ECN Forum Forums General Topics of the Trade General Discussion Area 3 Phase Power (490 volts on a 480 volt system)

Help,

I'm a residential electrician and don't work much with 3 phase power. My question is on a 3 phase 480 volt system is it abnormal to read 490 volts? I have a ac unit rated for 460 volts (nominal voltage) however as stated we are reading 490 volts at the unit. Is this a reason to be concerned? I should mention there are 3 other units on the roof also rated 460 volts and are operating normal.

Any feedback would be greatly appreciated.....

It's very likely a 480 volt supply system and 490 volts is not that bad.

The AC unit is labeled 460 but is designed to work on 480 volt supply system.

They do not make Motors actually labeled 480 volts.

Also most power supply system are useally ± 10% range so with the 480 system you got a bit leeway with it so few voltage over the system is not bad at all

Merci , Marc

"leeway" thats a good thing.
N-Star here, has been running "questionably" low lately.

That makes you worry.

You have a fine stable voltage, no worries.

Leland, frenchele,and iwire,

Thank you for all your responses. It's greatly appreciated. I didn't think it was a problem, but I have a customer and his A/C mechanic is advising him not to run his unit. I feel bad for the customer because he's at a loss for what to to do.

Thanks again!!!!

Click on the link for a pdf you can print out with a lot of official information to teach the A/C mechanic, maybe you will print out two and give him one.

Air Conditioning Equipment Voltage Ranges

Basically a 460 tagged unit is designed to run from 432 to 508 volts.

3- one for the customer to rest their worried mind.

Bob said

"Basically a 460 tagged unit is designed to run from 432 to 508 volts."

That is news you can use. I was trying to do the math in my head from the earlier 10% post and I got "brain feeeze" like when you eat ice cream on a hot day.

Bob,

Thanks again for being so helpful. I printed out the document and filed it away. It was of great use to me as well as the ac mechanic. Thank you again for reaching out.

Respectfully,
Bob Hanifin
Owner, ACE Electric

Glad I could help, I would like to say I have all this knowledge in my head......but really I am just a good 'Googler'.

Doing the math isn't that bad.

1% is 1/100, i.e. 1% of 460V is 460/100=4.6.

So 10% is 4.6*10=46.

460V -10% is 460-46=414.
460V +10% is 460+46=506.

That's not the shortest way, but the easiest to explain and remember.

iwires figures confuse me a bit though, since 432 is 480 - 10% and 508 is close to 460V + 10%, but only close.

Be sure to check the control transformer, many have taps on them that need to be set for the line voltage.

If the voltage is too high it usually can be corrected by reconnecting the primary taps on the transformer which feed the system. It is common for transformers to have +(2)2-1/2%FCAN (full capacity above normal)and (2) or (4)2-1/2% FCBN taps (full capacity below normal.)
Taps would be +5%, +2-1/2%, 0%, -2-1/2%, -5%, -7-1/2%, and -10%.
490v is very close to 2-1/2% high so reconnection of the primary tap to the +2-1/2% position would bring the secondary voltage to slightly below 480v. Remember that a transformer does not regulate and the voltage often varies as the load changes. And also the utility supply voltage may also especially at different times of the day.