How many volts at the outlet?

[slowthump]

My air compressor motor burnt out recently after not very long of use. I checked the voltage at a standard outlet in my new garage at it read 124.5 volts. I'm not sure how to measure the 220 compressor outlet but I get 124.5 on each leg when I probe the ground and one hot. I get 0 volts when probing each hot leg of the outlet. The compressor motor info tag says volts = 208 - 230. I assume I'm getting 251 volts out of the compressor outlet. Would that cause the motor to burn out?

Also, since there is no neutral on the 220 outlet, does the electricity follow the ground back to the panel to complete the circuit?

On a side note, I'm also burning out 500 watt bulbs real fast on my two halogen work lights.

 

[mrb]

its 240 not 220. the current does not flow out the ground, it flows between the two hots. Put your meter between the two hots and see what you get.

 

[slowthump]

Thanks for the reply MRB. I get 0 when simultaneously probing each hot leg of the the "240" outlet.

 

[brewchief]

Thanks for the reply MRB. I get 0 when simultaneously probing each hot leg of the the "240" outlet.

How is it wired at the panel? Sounds like you have two hots on the same phase.

 

[ForceFed70]

Thanks for the reply MRB. I get 0 when simultaneously probing each hot leg of the the "240" outlet.

If that is the case then you have an electrical problem and your compressor won't work at all.

Any chance you are out of range on your meter or something?

 

[MrMark]

You should get 124 on each leg to neutral or ground (neutral and ground are tied together at the entrance). That is the modern voltage level. Has been since before you were probably born. To measure the 250 you go ACROSS (VOLTAGE IS ALWAYS ACROSS TWO POINTS as it is the potential difference) the two phase (hot) legs. If you had 0V your compressor would not have run. Sounds like your system is normal.

 

[Mstrfxit12]

How is it wired at the panel? Sounds like you have two hots on the same phase.

+1 on that. Where are the breakers in your panel? Are they on top of each other, side by side , or located away from each other in the panel?

 

[Mstrfxit12]

+1 on that. Where are the breakers in your panel? Are they on top of each other, side by side , or located away from each other in the panel?

Sorry I should have specified. Where are the breaker(s) for this outlet in question? And is there one breaker or two to shut if totally off?

 

[slowthump]

I tried my tester at the panel on the two hot wires entering the panel. My tester still measured 0. Fiddled with the tester and figured out I had it set wrong. After adjusting tester I had 247 - 250 volts on the the two hots entering the panel and between the two hots on the compressor outlet. So it looks to be no problem for the outlet. Oh yea, the two hots to the compressor outlet are on separate legs.

I'm still wondering about the motor info stating voltage = 208-230. Is that normal and does the 250 volts it is getting cause any harm? Or did I just get a dud of a motor?

If 125 volts has been standard for a long time, why is it called "240" volt when the outlet is getting two 125 volt feeds and thus 250 volts. Should we not call the outlet a 250?

 

[PT Doc]

My guess is that your amperage is too small and the spike on startup isn't pulling the proper amps. Read about a guy with ir compressor that wired out to a 30 amp circuit because it was 23fla. He burned the motor out and then talked to ir and they stated out should have been 50a.

What size breaker is this circuit on?

 

[slowthump]

The compressor outlet is on a 20 amp breaker. Breaker tripped only after compressor slowed down considerably and started chugging. This did not happen durring startup but after the compressor had been running normally for about 20 seconds.

 

[mrb]

the nominal voltage in the usa is 120/240. thats why its called 240, not 220. your voltage is a little high which is why your halogen lights are eating bulbs. Buy 130v lamps for it and that problem should go away. What size circuit does the compressor say its supposed to have? Is there any possibility you could have a bad connection causing excessive voltage drop to the compressor when the circuit is under load? Measuring with meter and no load is not going to tell the whole story.

 

[slowthump]

I will check the outlet, power supply cord and any other connections. I sure was not overworking the compressor as it was just running my framing nailer and ran about every 5 minutes and then only intermittently.

 

[PT Doc]

What type/model compressor? Hp? Model?

So piecing together the details: probably 230v compressor on a 20 amp circuit. I'm not sure what the full load amps would need to be but 20a is fairly low. What gauge wire was used and how long a run to the outlet from the sub panel?

I think these answers will solve the mystery.

