electrical wiring question

[elevmike]

WHY? IMO the poster is WAY over his head.

Why do you ask Why?

I would agree that if the poster where actually working on the circuit, he may be in deeper then he should be, but it seems that he just observed something and is courious.

 

[rsdoran]

Why does something have to be changed?

 

[Jimmie_Ohio]

Why does something have to be changed?

I agree. The whole arrangement needs to be evaluated.

If there is an upstream breaker that is sized to protect the 10 gauge wiring, the 60 amp breaker might have been used as an intermediate switch for convenience only.

However, this is not good, because someone might add something later figuring the 60 amp breaker is sized appropriately (but the wire isn't).

This is what you get when people "use what they have" or "sweep the floor for parts".

 

[Tark]

I am not seeing an exception that allows the wiring to be rated less than the circuit protection and dont think I would EVER use wire rated less than the load and circuit protection even if there is some kind of exception.

I made the fatal mistake of thinking in general terms where THxx or equivalent is used, which at 10ga is rated 30 amps.

The reason you’re not seeing an exception is because it’s not an exception, but part of the code. And THxx is not rated at 30 amps, it is 35 or 40 amps depending on the type.

240.4 Protection of Conductors – Conductors … shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G)

240.4(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)


​

Note in Table 240.4(G) that Article 440 (Air Conditioners), Parts III and IV are listed, along with many others.

Here is an example on how you size the circuit breaker and conductor for an AC unit, I’ll size a single AC unit with a nameplate of 30AMPs.

440.6(A) …the rated-load current marked on the nameplate of the equipment in which the motor-compressor is employed shall be used in determining the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection…

440.22(A) The motor-compressor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor. A protective device having a rating or setting not exceeding 175 percent of the motor-compressor rated-load current…

​

So 30 x 1.75 = 52.5 amps. So I need a 52.5 amp breaker, I look at 240.6(A) and notice that 52.5 is not listed, but there is 50 and 60. Now I go to 240.4(B) and see that I will use a 60 amp breaker.

240.6(A) The standard ampere ratings for fuses and inverse time circuit breakers shall be considered ... (shows listing of standard sizes)

240.4(B) Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all the following conditions are met:…(which they are in this case)


​

Now to size the conductor.

440.32 Single Motor-Compressor. Branch-circuit conductors supplying a single motor-compressor shall have an ampacity not less than 125 percent of either the motor-compressor rated-load current …


​

So 30 x 1.25 = 37.5 amps. So I’m going to assume that I am using THHN conductors and that the equipment has connections that are rated at 90C. So I’m going to use 10AWG which is rated at 40amps.

So, by following the NEC I have 60amp breaker supplying a 30amp AC unit through 10AWG conductors. Would I do it this way on a job? You bet I would. First of all, you need to install a circuit breaker that is capable of handling the starting current, as stated in 440.22(A). Do I know what the starting current is? Nope, but the code will let me go up to 175 percent of the nameplate, and more if needed. So I go up to 175% because I’m not going to place a 30amp breaker in there only to find out that in the middle of summer the unit is tripping the breaker when it starts. Am I going to use a 60amp breaker and 6AWG to supply a 30amp AC unit? No that would be stupid.

So is this safe? Sure is. Conductors need to be protected from short-circuit, ground-fault, and overcurrent (and physical damage). Generally the circuit breaker is used to provided all three of these protections, like in your house. But in the case of motors the circuit breaker is only providing short-circuit and ground-fault protection. The overloads in the starter are used to provide the overcurrent protection.

Back to the poster's question. Is it possible to place a 10awg conductor in a 60amp breaker? Yes, under certian conditions.

 

[russrmartin]

Right on Tark, I agree with you whole heartedly on this one. The circuit is probably fine exactly how it is.

 

[darrenj]

I somewhat agree with Tark on this one..I quite ofter put a 40 or 50 amp breaker on a #10 wire...It is quite common and acceptable to do so.. I think 60 is a little excessive, As for over his head..i dont think so..he saw something that didnt sit right with him and asked a question..Good thinking..

