# 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 480/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. Oct 13, 2018 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... Oct 13, 2018 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, 2018 at 5:33

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.