Maximum Size Breaker for #12 Branch

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tryinghard

Senior Member
Location
California
I have a project where the engineer is asking for branch circuit of #12 THHN wire from a 25A breaker in a panelboard to an HVAC unit with a fused disconnect, this is not control circuit and distance is not an issue. I cannot locate any code to qualify this. 310-16 takes me to 240-4(D) & (G) then to 440 parts III & VI, whew!
Are there any exceptions to this circuit allowing 25A breaker on #12 conductors?
 

david

Senior Member
Location
Pennsylvania
Re: Maximum Size Breaker for #12 Branch

The engineer would know what the name plate of the HVAC equipment is calling for as far as min circuit ampacity and Max over-current protection.

Unless you have the equipment their with a name plate aaking for something different run what the engineer spec.
 

jimwalker

Senior Member
Location
TAMPA FLORIDA
Re: Maximum Size Breaker for #12 Branch

Yes and you just said you found 240-4.Thats where it says air conditioners (motors) are permitted.While i don't like it ,you can fuse at 25
 

wyatt

Senior Member
Re: Maximum Size Breaker for #12 Branch

it all comes down that sec 440 allow you to use 310.16 table amps of 25amp on 12 awg with out the derating of 240.4(d) to 20 amps look at 240.4(d) it says unless permitted in 240.4(g)
 

allenwayne

Senior Member
Re: Maximum Size Breaker for #12 Branch

This is not an unusual install,wire for min. and fuse for max on a/c`s has been the norm for along time.Some argue that the ampacity rating is being exceeded but the max. fuse is for start up and not the operating range.
 

tryinghard

Senior Member
Location
California
Re: Maximum Size Breaker for #12 Branch

Originally posted by david:
The engineer would know what the name plate of the HVAC equipment is calling for as far as min circuit ampacity and Max over-current protection.

Unless you have the equipment their with a name plate aaking for something different run what the engineer spec.
This project is really typical in that the Architectural team is still qualifying the submittals and that the project also went into a ?value engineering? mode after it awarded. With all this said the Engineer does not know the name plate rating as the VE came from the Mechanical Contractor and I received the nameplate from them. One of his units has a FLA of 14 and will have a fused disconnect at the unit and another has a FLA of 19.2 also with a fused disconnect at the unit.
 

tryinghard

Senior Member
Location
California
Re: Maximum Size Breaker for #12 Branch

I am calling these circuits branch but they are really feeders in that each circuit originates from a branch panelboard breaker (short circuit/over current protection device) and then feeds a fused disconnect (over current protection) at each unit. Am I correct in viewing these as feeders?

If so I would think 240-4(D) would qualify the feeder and 240-4(G) along with 440 would qualify the branch control circuit, from the fused disconnect to the unit. With this 240-4(D) tells me I cannot exceed 20A protection for #12.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Maximum Size Breaker for #12 Branch

That is a good point, the feeders must be run under the 'normal' rules.

Looks like 10 AWGs at least for the feeders.
 

peter d

Senior Member
Location
New England
Re: Maximum Size Breaker for #12 Branch

Originally posted by jimwalker:
.While i don't like it ,you can fuse at 25
I guess you would really hate the 40 amp breaker I installed on 10/2 NM (to feed an AC of course) :p :p
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Maximum Size Breaker for #12 Branch

Why are you using a fused disconnect? I know that some AC units require the use of a fuse for the OCPD, but if it doesn't then the branch circuit starts at the breaker. I'm also not sure that I would call the fused disconnect the branch circuit OCPD. I would call the fuses supplementary overcurrent protection and run #12 from the breaker.
Don
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Maximum Size Breaker for #12 Branch

Originally posted by don_resqcapt19:
I'm also not sure that I would call the fused disconnect the branch circuit OCPD. I would call the fuses supplementary overcurrent protection and run #12 from the breaker.
Don
Don, can we do that?

I sure would like to.

