Electrical Wiring in the Home/AMPS


QUESTION: I am having a problem with a compressor I just bought.
I have been doing home wiring for many years. I have added receptacles, lights, and hard-wired appliances. Last summer I ran 2 lines to my out building. One was a dedicated line for an AC unit and it works well. I disconnected the ac and plugged the compressor in that line. On start up the compressor does fine but causes the lights to dim for a second and they return to normal. When the pressure in the tank drops and it kicks back in it dims the lights and motor cannot engage. I assume it is not getting the 11.5 amps it needs to run at higher PSI. My house has 200amp service so I do not know why I am having this issue. Can you help please?


Due to the high starting current that compressors typically have there are 3 critical factors  that come into play. Distance. The size of the line going out to the shed (gauge) and the operating voltage of the compressor. The size of your service is not a factor here. You could have 400 amp service and still have this problem. The issue is maintaining the voltage to the compressor which is effected by the factors below.

Being that your having trouble its probable that one or all of these factors is contributing to the problem. 1.) distance.  2.) Size of the cable. 3.) Operating voltage of the compressor.

It sounds like you have a 120 volt compressor, the size of your line is 12 gauge and the distance may be around 100 feet or more.

If it is possible to reconnect this compressor (inside the compressor connection box) to operate at 240 volts this is the fastest and easiest solution to your problem with out doing anything major. It would say some where on the compressor motor and or the owners manual that its 120/240 connectable. If you only see 120 volts then your stuck but if so you would change the connections and install a 2 pole breaker at the panel assuming of course the compressor is the only thing on that circuit.

Your next possible solution is running a bigger line to the shed but I will assume this to be the most difficult way to solve the problem.

The next possibility is a "buck / boost" transformer. You would have to measure what the voltage is at the shed before turning the compressor on, then measure what the voltage drops to after the compressor starts and is running. Do this measurement with no pressure in the tank so the compressor will start and run. The biggest problem is getting the compressor over the hump. Once you know the difference between the voltage when the compressor is on VS off you will get an idea of how much the voltage needs correction. You could easily be losing 10 volts or more depending upon the distance and the size of the cable. A "buck boost" transformer is not that expensive and it may be your best way out all things considered or it may be better and not necessarliy cheaper to convert the line to 240 which is the easiest thing to do and just buy a compressor that operates at 240 volts if you can't reconnect the exisitng one to 240 and running a bigger line just isn't practical. You have to weigh all these things out and see whats best to do. If you just bought the compressor maybe you can return it for one that runs at 240. HD and Lowes have very liberal return policys.


---------- FOLLOW-UP ----------

QUESTION: i have a 2 pole breaker i can use and am under the limit of 42 breakers in my box. my question is:
aren't 2 pole breakers meant for 240v = coper to ground / white to one screw and black to the other = 240V?
how do you wire it for this 120V setup like mine?

and thanks again for your time.


You would have to change the receptacle on the other end to 240 volt, reconnect the compressor
(assuming you can do that) to 240 volt and put a 240 volt male plug on it.

Or take your 120 volt compressor back and get a 240 volt compressor in which case the only changes you have to make is the breaker and the receptacle. This is probably what you will end up doing because its easy and you will have no problems.  
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Licensed Master Electrician (32 years) 6165B Contractor in Closter New Jersey with 40 years total experience in Residential - Commercial - Industrial work. Emergency Service - General trouble shooting and repair work - ONAN & GENERAC generator installation - Paddle fans - Kitchen & Bath exhaust fans - AC lines - Smoke detectors - Telephone lines - Cable TV lines - Computer network cabling - Exterior lighting - Recessed lighting - Security lighting - (200 - 150 - 100 amp service up grades) - Electric Heat - New circuits - New appliance hook ups - and more ! FREE ESTIMATES 201-358-1552 Fully Insured


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"The Frank Williams School of Electrical Construction". My dear old boss is 100 ! and still going God Bless him. I started working for "Tenafly Electric" right out of high school at 19. He was tough but I learned more working for him in 8 and half years than most guys. We did mostly industrial work while doing some commercial and residential so I was fortunate to be exposed to all 3 areas of electrical construction. The guys I worked with were all good guys and I stay in touch with some of them still. Most of us went into our own buisness's which says something about the caliber of the guys I worked with. We had some interesting clients : Wella Corp. of shampoo fame, Farah Fawcett etc, I didn't buy shampoo for 10 years. It was a great place to work. Pan Am the former airline just before they went under, another great place to work, nice clean environment. C&C Metals, the largest button manufacturer in the US at that time, a not so clean environment but a very interesting place to work, lots of machine's cranking out buttons of all kinds but you had to be on your toes, it was a potentially dangerous place to be . All kinds of action going on around you.

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