#10 conductor on a 50amp breaker 220/240VAC

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knightstar

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Maybe someone can clarify if a Air conditioner that requires a 50 amp maximum breaker can be serviced using #10 conductor.

In this case the breaker in the service panel is a 50 amp breaker. The conductor is #10.

From my knowledge this is incorrect.
 
It depends what the minimum ampacity of the unit is and what the maximum OCP on the nameplate states. I would think 50 amps is probably high but may be allow in some circumstances.
 
It, as Dennis states, is possibly with Code based on 240.4(D) &440 Part III.
Where 240.4(D)((7) would normally limit the overcurrent device on a #10 to 30 amps, 240.4(G) allows instances where that may not be the "rule"
Art 440 is one of those areas listed in 240.4)(G) and 440.21 Part III sets forth the rules for selecting the circuit conductor ampcity and GFSC rating. Those are normally listed on manufacturers nameplate.
The rationale, I believe, is that the AC factory overload will protect the #10 and the inrush requires a larger OCP which will still provide short-circuit protection.
 
You need to check the MCA for the conductor size to answer your question. Do you have that information? The OCPD is only there for short circuit and ground fault protection and is not there to protect the conductors from overload.
 
This still blows me away.......say 10's on a 50 or 12's on a 40.

All these 40,50,60 amp circuits feeding AC units (residential 240v) could have been done with 12's and 10's instead of 8's and 6's......???

I mean, an ac is an ac, this just doesn't apply to industrial or commercial ac's right ?

In residential ac, the conductors would be based on the min. circuit ampacity x 125% of the condenser, and if 12's ampacity are rated , they can be installed on a larger than 20 amp circuit rated at the max overcurrent protection of
the condenser. Am I correct that it would not apply to the air handler. The air handler would be rated for 125% of the fla.........12's on a 20amp breaker max....correct?


Is what I stated correct?

Would this also apply to window ac units....

Thank you
Rich
 
ritelec said:
This still blows me away.......say 10's on a 50 or 12's on a 40.

All these 40,50,60 amp circuits feeding AC units (residential 240v) could have been done with 12's and 10's instead of 8's and 6's......???

I mean, an ac is an ac, this just doesn't apply to industrial or commercial ac's right ?

In residential ac, the conductors would be based on the min. circuit ampacity x 125% of the condenser, and if 12's ampacity are rated , they can be installed on a larger than 20 amp circuit rated at the max overcurrent protection of
the condenser. Am I correct that it would not apply to the air handler. The air handler would be rated for 125% of the fla.........12's on a 20amp breaker max....correct?


Is what I stated correct?

Would this also apply to window ac units....

Thank you
Rich
Click to expand...

The MCA already has the 125% factored in so you would use that number to determine the conductor size. The OCPD has to be large enough so that the unit can start meaning that it needs to be larger than the running current of the unit. This could mean that a unit with a 25 amp MCA and 40 amp OCPD could use a #12 conductor rated at 25 amps.
 
OK thanks Rob.......I see it, branch circuit conductors supplying a "single motor-compressor" shall have an ampacity of not less than 125% of either the motor compressor rated load current or the branch circuit selection current,
which is ever greater.

Would this include a window unit? Is a window unit a single motor compressor? Probably a different beast but....

I will search.

Thank you.

Rich
 
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ritelec said:
This still blows me away.......say 10's on a 50 or 12's on a 40.

All these 40,50,60 amp circuits feeding AC units (residential 240v) could have been done with 12's and 10's instead of 8's and 6's......???

I mean, an ac is an ac, this just doesn't apply to industrial or commercial ac's right ?

In residential ac, the conductors would be based on the min. circuit ampacity x 125% of the condenser, and if 12's ampacity are rated , they can be installed on a larger than 20 amp circuit rated at the max overcurrent protection of
the condenser. Am I correct that it would not apply to the air handler. The air handler would be rated for 125% of the fla.........12's on a 20amp breaker max....correct?


Is what I stated correct?

Would this also apply to window ac units....

Thank you
Rich
Click to expand...


I think the over sizing of conductors to a/c condensing units has got to be one of the all time greatest urban legends of the electrical trade. It pops up here on the forum again and again. Even some inspectors don't understand the concept. I just saw one the other day with an MCA of 28, Max OCPD 40-it was wired with #6 NM-could have been done with 10.
 
texie said:
I think the over sizing of conductors to a/c condensing units has got to be one of the all time greatest urban legends of the electrical trade. It pops up here on the forum again and again. Even some inspectors don't understand the concept. I just saw one the other day with an MCA of 28, Max OCPD 40-it was wired with #6 NM-could have been done with 10.
Click to expand...

