have to turn the phase converter on. If you have 3 phase that is on full time it would be different.

What electrical system do you have? If you have a 240V high leg delta, I think you can do it. I don't know how it agrees with the NEC, but what I have seen is you need to pull a 4 wire system plus a ground. This would give you 120V, neutral, and ground for your plug, and three phase and ground for your machine. You do need to ensure the plug is only fused to 20A.

The transformer is the only way if there is not a separate neutral pulled for the 120V.

NEC requires a transformer for this application. This way if phases are swapped it won't have any affect on the 120v power, plus you get a neutral in the deal. However, on something I owned, I wouldn't hesitate to run a recep off one of the 120v legs.

If you have a 208 Y and you've pulled all 4 wires plus ground... you can get 120 between any phase and neutral. Obviously.. it needs to be fused/breakered appropriately.

But... assuming you have a 3 wire plus ground system, you need a transformer. Easy to get, inexpensive and more importantly - safe. Modern machine control transformers will have integral fused input/outs, but you can add them. It is neither safe or legal to pull 120v if it's referencing ground; it must reference neutral. Make sure you buy a transformer big enough for the intended purpose.

Someone will no doubt pop up and say they are the same 'cos they are joined at the panel, but just ignore them


IF you are running a 240v RPC, you can just extend the neutral (all the way from the panel to the rpc and then to the machines, and reference L1/l2 to neutral for 120v, fused of course.

Big B said:

I have several 3 phase machines running off of a 240 volt rotary phase converter. All of them that have 120 volt outlets have transformers. They can be set for 240 or 480 input.

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Be sure you use only the power company 240V and not the RPC generated phase for the transformer power.

4 wires - A ground, 1 high leg & 2 wires 110v.

If it is a ground, then you can not use that for a 115V circuit. You MUST have a neutral wire from the breaker box. Ground wires are for safety: neutral wires are for current return in an active circuit. So using a ground wire for a circuit may seem to work, but it is not safe to do so.

A transformer should be fairly inexpensive so just get one. And wire it according to the NEC.

blake in spokane said:

4 wires - A ground, 1 high leg & 2 wires 110v.

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So how does the transformer create a neutral?

Our elecrician put plugs on ours, they are 220V 3 phase machines. No tranny needed.

A transformer will totally isolate the 115V circuit from the AC power mains. It is, in effect, it's own local power source and the Voltage that it creates will be floating with respect to the building power. At that point, the terminology "hot" and "neutral" becomes meaningless. Either wire from the secondary of the transformer could be considered the hot and the other the neutral. They are completely interchangeable. The current is generated in that secondary winding and returns to it and it does not matter one bit which wire is considered which.

I do not know what the code requires for such a circuit and it should be consulted. Both of the transformer's wires could be left complete isolated from any local ground or neutral and it would still function just fine: I have seen it done this way and that is the way I would do it. In such an isolated circuit there is no actual neutral.

On the other hand, either one of the wires of the secondary could be connected to a local ground and the circuit would still function just fine. I would NOT use the safety ground wire to the machine for this: a separate ground would be better. Again, the code should be consulted. And this ground connection, not the transformer, would create a neutral for that previously isolated, floating circuit. It would not necessarily be exactly equivalent to the building's neutral wiring and should not be connected to it unless the code so specifies.

As I said, consult the code and follow it for connecting such a transformer. In any case, I would strongly recommend that the outer metal frame of whatever devices that these wires are within should be properly grounded just like the rest of the machine.

Ironwoodsmith said:

So how does the transformer create a neutral?

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Sounds like an extension cord would solve your problem a lot easier than a transformer.

"So how does the transformer create a neutral? "

Good question.

The primary of the transformer will be between two hot legs of the incoming three
phase service. The secondary is electrically insulated from the primary, and for the
purposes of the NEC codebook, it is called a "separately derived service."

They treat the secondary as a brand new service entrance, and one end of the secondary
is tied, or bonded to chassis ground in the machine. So the high leg of the secondary
becomes the "black" wire, and the grounded end is where the green groundING wire and
white groundED wire are tied, or bonded, together.

Because it's a separately derived service, you MUST bond the neutral to ground at one
end of the secondary. This is how the neutral is magically created.

As others have said, be sure to provide overcurrent protection for your receptacle.

Also, very carefully size your transformer because if you want to fully equip a 15A receptacle, you'll need a 2KVA transformer. A panel mounted 2 KVA transformer is about 65 lbs and roughly 10" cube, where a 500 VA (generally speaking, 4A 120V circuit) is about a 4" cube and probably 7 lbs and probably 1/4 the cost. If you go the undersized route, it is wise to label the recep as "(some number) Amps max load".