Heat Pump wire correction

[crtemp]

I'm not going to get into the NM usage as we all should know NM is not allowed in a wet location and yes it is required to be sized at the 60?c column of table 310.16.

What I would like to discuss is what we protect conductors from and what methods are allowed by the NEC.

First there are two things we protect conductors from and these are two very separate and different requirements that we use to protect conductors.

First one is overcurrent.

Overcurrent is long term and can be quite a few minutes if just over the rating of the conductor, most are used to over current being provided at the supply end of the circuit by the breaker or fuses that supply a conductor, this is always true when you have a circuit supplying an unknown amount of load such as receptacles or branch circuits in a house or other, since we can't control what people might plug in or connect to this type of branch circuit we make sure the conductors are protected at the supply end.

But there are cases where the overcurrent protection is allowed to be at the load end and is just as safe as long as more loads are not likely to be added, the three most common times we will see this is: taps as we see in 240.21: HVAC as in 440: and motors: 430 Also if we look at table 240.4(G) we will see other places where the NEC allows end of circuit over current protection.

Now the second thing we protect conductors from is short circuit and ground fault.
Short circuit and ground fault protection is always at the supply end of a circuit and always has to be, a conductor can be protected from SCGF by OCPD many times the conductors rating because the available fault current in most circuits will always be in the instantaneous region of the trip curve of most all breakers and fuses with instantaneous trip, usually within 5 cycles,
the only time this can be a problem is if the circuit is too long and the resistance of the circuit limits the fault current to a level out of the IT of the breaker curve this is why we must pay attention to how long a circuit is run when we chose the conductor rating and the SCGF protection for that conductor.

Now if we look are the requirements in 240.4(D) we will see that it allows us to use 240.4(E) or (G) for small conductors instead of the normal 15,20,30 amp ratings for small conductors (14-10awg) also 240.4(G) tells us to use the articles in table 240.4(G) for the applications listed in that table for conductor protection, most of these articles will also direct us to use table 310.16 for the conductor Ampacity instead of the Ampacity listed in 240.4(D) for small conductors.

With all that said, if we look at 440 for Air-conditioning and refrigerating equipment, part III will tell us what we must do for SCGF protection (440.21-) you will find out that in most cases that this is the max circuit protection that we find on the label on the equipment.

For overload protection we will have to look at part IV (440.31-) we will see that like above we will chose the conductors based upon the minimum circuit size on the label, again this is the end of the circuit overload protection I mention above, the overload is provided by the overload protection built inside of the compressor.

As far as the 125% this is always provided in the minimum circuit size because the manufacture has already included it by UL requirements.

Heres the commentary from the 2011 NEC hand book on this:



So if the air-conditioning equipment has a label that gives you the minimum circuit size you use this for sizing the conductors, and the max is to size the OCPD for SCGF.

It's a very common saying for AC equipment you wire for the minimum and breaker for the max.

Also 440.31 directs us to article 310 for the sizing of the conductors and we would use table 310.16 for this in most applications, again as 240.4(D) stated that this article will over ride the small conductor requirements of 240.4(D) as 440 is in the list of 240.4(G)

I hope this helps with a little better understanding of why we see smaller conductors being protected by larger OCPD's they are still being protected, just not in a way we are use to, in this case they are protected by the overloads built in each compressor and fan motor in the air-condition equipment, if it is listed then go by the label on it.

awesome explanation!!
Understand perfectly now. Thank you for taking time to explain it clearly to me. I appreciate it

 

[augie47]

agree, excellent presentation, Wayne !

 

[iwire]

Nope, conduit(s) and THWN.

The areas in a 3R cabinet that allow K/Os are wet. You will note in the design of a 3R cabinet with devices, that all K/Os (not counting the water resistant hub) are below any connections. The area above the K/O is probably "damp".

I hate UF.

:lol:

 

[asphalt]

Just to clear things up, the NM was pulled through the attic to a bell box then piped with pvc to the disconnect and wired with THWN.

Thank you for all the input, I just wanted a little information before I called the inspector this morning. I've wired a lot of heat pumps and this is the first time the 80% rule has ever come up.

 

[fmtjfw]

Just to clear things up, the NM was pulled through the attic to a bell box then piped with pvc to the disconnect and wired with THWN.

Thank you for all the input, I just wanted a little information before I called the inspector this morning. I've wired a lot of heat pumps and this is the first time the 80% rule has ever come up.

Thanks, you posed an interesting topic.

 

[asphalt]

I finally spoke to the inspector yesterday afternoon. He told me that he considers the outdoor unit to be a continuous load and the 12-2 needs to be derated for that. I have to go back and re-pull 10-2 and rewire the whole thing now.

 

[texie]

I finally spoke to the inspector yesterday afternoon. He told me that he considers the outdoor unit to be a continuous load and the 12-2 needs to be derated for that. I have to go back and re-pull 10-2 and rewire the whole thing now.

