I'm going to purchase a air compressor which as a 7hp rating so I guess it's
really a 5hp compressor. My shop has a 40amp 240v feed to my subpanel.
Question, how many amp breaker is needed and typically what current does a
compressor like this draw.
Ask the Vendor about the AMP rating of the motor is the only REAL way
Frankly, if this is more than a real 3hp motor, I would be very
On Wed, 18 Aug 2004 21:35:22 -0400, "Woodchuck"
It may be less than you think! I have a 7 HP Porter Cable compressor, it is
in reality about 3 HP.
The specs you need to know will be on the motor. You need to size the
breaker so the motor draw as lower than 80% of the breakers rating. In my
case the motor was rated at 15 amps@ 240 volts, I am running it on a double
pole 20 amp breaker.
I have an Ingersol Rand 60 gal and it only draws 15 amps @ 240v. I was only
going to run it off our secondary clothes dryer feed (we no longer use it).
It would have been an easy hookup, but my electrician said that the 40A
could possibly be too much for it. For the price of a new breaker I have a
better piece of mind that I am not going to overload the motor with 40 A
when it only requires 15A. So I gave it a 20A breaker/
That is a common misunderstanding of the relationship between a c'bkr and a
The c'bkr does NOT protect the motor, only the insulation on the wires
feeding the motor.
There are conditions where a 20A c'bkr will nuisance trip on a 5 HP motor.
If you truly want to protect the motor, you must add an overload relay.
Most motors in the consumer market have internal overload protection. The
allowable overcurrent device is generally allowed to be up to 250% of the FLA
on the nameplate, rounded UP to the next standard breaker size.
Legally you could put a 15a FLA motor on 14 ga wire (you CAN use 310-16 not
240.4(D)) with a 40a breaker (430.52). At that point voltage drop would be a
bigger consideration than anything else.
Not necessarily so. His electrician may have meant that he should protect
both his wiring and his motor with the proper breaker. Yes, the breaker
primarily protects the wire, but it also protects the device at the end of
the wire. A failure in the motor that won't trip the breaker until it draws
40A will be more catastrophic than one that will trip the breaker at 20A.
His electrician is correct in stating that he should size the breaker to the
load that will go on it.
The breaker is there to protect the wiring, and that's all. If you believe
as you say, anytime you go to use an appliance, turn them all on as that
circuit will be underloaded if you don't. Never use a single corded drill in
your shop. Best turn on the dust collector too in order to load the circuit.
How's your power bill? Things must work in strange ways around your house.
What he is talking about is the different facets of protection in a motor
circuit. The branch circuit breaker is only there to protect from a short
circuit. Overload protection is another part of the system and that is what
protects the motor. It is also what keeps the wire from overheating.
That is why you can have things like #14 wire on a 40a breaker and still be
See the diagram at the beginning of NEC 430 to get a better understanding.
Not actually. Greg responded with something intelligble while your response
both ignored what I had originally posted and jumped way too fast at the
chance to post a wise ass comment. Neither of those characteristics served
to credit you.
So let me ask you a question - would you connect your 220V tablesaw directly
to the main breaker in your panel? Besides the code requirement for a
branch circuit, and wire size, why not? Isn't it because circuits are sized
to the application?
Maybe we should just tell people to follow the 240.4(D) rule (15a = 14ga, 20a
= 12 ga, 30a -ga) and ignore the 430 rules altogether since they seem to be
over everyone's head.
It just isn't what the code allows.
430 "motor" rules do require the circuit to be sized to the motor served.
Not at all Greg - I appreciate you mentioning the 430 rule - something I'm
not familiar with and can stand to learn from. My reply was more in
response to CW's condescending tone than it was related to anything factual
You do seem to have a bit of a comprehension problem. Breakers are sized to
protect the wiring. How many 350ma breakers do you have for electric clocks?
You must have a whole bank of them for individual light. Do you have a
separate building to house all these?
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