The circuit breaker wiring confict!

[Russ J. Hensel]

I was on a condo inspection the other day and found the following issue. I found a HVAC compressor without a heat pump with a 50 Amp breaker wired with #10 awg copper wiring. I stated in my report that the wiring was to small and was a safety issue and that an electrical contractor should evalaute and further evalaute and cure the situation.

The response from the contractor was that in this particualr case, this was acceptable given that it was a motor and peak power...blah blah blah....Is any of this right? I think I was getting some smoke up the keister because if this is the case in the 725 condo's then he is gonna have some splaining to do Lucy.........Just need to know if that in ANY case is a 50 amp breaker allowed and safe with a #10 AWG copper wire? Thanks!

[Speedy Petey]

Yes. It is very likely legal and safe...in this case.

[Greg Fretwell]

What did the label on the condenser say?
If it said something like "minimum branch circuit ampacity 30a, maximum over current device 50a" he did the right thing.

[Marc D. Shunk]

For motor loads (such as an AC compressor) the breaker only provides short circuit protection. The overcurrent/overload protection is built into the AC unit. There often appears to be a great disparity between the wire gauge and the rating of the breaker for motor loads. Motor loads are "the exception to the rule", many times.

Greg rightly points out what is probably written on the AC unit data plate. The argument starts and ends with the data plate on the unit. That is your next stop.

[brepanshek]

Marc, not quite sure what you mean. If you have a short circuit couldn't the smaller guage wire burn up before the breaker trips????? sorry I have trouble with understanding this one.

[brian winkle]

The current available during a short circuit will be so high the larger breaker will open in nanoseconds.

[Marc D. Shunk]

Originally Posted by brepanshek

Marc, not quite sure what you mean. If you have a short circuit couldn't the smaller guage wire burn up before the breaker trips????? sorry I have trouble with understanding this one.

No. In fact, with regard to short circuit protection, the trip curve for most breakers 20 amp through 100 amp is nearly the same. Overload is a different story. In the case of an AC unit, the overload protection is built into the AC compressor. The breaker is only providing short circuit protection, and is purposely oversized to assure that it will not nuisance trip during compressor startup. There are rules as to how much oversized it may be, on the off chance that there would be an overload (partial low-level short circuit) in the wiring between the breaker and the AC unit. They only want the breaker a few sizes larger than "normal" to increase the chances that it will trip if there's a wiring malfunction that is not related to the compressor itself.

[Charley L. Bottger]

Originally Posted by Marc D. Shunk

No. In fact, with regard to short circuit protection, the trip curve for most breakers 20 amp through 100 amp is nearly the same. Overload is a different story. In the case of an AC unit, the overload protection is built into the AC compressor. The breaker is only providing short circuit protection, and is purposely oversized to assure that it will not nuisance trip during compressor startup. There are rules as to how much oversized it may be, on the off chance that there would be an overload (partial low-level short circuit) in the wiring between the breaker and the AC unit. They only want the breaker a few sizes larger than "normal" to increase the chances that it will trip if there's a wiring malfunction that is not related to the compressor itself.

Marc I agree with what you are stating but you left out one word that might make it easier to understand for the non-A/C folks. The compressor is protected by a thermal overload embedded in the windings of the compressor sensing heat only which can be generated from faulty electrical current or excessive heat from the freon side of the compressor.

I know of No A/C MFG that does not list the minimum and maximum circuit amps on the name plate data. I can not tell you how many times I have found a 60 amp breaker installed on a A/C unit when it stated right on the name plate Max 40 amp

[Greg Fretwell]

You will still probably have short circuit protection with the 60 (depending on circuit length) but what you won't have is protection for the compressor in a locked rotor situation. In a case where you could have just fixed this with a $10 capacitor the compressor will burn up and you will have a huge bill (assuming the tech doesn't sell you a new system anyway).
If the manufacturer is doing his job the LRA will trip the breaker if it sits there for several seconds but it won't trip on a normal start where LRA is less than a second. There is quite a bit of engineering that goes into that label.
The only place in the NEC where they say you can set the O/C device to hold LRA indefinately is for a fire pump. That is another breed of cat entirely. The thinking there is you can suck a bass up out of the lake, lock the firepump rotor, cook it down to bouillabaisse and send it on down the line.

