Measuring Three-Phase 480-Volt Shop Voltage

      Understanding how three-phase power works. January 24, 2006

Question
We are setting up our new shop and the current coming into the building is 480 volts. I tested the power at the 3 leads coming in and they were 277 volts each, with a neutral and ground, so how does one hook up a machine that requires 480 volts to run? I have wired all my buildings before with 3 phase 220 and I know that I will require a step down transformer for 110. Can anyone help?

Forum Responses
(Cabinetmaking Forum)
From contributor B:
You would check voltage between two hot legs, not one leg to ground. You will need a transformer to run 220 volt three phase and of course single phase. Other than the voltage difference, it is wired the same except that smaller wire will be required using 440 volts. If you screw up and wire a 220 volt machine to the 440 system, it will run real fast for a very short period of time.



From the original questioner:
Thanks. You are right. I did measure across one leg to ground.


From contributor L:
I'm not positive about this, but you should find 120 VAC across a hot and a neutral.


From contributor T:
The 480 VAC will be 277 from each hot leg to neutral. In three phase applications, the 480 typically is stepped down to 208 VAC. Then, each hot leg off the step down transformer will be 120 VAC hot to neutral.


From contributor J:
You have what is called a 480Y service coming into your building. It is a 3ph 4 wire service. You have 3 hots, a neutral, and a ground wire. The voltage between any one leg and ground will be 277 volts, and between any 2 hot wires will be 480 volts.

3 phase machinery runs off Delta voltage, which means you only run 3 hot wires and no neutral wire. If you mistakenly wire a 230 volt machine at 480, it will not run faster. It will most likely burn the motor up. Voltage does not affect the RPM of a motor, but the frequency of the voltage does.

You will need a transformer for your 120/208 1 and 3 phase loads. You will need a 480Y primary x 208Y secondary transformer.

In my shop, I used another transformer for my 230 volt 3 phase loads. I use a 480D x 240D transformer. All my motors are rated at 230 volts. Your motors will run a little cooler and draw slightly less amperage if you run them at 240 volts as opposed to 208. 208 only really works well with motors that specifically say 208-230 on the nameplate. Generally foreign motors are rated at 220-230 volts.



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