The last hook up i did realy threw me for a loop..i had to install a #1 (single conductor cable) under an 800amp breaker...Yep you can do it..However you have to look under FIRE PUMP..

 

[leitmotif]

The reason you’re not seeing an exception is because it’s not an exception, but part of the code. And THxx is not rated at 30 amps, it is 35 or 40 amps depending on the type.

240.4 Protection of Conductors – Conductors … shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G)


240.4(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)



​

Note in Table 240.4(G) that Article 440 (Air Conditioners), Parts III and IV are listed, along with many others.


Here is an example on how you size the circuit breaker and conductor for an AC unit, I’ll size a single AC unit with a nameplate of 30AMPs.

440.6(A) …the rated-load current marked on the nameplate of the equipment in which the motor-compressor is employed shall be used in determining the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection…


440.22(A) The motor-compressor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor. A protective device having a rating or setting not exceeding 175 percent of the motor-compressor rated-load current…

REPLY the key word here is not EXCEEDING 175%. It can be therefore less


​

So 30 x 1.75 = 52.5 amps. So I need a 52.5 amp breaker, I look at 240.6(A) and notice that 52.5 is not listed, but there is 50 and 60. Now I go to 240.4(B) and see that I will use a 60 amp breaker.

240.6(A) The standard ampere ratings for fuses and inverse time circuit breakers shall be considered ... (shows listing of standard sizes)


240.4(B) Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all the following conditions are met:…(which they are in this case)



​

Now to size the conductor.

440.32 Single Motor-Compressor. Branch-circuit conductors supplying a single motor-compressor shall have an ampacity not less than 125 percent of either the motor-compressor rated-load current …

REPLY continues to say "or branch circuit selection current" Gotta confess I don't know what this is - adjustable trip setting maybe ??.






​

So 30 x 1.25 = 37.5 amps. So I’m going to assume that I am using THHN conductors and that the equipment has connections that are rated at 90C. So I’m going to use 10AWG which is rated at 40amps.


So, by following the NEC I have 60amp breaker supplying a 30amp AC unit through 10AWG conductors. Would I do it this way on a job? You bet I would. First of all, you need to install a circuit breaker that is capable of handling the starting current, as stated in 440.22(A). Do I know what the starting current is? Nope, but the code will let me go up to 175 percent of the nameplate, and more if needed. So I go up to 175% because I’m not going to place a 30amp breaker in there only to find out that in the middle of summer the unit is tripping the breaker when it starts. Am I going to use a 60amp breaker and 6AWG to supply a 30amp AC unit? No that would be stupid.

So is this safe? Sure is. Conductors need to be protected from short-circuit, ground-fault, and overcurrent (and physical damage). Generally the circuit breaker is used to provided all three of these protections, like in your house. But in the case of motors the circuit breaker is only providing short-circuit and ground-fault protection. The overloads in the starter are used to provide the overcurrent protection.

Back to the poster's question. Is it possible to place a 10awg conductor in a 60amp breaker? Yes, under certian conditions.

GENERIC REPLIES
I do not argue that Tark is correct for most part on interpretation of NEC
EXCEPT as I noted above the overcurrent device ie (60 amp breaker) CAN be a MAXIMUM of 175%. It can therefore be less IF it can handle starting current.

Does anyone know what branch circuit selection current is and explain it to me please?

If the motor draws 30 amp full load I would tend to use # 8 THHN or MTW. Something about # 10 protected witha 60 amp breakr just bugs me. IF I had to use a 60A to get this to start I would use #6 because
I like beefy wiring (overkill I know)
Keep starting current voltage drop to a minimum - especially if there is any length to the circuit. We did not get into sizing for voltage drop -- I know.

NEC is a bare minimum code
Art 90 - 1 (b)
"This code contains provisions that are considered necesary for safety.
Compliance therewith and proper maintenance will result in an installation that is essentiallly free from hazard
BUT
not necessarily efficient, convenient or adequate for good service or future expansion of electrical use"

Now then what is essentially "free from hazard" mean? In my mind it is recognition they cannot guarentee that it is safe for all applications - another liability catch phrase - but they cannot guarantee total safety I know.