 

[slowthump]

The compressor is a Grizzly 3hp 240 volt. The original motor burned out soon after I bought it and I was pretty sure at the time that it was due to a crappy chinese motor. Replaced the motor with a Chicago Electric Motor that is labeled as 208-230 volt, and I believe 16 amps. That motor has been doing fine for the last 7 years of occasional use. This was running in my attached garage. I am in the process of building a new detached garage and have wired a 60 amp subpanel in it. Underground wired with 2-2-2-4 aluminum and it took 267 feet of cable. The 240 outlet is wired with #12 wire and is about 2 feet from the subpanel. I checked out the wiring connections on the compressor, panel, outlet and they all look good. The compressor was the only thing running off the subpanel when the compressor fried. I am nervous about running my 240 volt 3hp table saw in the new shop. How do I test the voltage under load?

And thank you all for the replies!

 

[PT Doc]

The compressor is a Grizzly 3hp 240 volt. The original motor burned out soon after I bought it and I was pretty sure at the time that it was due to a crappy chinese motor. Replaced the motor with a Chicago Electric Motor that is labeled as 208-230 volt, and I believe 16 amps. That motor has been doing fine for the last 7 years of occasional use. This was running in my attached garage. I am in the process of building a new detached garage and have wired a 60 amp subpanel in it. Underground wired with 2-2-2-4 aluminum and it took 267 feet of cable. The 240 outlet is wired with #12 wire and is about 2 feet from the subpanel. I checked out the wiring connections on the compressor, panel, outlet and they all look good. The compressor was the only thing running off the subpanel when the compressor fried. I am nervous about running my 240 volt 3hp table saw in the new shop. How do I test the voltage under load?

And thank you all for the replies!

My guess is the 16 amp rating on the 20 amp circuit. I would guess at least 30 amps on startup. Maybe there is a chart that you can track down that lists full load amps and breaker recommended. Quincy stated 50 amp breaker for 5 hp that has about 22 amp rating.

 

[slowthump]

I just checked the motor info and it says input = 3.1 kw. If I am figuring right at 3100 watts divided by 250 volts that equals 12.4 amps. I just plugged compressor into outlet briefly and it makes a noise and tries to start and then attempts to spin but then trips breaker. Motor and the compressor spin freely. Might it be worth the try to replace capacitors?

 

[71flh]

I think its a myth that a device will fry if the circuit is rated too low. A device will draw the current it needs.

If its too much current for the breaker, it'll trip. If its too much current for the conductor feeding the circuit, but not enough to trip the breaker, the conductor will get hot--maybe too hot.

At startup the current drawn is theoretically approaching infinity for a very short period of time. Breakers can't trip in the short time the current is high, so its not necessary to vastly oversize breakers as is sometimes suggested.

You may want to check the voltage on the motor while its running to see if its substantially lower than it is w/o a load.

 

[MrMark]

My guess is that your amperage is too small and the spike on startup isn't pulling the proper amps. Read about a guy with ir compressor that wired out to a 30 amp circuit because it was 23fla. He burned the motor out and then talked to ir and they stated out should have been 50a.

What size breaker is this circuit on?

If IR told him that they were selling him a tall tale. I hope he wasn't buying that pile. The breaker your friend had was right, and it can handle the inrush current on startup as another poster just stated. These are inverse time delay breakers and they will not come close to tripping on inrush, which is well over 50 amps.

 

[theoldwizard1]

One thing I don't recall seeing any one mention is voltage during starting and under heavy load (compressor "chugging"). These readings are somewhat difficult to get because most DMM do not react fast enough to get a decent reading.

If the voltage is dropping down to say below 210V during these time, it is an indication that somewhere between the load and the power company's transformer there is an undersized conductor, or possibly too much load on the transformer.

 

[MrMark]

or more likely the motor is seizing or shorting.

 

[foolishpride]

I just checked the motor info and it says input = 3.1 kw. If I am figuring right at 3100 watts divided by 250 volts that equals 12.4 amps. I just plugged compressor into outlet briefly and it makes a noise and tries to start and then attempts to spin but then trips breaker. Motor and the compressor spin freely. Might it be worth the try to replace capacitors?

Yes, check your capacitors. I would have said maybe a bad unloader valve on your compressor, but you said the motor and compressor spin freely, so I would rule that out.

 

[Milton Shaw]

The way I test voltage under load is to pull the plug 1/8" out of the socket and probe with test leads(push plug back in to hold test leads in place), turn compressor on and see what the voltage is. Also could do this after you turn oven/stove on and see if the whole house is getting low voltage on one of the legs under load. Could be a bad connection anywhere from street to house if this is happening. Also have seen house lights dim when a/c comes on from bad connections, and no ground rod. (neutral would show a momentary voltage).