I agree I see this very often too. It's hard for some to understand that a #12 conductor can be used with an MCA of 25 amps and protected at 35 or 40 amps.
 
A window air conditioner is an appliance and it plugs into a receptacle. The wiring to the receptacle must be protected at or below its ampacity.
An air handler that is just an air handler would be covered by the rules in Article 430 and like for ACs in Article 440, the wire ampacity is often smaller than the breaker rating.
 
It could also be that the 50A CB is feeding just a single motor load and there is another Thermal Over Load device down stream of the breaker so that it is protecting the conductors for running current and the 50A CB is there just for Short Circuit protection, per 430.52.
 
AC verses Breaker verses conductor.

AC verses Breaker verses conductor.

The way I read this in the NEC is a breaker must be the weakest link in the chain. Should a current drain occur on the conductor, regardless of what connects to the conductor, the breaker is suppose to protect the device and the conductor.

The breaker size is suppose to match the name plate on the ac condenser. The conductor is suppose to match the breaker.

It shouldn't matter device is on the conductor?

I understand that there are various insulations on the conduct and heat variations that change the ampacity of the conductor. However, when I go to the site, I can not tell what type of insulation is on the wire. Its cut off, and I don't know about anyone else but I can't tell unless I read what is written on the conductor jacket.

Thanks guys and if you have any more thoughts I would appreciate to hear them.
 
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augie47 said:
It, as Dennis states, is possibly with Code based on 240.4(D) &440 Part III.
Where 240.4(D)((7) would normally limit the overcurrent device on a #10 to 30 amps, 240.4(G) allows instances where that may not be the "rule"
Art 440 is one of those areas listed in 240.4)(G) and 440.21 Part III sets forth the rules for selecting the circuit conductor ampcity and GFSC rating. Those are normally listed on manufacturers nameplate.
The rationale, I believe, is that the AC factory overload will protect the #10 and the inrush requires a larger OCP which will still provide short-circuit protection.
Click to expand...


If I read this correct, the breaker is not there to protect the conductor from over heating? What is suppose to protect the conductor?
 
knightstar said:
If I read this correct, the breaker is not there to protect the conductor from over heating? What is suppose to protect the conductor?
Click to expand...

The overload device in the A/C unit.

It might help to think of it as you might think of a "tap". If you have a 100 amp switch tapped from a 400 amp conductor protected by a breaker, the 100 amp switch feed is allowed to be rated at 100 amps, not 400.
The fuse protects the tap from overload just as the HVAC overload protection device protects your #10. If the wire were to have a fault ahead of the overload (or fuse) the current flow will be large enough for the breaker to open.
 
knightstar said:
The way I read this in the NEC is a breaker must be the weakest link in the chain. Should a current drain occur on the conductor, regardless of what connects to the conductor, the breaker is suppose to protect the device and the conductor.

The breaker size is suppose to match the name plate on the ac condenser. The conductor is suppose to match the breaker.

It shouldn't matter device is on the conductor?
Click to expand...

The part that you have wrong is that in this installation is that the OCPD is not protecting the conductor from overload, it only is protecting the circuit from a ground fault or short circuit condition.
 
Look at article 440.22. You may go 175% to 225% of the motor compressor rated current or the branch cir. selection current- whichever is greater.

Not sure how this all plays out but my understanding would be that in some cases the OCPD can be greater than the nameplate.
 
knightstar said:
If I read this correct, the breaker is not there to protect the conductor from over heating? What is suppose to protect the conductor?
Click to expand...

I think you editted this post after you posted, :0... Ok, I was to say something else but anyways; 9/10th of the time one is only protecting the circuit. It's when you get into a specific disconnect that your protecting that application, forward from this point. Your equipment is cutoff via the breaker/disconnect making it the protection to this device. The circuit to this service is still protected itself by some kind of breaker.

Also keep in mind the math and ratio balancing that within electrical equations, generally if the voltage goes up the amps usuage goes down. In industrial it's not unusall to see tons of 20 AMP /480 motors, most only draw 5-7 continually (mostly less) well below when used to apply any required summary equation. MCA is the way to go if present and a good reminder.

The NEC tells us to address the requirements of each type of circuit service, with-in are the specific's as presented
with exceptions, use them. :)
 
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Stick has provided a good graphic. I will note one thing about this one it's based on the 2008 and earlier version of the NEC. This would no longer be applicable under the 2011.

attachment.php
 
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