 

[Tark]

True, the branch-circuit short-circuit and ground-fault protective device (not the overcurrent device) can be less than 175%. The maximum is in fact 225%, if 175% is not sufficient for the starting current of the motor. Remember I was showing how it would be possible (and legal according to the NEC) to place a 10AWG conductor in a 60amp breaker.

Let’s not forget you have to keep the real world in mind as well. If you send the hook-up of this AC unit out to bid, the contractors will more than likely bid it with a 50 or 60 amp breaker with 10AWG conductors. First the contractor wants the job, and in most cases jobs are awarded to the lowest bidder. Second, being the lowest bidder you better make sure you do it right the first time else you aren’t going to make any money. So the contractor determines he can use a 60 amp breaker and that’s what he will install, or maybe a 50 amp. It’s more cost effective to install a 60 or 50 amp breaker today as apposed to installing a 30 amp breaker which you might have to change out at a later date. He’s going to use 10AWG conductors because of cost, it’s cheaper in materials and installation cost. Of course the contractor will be more than happy to run 6AWG wire for you after he is awarded the job. Just as soon as you sign the change order. After all he did bid the job according to the NEC, if you want something more it’s going to cost ya.

Branch-Circuit Selection Current is in 440.2 “The value in amperes to be used instead of the rated-load current in determining the ratings of motor branch-circuit conductors, disconnecting means, controllers, and branch-circuit short-circuit and ground-fault protective devices wherever the running overload protective device permits a sustained current greater than the specified percentage of the rated-load current. The value of the branch-circuit selection current will always be equal to or greater than the marked rated-load current.” This rating is placed on the equipment nameplate by the manufacturer.

Basically as I understand it, it is possible for the manufacturer to use an overload device for the compressor motor that has a higher rating than that permitted by the NEC, in doing so they must place a branch-circuit selection current rating on the nameplate that reflects this. I think this comes into play when the AC equipment has more than just a compressor-motor in it that needs power.

 

[bwiring]

Ac Wiring

If You Look On The A/c Nameplate It Will Say "min. Ckt. Ampacity" And That Is What Determans The Wire Size. The Nameplate Also
Will Say "max. Fuse (or Harc Circiut Breaker)" And This Is The Size Fuse Or Breaker That You Need To Use.

 

[BDKuhns]

Do we need to keep in mind length of run and if the conductor is in an attic. Does the conductor need to be up-rated for lenght or ambient heat during summer?
My neighbor installed a heat pump on his house. His inspection failed simply because the line was run in the attic and he did'nt up-rate the conductor.

 

[Tark]

If You Look On The A/c Nameplate It Will Say "min. Ckt. Ampacity" And That Is What Determans The Wire Size. The Nameplate Also
Will Say "max. Fuse (or Harc Circiut Breaker)" And This Is The Size Fuse Or Breaker That You Need To Use.

No, you do not have to use the Max size, you can use a smaller size. You just can't use one that is larger than that.

 

[darrenj]

Do we need to keep in mind length of run and if the conductor is in an attic. Does the conductor need to be up-rated for lenght or ambient heat during summer?
My neighbor installed a heat pump on his house. His inspection failed simply because the line was run in the attic and he did'nt up-rate the conductor.

Never heard of that one..But i guess it would be covered under "Ambiant temp"..Wire is rated at a certain ambiant temperature..If the ambiante is higher you have to derate..I have never seen that yet in a residential project but it does make some scence ..specialy if you live in a hot climate..

D

 

[leitmotif]

Do we need to keep in mind length of run and if the conductor is in an attic. Does the conductor need to be up-rated for lenght or ambient heat during summer?
My neighbor installed a heat pump on his house. His inspection failed simply because the line was run in the attic and he did'nt up-rate the conductor.

The circuit should have only 3 (5%) volt drop at max load. Therefore yes you do need to keep this in mind. Probably for runs less than 50' no problem. 100' I would do a set of calcs to ensure no problem.

You have to install wire that meets the temp expected in the installed environment. IN an attic I would install 90 C conductors.

CAVEAT as I mentioned previously I do not worry about complying with NEC - I exceed it.

Dan Bentler