Line Voltage vs. Motor Namplate

[LG63]

Hi- What a great forum you have here. I hope it wasn’t a mistake to register, I’m already tool poor and I can’t imagine hanging out here will improve that situation.

So my first question is an electrical one. A few years ago I noticed that my shop voltage is consistently 125/250V and I’ve always wondered how that effects motor current. If I have a compressor motor that is nameplated 22 amp at 230V and my line voltage is actually 250V, will the actual full load amps be more or less than the nameplate amps, assuming the motor is exactly fully loaded?

 

[Stuart in MN]

Nominal voltage from the electric utility these days is 120/240vac, at least in most parts of the US (there may be some variations here and there.) They also have a tolerance, normally +/- 5 per cent, so the voltage can vary from 114/228 to 126/252 and still considered to be OK.

That said, at a constant load the current will go down as the voltage goes up, so your motor is probably running at a slightly lower current than the nameplate value. It's nothing to be concerned about.

 

[Gary S]

That said, at a constant load the current will go down as the voltage goes up, so your motor is probably running at a slightly lower current than the nameplate value. It's nothing to be concerned about.

That is a good thing for the motor. Motors overheat from excess current, not excess voltage. The slightly higher voltage makes the motor run with less current, so it should last longer.

 

[KinzeMech]

It is pretty common to see 240v service range from 220-250. Your line voltage that was at 250V will probably vary throughout the day. The amps being drawn by your motor will vary in response to the load you have put on it. On a day when your voltage is 250, you will be able to put more load on that motor without tripping the overload protector, than you would be able to on a day when it is 220V.

 

[EOC_Jason]

It can also depend on the meter you are using to read the voltage. If it's old and/or cheap it might not give you an RMS value so it will be a little higher than what it really is.

 

[LG63]

It can also depend on the meter you are using to read the voltage. If it's old and/or cheap it might not give you an RMS value so it will be a little higher than what it really is.

I have a Fluke 77, has never been calibrated since I bought it 20+ years ago, but I suspect it’s fairly close. There is a transformer on the pole for my meter drop so maybe that’s why I’m on the high side. I plan to use this little extra voltage cushion when I run temporary wiring to a compressor with a 22 amp motor. I’m planning to run about 25 ft of #12 cord from the panel to the compressor. I used an online voltage drop calculator and assumed 100A startup current and it gave my 4.2% voltage drop which sounded doable. I ran a 15A compressor on a #16 cord by accident (thought it was #14) for about 10 years with no ill effects so it would seem like #12 would be a slam dunk for 22A. Thoughts?

 

[KinzeMech]

I think 12 is good for 20 amps. You'll probably get by with it, but if it were me, I'd go 10.

 

[Charles (in GA)]

I’m planning to run about 25 ft of #12 cord from the panel to the compressor. I used an online voltage drop calculator and assumed 100A startup current and it gave my 4.2% voltage drop which sounded doable. I ran a 15A compressor on a #16 cord by accident (thought it was #14) for about 10 years with no ill effects so it would seem like #12 would be a slam dunk for 22A. Thoughts?

Well, you asked for opinions, and mine is that #12 is not suitable for 22 amps running or starting.

Charles

 

[Stuart in MN]

If the nameplate says 22 amps, you should be running it on a 30 amp circuit with #10 wire.

 

[KinzeMech]

It's also worth mentioning, if the nameplate lists full load amps and service factor amps, wire for service factor amps (it will be the larger number if it is listed, not all motors list a service factor).

 

[theoldwizard1]

Nominal voltage from the electric utility these days is 120/240vac, at least in most parts of the US (there may be some variations here and there.) They also have a tolerance, normally +/- 5 per cent, so the voltage can vary from 114/228 to 126/252 and still considered to be OK.

+/- 5% HA !

Most power companies wouldn't do a thing until you can show consistent variation of +/- 10%.

My Dad had an old cabin up north. The well pump would cause all the lights to dim and the refrigerator would trip the thermal overload several time before starting. Voltage was dropping to below 200/100 on starting loads !! PoCo installed a bigger transformer.

They do have recording equipment that they can hook up in your house to do around the clock monitoring.

 

[Wholesum]

Motors can more than double the load as the windings need to be energized. When they are turned off they through this back into the circuit and can cause problems for other devices that share it. Capacitors are used with AC units to minimize the power drain on the circuit.

For maximum motor efficiency a 3-phase circuit with a 3-phase motor is the most effective and all industrial motors and compressors are 3-phase.

 

[Falcon67]

If the nameplate says 22 amps, you should be running it on a 30 amp circuit with #10 wire.

X3 - my heater is rated at 20A, pulls 21 - #10 wire. My compressor is a 3.2HP 240V, which is about 10~11A, no problem on #12.

 

[Speedy Petey]

Motors and heaters are two different types of loads. Heaters are resistance elements which will change the watts and amps with the voltage. As the voltage goes up so do the watts and amps. The resistance is the only constant.
Check the rating on an electric baseboard heater some time.


Also, motors can use very different codes than other loads (see Art. 430). The old #14 on 15A, #12 on 20A, #10 on 30A all go out the window.
Personally though I DO NOT apply this in residential applications or for wiring motors that plug in to receptacles.