The inspector is wrong. The MCA already has the 125% built in to the calculation. So what he is asking is to add 25% to 25%. The MCA is to be used for conductor ampacity at the value indicated. In the absence of any other required derating such as # of CCC, etc, the #12 NM is fine. It is just that simple. He needs to get some training.

 

[asphalt]

The inspector is wrong. The MCA already has the 125% built in to the calculation. So what he is asking is to add 25% to 25%. The MCA is to be used for conductor ampacity at the value indicated. In the absence of any other required derating such as # of CCC, etc, the #12 NM is fine. It is just that simple. He needs to get some training.

I tried to explain that to him, but probably didn't do a very good job. I need to learn how to deal with inspectors better.

 

[david luchini]

The inspector is wrong. The MCA already has the 125% built in to the calculation. So what he is asking is to add 25% to 25%. The MCA is to be used for conductor ampacity at the value indicated. In the absence of any other required derating such as # of CCC, etc, the #12 NM is fine. It is just that simple. He needs to get some training.

:thumbsup:

 

[texie]

I tried to explain that to him, but probably didn't do a very good job. I need to learn how to deal with inspectors better.

I would suggest that you get a copy from the UL website of "Marking and Application Guide Electrical Heating and Cooling Equipment". While this covers more than condensing units, take a close look at section 16 and it explains the way MCA is calculated by the manufacturer and cites the related code references.

If it were me, I would not just roll over for an inspector if he/she makes a wrong call, especially if it is going to cost me money. We all make mistakes and are sometimes just misinformed. A good and fair inspector should be willing to learn along with the rest of us without being offended. If he won't cooperate, take it above him.

 

[qcroanoke]

Do you have the manual on the unit?
They sometimes list wire size required as well as the MCA and MOCP.
It sounds like you are looking at a lot of work just because the EI doesn't get it.
I'm not much in to fighting the EI but I think this warrants an argument.

 

[asphalt]

Looks like I'm going to have to learn how to explain this to inspectors better because I just got another correction from a different inspector for a different job that says you can't use 25A fuses on #12. My company needs to find more time for me to deal with all this.

 

[texie]

Looks like I'm going to have to learn how to explain this to inspectors better because I just got another correction from a different inspector for a different job that says you can't use 25A fuses on #12. My company needs to find more time for me to deal with all this.

Yep, and that is why I mentioned in my post to you that you should get to the bottom of this as this can have bigger implications. I'll bet these same inspectors would also have an issue with, for example, a motor that has a table value X125% of 18 amps and using #14 THHN with a 45 amp breaker.

 

[kwired]

It is not your job to educate inspectors, if you know they are wrong and they will not listen go to their supervisors, or to state level AHJ if possible and they will not listen.

I know some out there just do what an inspector tells them to do because it seems easier at the time, but if I know they are not right I am not doing myself any favors either by just letting them have their way, I want it right, not for just this job but also when similar situation comes up again. Maybe I bid a job using the 12 AWG because I knew it was allowed, now they are trying to make me fix something that is not wrong, and at my expense

MCA already has the continuous load factor built into it. OP's inspector needs to study art 440 a little harder, only ampacity adjustment necessary for this equipment is for ambient temp or number of conductors in raceway/cable if it applies.

 

[qcroanoke]

Looks like I'm going to have to learn how to explain this to inspectors better because I just got another correction from a different inspector for a different job that says you can't use 25A fuses on #12. My company needs to find more time for me to deal with all this.

Seems to me your boss should be the one explaining this to the EI.

 

[tom baker]

I'm looking at Art. 440 parts III and IV and I don't see where it says you can use #12 for over 20 amps. Am I not seeing it? I don't even understand how the OP can get away with putting the #12 romex on a 30 amp breaker.

Since its an HVAC unit, the NEC allows us to us a different ampacity for conductors

Start with 240.4
Then look at 240.4(G) which allows for specific applications. Right at the top it shows 440 Parts III & VI. This section allows you to use a wire sized larger than what you would normally per Art 310.
The motor compressor has internal overloads that protect the wire from overloads. The circuit breaker protects against short circuits and ground faults.

 

[kwired]

Since its an HVAC unit, the NEC allows us to us a different ampacity for conductors

Start with 240.4
Then look at 240.4(G) which allows for specific applications. Right at the top it shows 440 Parts III & VI. This section allows you to use a wire sized larger than what you would normally per Art 310.
The motor compressor has internal overloads that protect the wire from overloads. The circuit breaker protects against short circuits and ground faults.

I have to disagree with your first sentence as written. I think I get your intention though.

A 12 AWG conductor with 60C rating has an ampacity of 20 amps whether it supplies a lighting circuit, heating equipment, motor, or air conditioning equipment. Allowable overcurrent protection of this conductor could be different for each of those examples though, but in general when the fuse or breaker is allowed to be larger it is for starting purposes, but motor overloads still protect the conductor from overloading as well as protecting the motor.