[David A. Andersen]

Overcurrent protection devices must have an interrupting rating sufficient for the maximum possible fault circuit available on the line side terminals of the equipment.

Conductors must be protected against overcurrent in accordance with their ampacity. For example, a 10 AWG conductor is protected by a 30 amp breaker

Overload devices must be provided to protect motors, motor control apparatus and branch circuit conductors against excessive heating due to motor overloads and failure to start. That overload protection device is sized between 115% to 140% of the motor name plate current rating.

The branch circuit, short circuit and ground fault protection device should be sized sufficiently large enough so that the motor circuit, short circuit devices are capable of carrying the motor starting current. This is accomplished by sizing a circuit breaker at 250% of the motor full load amp rating.

A 5 hp, 240 V single phase motor the service factor of 1.16 may have a current rating of 28 amp's.

The overload protection device must be sized no more than 125% of the motor nameplate current.

Breaker size: 28 amps X 1.25 = 35 amps

The short circuit protection device is sized no greater than 250% of the motors full load current rating.

Branch circuit short circuit device: 28 amps X 2.50 = 70 amps


This explains why larger circuit breakers are used on appliances that do not draw that much current during operation and how nameplate data is achieved.

However, branch circuit protection must protect the conductor from overheating during the startup amperage draw or overloaded condition. You cannot undersize the wire and oversize the breaker.

[John Allingham]

Could someone with a copy of the electrical code post what it has to say regarding the relationship between breaker size and wire size for A/C feeds.

[Marc D. Shunk]

Originally Posted by jallingham

Could someone with a copy of the electrical code post what it has to say regarding the relationship between breaker size and wire size for A/C feeds.

No, because every time someone does, somebody else chimes in and condemns that person. Sorry, but the precedent has been set. Buy your own copy.

[Michael Larson]

Don't let the code whiners get you down marc.

[Paul W. Abernathy]

(D) Small Conductors.

Unless specifically permitted in
240.4(E) or 240.4(G), the overcurrent protection shall not
exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG,
and 30 amperes for 10 AWG copper; or 15 amperes for
12 AWG and 25 amperes for 10 AWG aluminum and
copper-clad aluminum after any correction factors for
ambient temperature and number of conductors have
been applied.

ok...so here is (E)

(E) Tap Conductors.

Tap conductors shall be permitted to
be protected against overcurrent in accordance with the following:
(1) 210.19(A)(3) and (A)(4) Household Ranges and Cooking
Appliances and Other Loads
(2) 240.5(B)(2) Fixture Wire
(3) 240.21 Location in Circuit
(4) 368.17(B) Reduction in Ampacity Size of Busway
(5) 368.17(C) Feeder or Branch Circuits (busway taps)
(6) 430.53(D) Single Motor Taps

Ok...here is (G)

(G) Overcurrent Protection for Specific Conductor
Applications.

Overcurrent protection for the specific conductors
shall be permitted to be provided as referenced in
Table 240.4(G).

Table 240.4(G) Specific Conductor Applications

Conductor Article Section

Air-conditioning and
refrigeration equipment circuit conductors
440, Parts III, VI
Capacitor circuit conductors
460 460.8(B) and 460.25(A)–(D)
Control and instrumentation circuit conductors (Type ITC)727 727.9
Electric welder circuit conductors 630 630.12 and 630.32
Fire alarm system circuit conductors
760 760.23, 760.24, 760.41, and Chapter 9, Tables 12(A) and 12(B)
Motor-operated appliance circuit conductors
422, Part II Motor and motor-control circuit conductors 430, Parts III,
IV, V, VI, VII
Phase converter supply conductors 455 455.7
Remote-control, signaling, and
power- limited circuit conductors 725 725.23, 725.24,
725.41, and Chapter 9, Tables 11(A) and 11(B)
Secondary tie conductors450 450.6

OK....see.......CODE made that more than it needed to be...if the Nameplate says Minimum Ampacity 20A and Maximum OCPD is 40A....then you can size the conductors to it for 20A and size the OCPD at 40A........be done with it.

Posting CODE on that one just...well.....is way to involved....

[John Allingham]

Thanks Paul. Now I just have to find out if the same exception applies up here in the great white north.