# 480V/3P Motors on 277/1P ?



## Bionic Sparky (Jun 22, 2012)

Can 3P 480 volt hvac motors run temporarily on 277v single phase ?


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## micromind (Aug 11, 2007)

Bionic Sparky said:


> Can 3P 480 volt hvac motors run temporarily on 277v single phase ?


They sure can; at greatly reduced HP, they'll need a bit of help getting started, and the key word here is temporary.


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## brian john (Mar 11, 2007)

In line with ridiculous questions BUT

They can run on 480 single phase IF the motors are spinning at the time you lose a phase. But try and start them No, :no:No, No.:no::no:


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## hardworkingstiff (Jan 22, 2007)

Bionic Sparky said:


> Can 3P 480 volt hvac motors run temporarily on 277v single phase ?


What's the rest of the story?


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## Bionic Sparky (Jun 22, 2012)

Ok. That makes more sense then. This was last week when the big storm outage hit us. We were just in doing a few circuits and the hvac/mechanical guys were they. We got to talking and he said then power went out they lost 2 phase of 3 but the RTU's kept running and keeping everything cool til they totally went out. They had to ship in a 900kw diesel generator but as it turns out by the time they got it up and running the power came back on. There goes 200k in product and $25 for the gen and electricians. Is there a possibility of long term damage to the RTU's ?


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## Bionic Sparky (Jun 22, 2012)

I forget to add. I asked the tech at least 3 times about the phase loss. I said you mean it single phased. He kept saying no. It lost phase A and B only ran on C ?? I didn't think that was possible. Maybe he meant to say single phased? I'm not a motor person though. It was on a 2000 amp service by the way with at least 15 RTU's. I will also add that now they are started to notice electrical problems that they don't remember having last week as well. contactors chattering, things tripping breakers etc


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## Big John (May 23, 2010)

No f'ing way.

-John


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## Hippie (May 12, 2011)

Bionic Sparky said:


> I forget to add. I asked the tech at least 3 times about the phase loss. I said you mean it single phased. He kept saying no. It lost phase A and B only ran on C ?? I didn't think that was possible. Maybe he meant to say single phased? I'm not a motor person though. It was on a 2000 amp service by the way with at least 15 RTU's. I will also add that now they are started to notice electrical problems that they don't remember having last week as well. contactors chattering, things tripping breakers etc


There's no way they could have run on only one phase, and allowing them to run very long on only 2 could definitely cause trouble, motors will overheat quick when they lose a phase, the increased temperature and current draw will damage motors, conductors, etc


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## micromind (Aug 11, 2007)

If two phases or a 3ø system are lost during normal operation, and there are phase to neutral loads, the 3ø motors would indeed see phase to neutral voltage, but not quite full voltage. 

This due to the hot phase backfeeding through the phase to neutral loads, and the impedance of those loads will determine the voltage the motors will see.


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## user4818 (Jan 15, 2009)

You guys walked into the Cletis trap again.


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## stuiec (Sep 25, 2010)

*why*



Peter D said:


> You guys walked into the Cletis trap again.


whatever could you mean? :laughing:


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## bobelectric (Feb 24, 2007)

www.time-mark.com


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