Sizing Circuit For Compressor

waynet

Clamp meter on 230V, 6HP compressor power cord shows brief spike of approx 40amps at startup, then about 14amps while running. Electrical plate states 15amp SFA. If I size the breaker/circuit for the SFA rating, say 20amps, will the startup spike trip the breaker?

DecksEtc

I sometimes have problems with my compressor on a 15A circuit but have never had problem running on a 20A circuit.

Teetorbilt

A 20 A breaker will trip at 27A or before. If you're start is 40A, you need a 50.

stars13bars2

everyone knows that with #12 cu conductors a 20 amp breaker is the largest that, according to the electrical code, can be used. some electricians know that these rules change where there is a motor involved that has thermal protection. you are then allowed to use 250% of the full load. 250%X15A= 37.5A and to go to the next standard size breaker which would be a 40 amp. your breaker will not trip on startup and you will not violate electrical code. if you did not know that, there are probably a lot of other rules that you will violate if you do the installation yourself. i assume you are comfortable with the liability. grounding and bonding are the most often misapplied rules. but whats a dead body or two among family anyway???

DMC

The wire size should be 12 ga to handle the 14 - 20 A of run current. The inrush motor current is insignificant in duration and will not trip the breaker. If you were to use an o'scope you would see that the inrush current probably is closer to 60-100 Amps but may only last usec to msec and therefore is not seen on a current clamp. Only the steady state current is of a concern. 12 ga wire and 20 amp breaker and you're good to go.

paintr56

Did I read this right? I am a painter not an electrician But I have some training in electrical work and do my own wiring when needed. Are you saying that it is exceptable to run #12 wire to a 40 amp breaker? What happens when the original compressor is unplugged and a welder is plugged in instead? I allways thought the breaker was sized to protect the wire and the wire was sized to the expected load. Wouldn't a delay breaker be the correct thing to use in this example?

Jim Bunton

stars13bars2

Jim
You read it correctly, but the scary part is you are not willing to accept it, while you seem to be fine with the advice offered by the throughly unqualified. If you were to plug a welder or anything other than a thermally protected motor of 15 or less full load amps into the 15amp 240volt receptacle, you are correct in that the breaker should then be changed. If you were worried about your 30 or 40 amp welder overloading the #12 wire, send me a picture when you get it plugged into the 15 amp receptacle, but you have electrical training and already knew that. I am not aware of a any delay breaker and the most correct breaker for this installation is called a high magnetic. It only has a magnetic trip device and no thermal trip device, as the thermal protection is included in the motor itself. This breaker would be a special order for most suppliers and the normal thermal / magnetic that they have in stock will work just fine. I also was not aware that we were using a motor starter. If we don't use a starter there are no heaters to upsize. If you choose him and upsize a heater by as much as 10% you loose 70% of the thermal protection offered by a motor starter. A motor starter provides two things: 1. thermal protection 2. the ability to turn the motor on and off. If you don't need the thermal protection it is a very expensive switch. That guy must have gotten his electrical training from his second wifes' stepsons' friend that worked the counter for two weeks at an electrical supply house. While this forum is not intended to teach all the requirements for installing a compressor, my last post said if you did not already know what I was saying that, YOU ARE NOT QUALIFIED TO DO YOUR OWN ELECTRICAL WORK!!! Almost all insurance policies have a clause that boils down to this; should your non code compliant electrical work be deemed the cause of the fire or injury, they will not pay any claims against them.This should not be considered a word to the wise, as they don't need it, this is for the stupid ones. Is this the DIY forum????? If you guys want to offer electrical advice you should first go take and pass a master code test. It ain't as easy as it looks.

jbfan

One thing that was not stated is this. Install according to the manufactors instruction!!!!

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We are the people (our parents warned us about) (Jimmy Buffett)

paintr56

What happens if a short occurs what protects the #12 wire?

Jim Bunton

Speedy Petey

stars13bars2 is absolutely correct as to the codes. Same goes for welders and a/c units.
While I say this I DO NOT agree with it's application in a residential setting. Mainly because of the reson stated.
Ok, true it still is a 15 or 20 amp receptacle, but in our DIY world who is to say the next guy will not pull 10 receptacles off this one for his new garage workshop???
In a commercial setting these things are set in place and specifically installed as far as circuiting. Also the majority of the time a real electrician would be doing any renovation or retrofit work.

There are thousands of these appliances running without problem. More can be done than to impliment codes which I personally consider inappropriate in a home.

*edited for spelling mistake, not to change content*

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stars13bars2

Ok the #12 wire is protected against a short by the magnetic trip unit in the circuit breaker. I know that the DIY'ers think if you have a 20 amp breaker that you can't have over 20 amps before the breaker trips. In a short circuit protected by a 20 amp breaker, your breaker sees thousands of amps before it trips. It's a thing called ohms law. Ohms' simple relationship between current, voltage and resistance is an awesome one that withstands the test of time. I can't tell if Petey is with me or not. He seems to be saying that everyone that thinks they are qualified should be allowed if not required to wire their own dwellings, and if they burn them down so what the codes are for commercial settings with real electricians because he personally considers codes inappropriate in homes. ELECTRICAL WIRING IS NOT A DO IT YOURSELF JOB !!! No offence to plumbers but the DIY'ers should stick to plumbing because I have never heard of anyone being watercuted and you will be able to see and smell most of your mistakes. THIS IS A CONTRACTOR FORUM WHERE ARE THE CONTRACTORS ???

Speedy Petey

WHOA! Hold it.
If you really think: "I personally considers codes inappropriate in homes.", you are out of your mind. PLEASE re-read what I wrote.
I do think SOME people should be allowed to do work in thier own homes, with the proper inspections, permits and at least some knowledge of what they are doing. This IS America last time I checked.
What I did say was that I agree with you about the sections of the code allowing the 250% rule with regard to motors and the similar codes with regard to welders. In an industrial/commercial setting they have a very valid place in our work. Why else would they be in the NEC.
My opinion is that I do not agree with applying these sections of the code to residential settings, for the reasons I stated. I think they should be wired in the standard method required by chapters 1-4 of the NEC.

A 14 amp running load appliance should not be tripping a modern 20 amp circuit breaker at startup. There is more going on here than meets the eye.

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paintr56

Thank you for your information. I am posting here in an attemped to learn not to give you a bad time.

Jim Bunton