60 amp circuit breaker and #10 ga. wire recommended??? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 01-19-2011, 05:26 PM
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60 amp circuit breaker and #10 ga. wire recommended???

I am considering the purchase of a compressor that has a 5hp motor with overload protection and an 80 gallon tank. Motor has a full load current of 28 amps. I E mailed the company for wire and breaker recommendations and they told me to use #10 THHN copper wire with a 60 amp breaker. Their explanation.

On a motor circuit the breaker needs to be over-sized, compared to full load current, to allow for current inrush on start up.

The amperage of the wire does not need to match the breaker rating as long as the motor has overload protection.

Does this sound right to all you electrical experts out there?
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Old 01-19-2011, 07:10 PM
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No it does not. The circuit breaker prevents an overcurrent condition in the conductors, and #10 AWG wire should be protected with no more than a 30 A circuit breaker. The motor may well have adequate overload protection, but a 60 A circuit breaker on #10 AWG wire is asking for trouble, and I'm certain that it would not pass a code inspection anywhere. If the compressor company says a 60 A circuit breaker is needed, you must size the conductors accordingly. I'm no longer a licensed electrician, but unless things have changed recently, I would not use their recommendations using #10 AWG wire. Please check with your local inspector to make certain you are complying with the NEC regarding your installation. Don't try to save a buck by using undersize conductors. Electricity can be your friend...or a fearsome foe.
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Old 01-19-2011, 07:26 PM
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Not only is #10 wire on a 60 amp breaker a bad idea, it is completely against code. Also, the 60 amp breaker is completely unnecessary. A 30 amp breaker (which IS correct for #10 wire) is adequate for this compressor. The motor start current might spike for a second or so at start, but that will neither trip the breaker nor overheat the wire. It certainly won't hurt to upsize both the wire and the breaker (#8 and 40 amp or #6 and 50 amp) but it's not mandatory.
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Old 01-20-2011, 01:42 AM
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i am betting it is rated at about 24-25 full load amps (FLA)- if it is a 240v- so a 35amp breaker will be needed (at 20% per NEC). No special wiring will be needed since it will not be rated at continuous duty. Would install #8 wire (less than 100' of wire)

if your plate indicates 23 full load amp this can be done with 30amp breaker with #10 wire. (less than 100' of wire)
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Old 01-20-2011, 05:48 AM
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2008 NEC
430.21- wire shall be 125% of full load current.
Table 430.52- Inverse Time Breaker (standard breaker) not to exceed 250% of full load current.
28a x 125%= 35a wire
Table 310.16- #10 copper THWN =35a
28a x 250%= 70a max fuse
The motor is protected by the thermal overlads installed in motor (reset button)
60a brkr is legal
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Old 01-20-2011, 06:25 PM
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ok

Cal 1320 nailed this one on the book. If you ignore the obelisk about not being able to run the #10 at 35a, (only 30a). Not taking away from Cal, but rather than running the #10 at ~.80%-90% of it ampacity rating, I think I would move up to a #8. May have to lug it(use connectors), to hook it up in the pressure switch. #10 wire is minimum engineering. I don't go there.olnolan
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Old 01-21-2011, 11:20 AM
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Quote:
Originally Posted by OLNOLAN
If you ignore the obelisk about not being able to run the #10 at 35a, (only 30a). rather than running the #10 at ~.80%-90% of it ampacity rating, I think I would move up to a #8. May have to lug it(use connectors), to hook it up in the pressure switch. #10 wire is minimum engineering. I don't go there.olnolan
Olnolan, I totally agree. The obelisk doesn't apply when sizing for motors, but I dont go there either. As crappy as the code is I cant believe states would omit parts of it. The NEC is a bare bones minimum.
The NEC is really only for safety. It states right in the book if you follow the NEC it will be free from hazard but not efficient, convienant, or adaquate for good service.
We should all strive to do better than code.

Hotrodman
BTW, kudos for buying a REAL 5HP compressor.


Hey, you never told us how far away from the power source the compressor is.
Voltage drop anyone????
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Old 01-21-2011, 07:39 PM
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Compressor is 30 feet away from the box

The compressor is only 30 feet away from the box. To start with I had planned on 8 gauge wire with a 40 amp circuit breaker. I was flabbergasted when he told me to put the 60 amp breaker on 30 amp wire.

I originally planned on a Quincy, but they had a Chicago Pneumatic there that had all the features of the Quincy, cheaper, and came with what they referred to as a bumper to bumper warranty for two years. Haven't made up my mind yet.

Anyone had experience with the Chicago Pneumatic compressors?
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Old 01-22-2011, 05:51 PM
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Quote:
Originally Posted by HotRodMan
Their explanation.

On a motor circuit the breaker needs to be over-sized, compared to full load current, to allow for current inrush on start up.

The amperage of the wire does not need to match the breaker rating as long as the motor has overload protection.

Does this sound right to all you electrical experts out there?
It may meet the technicalities of the NEC, but it isn't right. For motor loads you use a type HACR breaker which is designed to handle the additional current during startup without having to grossly oversize the breaker. Most 2 pole residential breakers carry the HACR rating, but its something you'd want to verify when purchasing since the standard type will work fine on resistive loads like the water heater or stove, but not on motor starting loads.

The same thing can be done in fuse protected circuits by using Fusetron time delay fuses.
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Old 01-24-2011, 08:55 AM
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Discussion

Here is some info on HACR (Heating Air Conditioning Refrigeration)breakers.
http://static.schneider-electric.us/...0600DB0102.pdf

It appears to me that all modern manufactured breakers carry the HACR rating. This rating does not guarantee it will allow inrush start up current without tripping. I believe it was intended for multiple motor installations(ac fan motor & compressor motor combos).

Typical motor inrush startup currents are ~6 times the running current. The amount of time the circuit sees inrush current varies according to motor design, attached motor load, e.g., compressor type, pump type, centrifugal/positive displacement, pressure regulation valve style, fan blade style, etc.,etc.,etc. As you can see, one size does not fit all.

As far as time delay fuse use, NEC only allows 175%. These fuses have a delay long enough to get the motor through inrush current and down to FLA without blowing. This is the rule used with motor starters having main fuses. If the motor starter uses a circuit breaker, instantaneous trip ratings can be 800%,1100% on type E motors(three phase motors). Some of these breakers have adjustable trip units that can be tailored to the specific startup curve time desired.

In my experience with compressor motors, most of the time 200% will get you through inrush current, but not always. You sometimes get a nuisance trip. If you watch a clamp on analog ammeter, you can actually time the inrush in alot of cases, digital meters are usually too slow to catch it(visually). In industrial settings we sometimes use a setup with a CT and a shunt connected to a chart recorder that will actually graph it to give the exact time curve so we can cure occasional starting problems.

In HotRods case, he has a compressor that is right at the borderline of homeowner use. This compressor will likely see an inrush starting current of ~170+ amps for ~2-5 seconds, depending on starting conditions. The company he spoke to, in all probability, has had problems with nuisance trips on startup with anything less than a 60amp inverse time circuit breaker. It could actually be pushed to a 70 amp, if required. If his home service isn't up to snuff, this motor will likely dim the lights a bit when starting, HA HA. olnolan
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