A/C Feeder and overload protection

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dbryan

Member
The FLA listed is 17.8 amps. The maximum breaker size is 35 amp.For a #10 wire the * refers to 240.4(D). This specifies that the maximum breaker size is a 30 amp unless specifically permitted in 240.4(E)-(G). 240.4 (G) refers to article 440, Parts III, VI. I cannot find permission to use a 35 amp breaker on a #10 wire.

Is there anywhere that allows me to use a 35 amp breaker and #10 wire?
 

jschultz

Member
Re: A/C Feeder and overload protection

some where that there is no code, may allow it. try some of the third world countries.
 

bennie

Esteemed Member
Re: A/C Feeder and overload protection

440.22(A) Branch circuit protection.

440.22(C) Protective device not to exceed the manufacturers values.

440.32 For wire size.

240.4(G) Specific conductor applications.
 

bphgravity

Senior Member
Location
Florida
Re: A/C Feeder and overload protection

jschulz??? :confused: :confused:

Let me explain in more detail. Typically, OCD's are selcted by the greatest amount of current-carrying capability of the conductor it will be protecting. The items in 240.4(G) have other means of protection and do not require the overcurrent aspect of a fuse or CB. Equipment such as A/C's require the branch-circuits and feeders to have short circuit and ground-fault protection. These OCD's may be sized much higher than what table 310.16 shows. Unlike general circuits, the conductor is found using 310.16, but the OCD is found using others multiples or other tables. This allows hard starting equipment to operate properly while still allowing proper clearing of electrical faults. A #10 conductor can see several thousand amps for a very short period before insulation or annealing damage. So in your case, a #10 is plenty fine for a 35-amp OCD.
 

jociha

Member
Re: A/C Feeder and overload protection

You say 35 amp is the max. If the load is 17.8 amps why not use a 30 amp ocp.
 

bennie

Esteemed Member
Re: A/C Feeder and overload protection

For the FLA indicated No.#12 wire will work with a 35 amp protective device.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: A/C Feeder and overload protection

Originally posted by jociha:
You say 35 amp is the max. If the load is 17.8 amps why not use a 30 amp ocp.
You should read Article 440, but in general the larger breaker is for start up, the conductors themselfs are protected by motor overloads in the unit.

The breaker in this case is for phase to phase shorts or phase to ground shorts.

As Bennie said even a 12 AWG would be code compliant.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Re: A/C Feeder and overload protection

iwire,

jociha is correct. The lable says the maximum permissible, not required, OCPD is 35A. You are free to use a smaller rating (i.e. 30A) as long as it is capable of starting the equipment.
 

bennie

Esteemed Member
Re: A/C Feeder and overload protection

I am speculating...The motor appears to be a 5 hp.

5 hp Motor has a locked rotor current of 122 AMPS.

The 35 amp fuse will have a time/trip curve to allow starting, but will shut down in the event of a failure to start within a determined time, to prevent damage to the winding.

For insuring proper operation the fuse should be as indicated by the manufacturer.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: A/C Feeder and overload protection

Jim I never said he had to use the 35 amp breaker, just that he could use that size with 10 or 12 AWG.(Which was the question) Many people get hung up on 310.16 and 240.4(D) when sizing the short-circuit, ground-fault protective device. I also suggested he read 440 himself to get the whole picture. :)

But I agree with Bennie I would use the size on the tag to prevent a call back for a tripping breaker.

[ April 29, 2003, 04:48 PM: Message edited by: iwire ]
 

dbryan

Member
Re: A/C Feeder and overload protection

I appreciate all of your comments. I am still confused. How can I use a 35 amp breaker on #12 wire when 240.4(D) Small conductors specifically states that "the overcurrent protection shall not exceed ... 20 amperes for #12AWG...copper".

If 125% of the rated load current required a #8 wire then I could use an overcurrent protection device of 175% for an A/C unit because it is not listed in 240.4(D).

In my application, a #12 wire will carry 125% of the rated load current for the A/C unit but according to 240.4(D) the maximum breaker size is 20 ampheres which may or may not start the motor.

For my application, I think that a #12 wire with a 35 amp breaker is perfectly fine but I cannot prove that it is not a code violation based on 240.4(D).
 

bphgravity

Senior Member
Location
Florida
Re: A/C Feeder and overload protection

The key is "Unless specifically permitted in 240.4(E) through (G)..." In Article 440 which covers provisions for A/C equipment. The OCD of the branch circuit is not determined based on the size of the required conductor but on what size is required to allow motor and compressor start and to provide short and ground-fault protection. Some calculations allow a 70-amp OCD on #14. 240.4(D) only applies to loads not specified in (E) through (G) like lighting general purpose outlets. :)
 
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