# When tapping single phase power from a 3 phase service, how much power can safely be drawn?

I'm setting up a piece of lab equipment that needs 220v-260v to operate properly. The room it's located in has a a 480v 3-Phase service, in the form of a California Standard 50Amp twist-lock connector. It has a large outer ground terminal, and what I assume are 3 "hot" terminals.

The device in question had a 3-prong 240v plug, which I have removed. One Ground, one Hot, one Neutral.

Based on my reading, I believe I can connect the device by wiring Neutral and Ground on the device to the service Ground, and Hot to one of the 3-phase leads. Each lead should be about 440/sqrt(3) volts relative to ground, which is correct for the device.

I've been told that using only one phase of the 3-phase power is not something the transformer is designed for, and so the power factor will not be good, and the full 50amps will not be available. Needless to say, I need about 10 amps, so I assume this won't be a problem.

Is this feasible? Is there anything i'm missing?

And don't worry, I'll someone look at the cable after I set it up. I'm an EE student so I thought I would give it a try at least.

• You should really have a talk to the house electrician. This may be a corner-grounded delta arrangement. In that case, one of the "phase leads" is in fact the ground lead, and the other two have 480V against that one and against each other. – Janka Oct 13 '18 at 0:49
• Hmm so based on google, if that were the case I would need to identify which phase was tied to ground and use that as the neutral supply? Otherwise i would be shorting two phases together... – Tim Vrakas Oct 13 '18 at 1:51
• If that was the case, there would be no neutral equivalent wire. You would only have 480v available to you. Also, is "440/$\sqrt 3$" a typo? Where did you pull the 440 from? – K H Oct 13 '18 at 5:33

## 2 Answers

It sounds like you are intending to use ground as a replacement for neutral. That is not proper practice even if you verify that you have 480 volt wye service and neutral is connected to ground at the service entrance. In addition, you have not said that you have verified the supply voltage, A 480 volt service could be a bit higher at the service entrance and fully 480 volts at your location. That would mean 277 volts line to neutral, above the high end of your required range.

You should ask someone in charge of the facility what can be done to provide what you need.

What you suggest is not advisable.

If you are not offered a neutral then you cannot assume that you have access to a 'Y' service. As a 'delta' service you are able to use the phases for power. Your phase voltage is also outside your control but over 400V.

The correct solution is to arrange a enclosed tapped 400-480V to 230V isolated transformer with a suitable receptacle for your instrument. The ground will be passed through and connected to the transformer casing and to the secondary centre tap and neutral. You will need suitable circuit breaker and over current and residual fault current switches. This will eat into your budget a bit, if permanently mounted you will likely need facility electrician to fit and connect as well.

You can draw current to the maximum that your supply circuit protection devices will supply. If your loading is heavy and always on the same phase (pairs) you may overload the phase(s) before you have used the full supply available. Unevenly loading a 'delta' service causes one live to have less current, unevenly loading a 'Y' service causes one live line to have extra load and the neutral to carry undesirable but not a fault current. If you have 50A service at 400+V you could have over 85A at 230V after your 20kVA transformer (this is a good size).

Facility electrician can probably wire a 230V outlet for less money than supplying and fitting a step down transformer with ancillaries.

If you had a 'Y' outlet you could use the neutral and one phase (live) to get your 260+V single phase supply at 50A.