This could be a major cost difference once we move up to larger RTUs with MCAs of 100, 200, 300 amps etc.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Maximum Size Breaker for #12 Branch

Bob,
Don, can we do that?
We can until we get a red tag. :D
I just don't see a problem. If the unit does not require a fused disconnect the conductors are branch circuits and we can use the smaller wire. As soon as we put a fused disconnect, some call the supply circuit a feeder and the rules change. How does adding an additional OCPD at the load end of a circuit make it less safe? As far as calling the fused disconnect supplementary overcurrent protection, I think that we can do that because that OCPD is not directly required by the code. Yes, if the nameplate calls for fuses, then 110.3(B) would require that protection, but the protection is not required for the branch circuit, it is required for the equipment making it supplementary overcurrent protection. It is not really part of the branch circuit.
Don
 

peter d

Senior Member
Location
New England
Re: Maximum Size Breaker for #12 Branch

Originally posted by iwire:
This could be a major cost difference once we move up to larger RTUs with MCAs of 100, 200, 300 amps etc.
Is that because large unitaries call for fuse protection?
 

pete m.

Senior Member
Location
Ohio
Re: Maximum Size Breaker for #12 Branch

Don,
I'm not sure but it seems that the commentary found in the 2005 NEC Handbook in the last sentence or so seems to suggest that supplementary OCP is limited in application....
(not arguing with you on this at all just trying to see it from all sides :) )

Pete

Supplementary Overcurrent Protective Device. A device intended to provide limited overcurrent protection for specific applications and utilization equipment such as luminaires (lighting fixtures) and appliances.

There are two levels of overcurrent protection within branch circuits: branch circuit overcurrent protection and supplementary overcurrent protection. The devices used to provide overcurrent protection are different, and the differences are found in the product standards UL 489, Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures, and UL 1077, Supplementary Protectors for Use in Electrical Equipment.
Provided as a generalization for understanding, the NEC requires that all branch circuits use only branch circuit ``rated'' overcurrent protective devices to protect branch circuits, but it permits supplementary overcurrent protection devices for limited use downstream of the branch circuit ``rated'' overcurrent protective device.
Added for the 2005 Code, the definition of supplementary overcurrent protection device contains two important distinctions between supplementary overcurrent protection devices and branch circuit overcurrent protective devices. First, the use of a supplementary device is specifically limited to only a few applications. Second, where it is used, the supplementary device must be in addition to and be protected by the more robust branch circuit overcurrent protective device.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Maximum Size Breaker for #12 Branch

Pete most of the large units we wire come with a factory installed disconnecting means that is also a breaker.

If we could call the run from the panel to this breaker a branch circuit and not a feeder then we could possibly run smaller conductors.

[ February 23, 2006, 04:57 PM: Message edited by: iwire ]
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Maximum Size Breaker for #12 Branch

Bob,
If the OCPD is part of the equipment then it is not providing protection for the branch circuit. I see no way that the supply for that type of equipment would be a feeder.
Don
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Maximum Size Breaker for #12 Branch

Pete,
Supplementary Overcurrent Protective Device. A device intended to provide limited overcurrent protection for specific applications and utilization equipment such as luminaires (lighting fixtures) and appliances.
That is exactly what we have when the nameplate on the equipment calls for fuse protection. It needs fuses to protect the equipment not the branch circuit.
Don
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Maximum Size Breaker for #12 Branch

Originally posted by don_resqcapt19:
Bob,
If the OCPD is part of the equipment then it is not providing protection for the branch circuit. I see no way that the supply for that type of equipment would be a feeder.
Don
Evening Don,

Many of the contracts here require mechanical contractor to provide the disconnects and / or motor controls for any equipment they bring to the job.

Often they will meet this by ordering all their equipment with disconnects included.

So the breaker is an option to the unit and installed inside it (with an external handle).

I have always looked at our supply to this as a feeder but am interested in your view.

I always wondered if the motor overloads are the final OCP?
 

tryinghard

Senior Member
Location
California
Re: Maximum Size Breaker for #12 Branch

Great dialog here, I really appreciate all your insights. I will say the information I received from the Mechanical Contractor does state the fused disconnect is needed, maybe they bought cheep units?

I guess we?re down to terminology then, between feeder and branch circuit. With all this said the supply from the panelboard is a feeder to the fused disconnect which then has branch circuit to the unit. Again the way I understand it is the feeder is qualified with 310-16 & 240-4(D) #10?s and the branch circuit is qualified with 240-4(G) & 440 as #12?s.

Am I missing something?
 

RUWired

Senior Member
Location
Pa.
Re: Maximum Size Breaker for #12 Branch

I don't want to do the math right now ( it's night night for me ) but.....240.3 and table 240.3 tells us Equipment SHALL be protected by article 440.
Rick
 
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