HVAC equipment as it applies to the NEC.

h1h2h3

Member
Location
MA
Hi All,

I had a few questions about an install I have coming up (and it would also be nice to know for future reference as some of this has always confused me a bit.)

We're installing a 3 phase 208 v condenser unit on the roof of one of our buildings. The name plate calls for a minimum circuit ampacity of 34 amps and a max fuse or HACR breaker rating of 50 amps. It will be about 8 ft from where my circuit emerges from inside to the disconnect I am mounting on the unit.

My questions are as follows:

1. Is it acceptable to size my wire according to the nameplate's minimum circuit ampacity and not the max ocpd used? I understand that in this case it is a mute point because it will be a #8 AWG regardless, but generally speaking is it? (ie: a 24 amp minimum ampacity (So a #10 AWG) protected by a max breaker size of 40 amps (per the nameplate.) Is this what the code is referring to in 440.6 (B) exception 1 and 430.52 (A-C)?

2. Since my wire run is just about 80' total and I'm right on the edge of the allowance permitted by 310.15 (A)(2), I've decided to derate my wire per table 310.15 (B)(3)(c) to ensure a quality install. I figured that the average high in the summers in New England is 96-104 and I'm using foam/strut pyramid roof racks that are about 3.5" high. This put's me at 104+40=144 degrees, a correction factor of .65 (I'm using THHN). Now I'm confused as to whether I apply this number based on the 34 amp minimum circuit ampacity I have or, if I apply it to the 50 amp rating of ocpd. If I apply it to the 34 amp minimum circuit ampacity, I'll have to run a # 6 from inside to the disconnect. If I apply it to the 50 amp circuit as just that, a 50 amp circuit, I have to run a #4AWG.

3. My final install is planned like this. A 50 Amp HACR bolt on breaker at the panel. A #8-3 MC from the panel to a metal splice box inside. 1 1/4" PVC Schedule 80 (lot of falling debris on this roof) from the inside splice box to right before unit (with an expansion fitting of course). Change over to liquitight to make tight sweep to disconnect. (3) #4 AWG THHN ungrounded (hot) conductors and one #8 AWG EGC. Unfused disconnect at unit because namplate specifies fuse or HACR breaker and I already have the HACR breaker at the panel (most breakers are HACR these day). Does this sound correct?

I just recently left construction and went to work for a property management company. I am the only electrician there, so they like to blame me for lots of things (lol). Its difficult not having anyone around to bounce ideas off of any longer and a bit overwhelming.

I sincerely appreciate you taking the time to read this thread. Hopefully you guys will have some insight for me.

Thank You.
 

stickboy1375

Senior Member
Location
Litchfield, CT
Hi All,

I had a few questions about an install I have coming up (and it would also be nice to know for future reference as some of this has always confused me a bit.)

We're installing a 3 phase 208 v condenser unit on the roof of one of our buildings. The name plate calls for a minimum circuit ampacity of 34 amps and a max fuse or HACR breaker rating of 50 amps. It will be about 8 ft from where my circuit emerges from inside to the disconnect I am mounting on the unit.

My questions are as follows:

1. Is it acceptable to size my wire according to the nameplate's minimum circuit ampacity and not the max ocpd used? I understand that in this case it is a mute point because it will be a #8 AWG regardless, but generally speaking is it? (ie: a 24 amp minimum ampacity (So a #10 AWG) protected by a max breaker size of 40 amps (per the nameplate.) Is this what the code is referring to in 440.6 (B) exception 1 and 430.52 (A-C)?

#10 AWG is good for 35 amps in this application, I always use the smallest wire size and the largest breaker, as long as voltage drop is not an issue. So I would run #10 AWG , with a 50 amp breaker.
 
Last edited:

h1h2h3

Member
Location
MA
May I ask how #10 is good for 35 amps in this application? 240.4 (D) says that it isn't. Can you explain so that I may understand this?

Also, do you have and insight in to the other questions?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
We're installing a 3 phase 208 v condenser unit on the roof of one of our buildings. The name plate calls for a minimum circuit ampacity of 34 amps and a max fuse or HACR breaker rating of 50 amps. It will be about 8 ft from where my circuit emerges from inside to the disconnect I am mounting on the unit.

My questions are as follows:

1. Is it acceptable to size my wire according to the nameplate's minimum circuit ampacity and not the max ocpd used? I understand that in this case it is a mute point because it will be a #8 AWG regardless, but generally speaking is it? (ie: a 24 amp minimum ampacity (So a #10 AWG) protected by a max breaker size of 40 amps (per the nameplate.) Is this what the code is referring to in 440.6 (B) exception 1 and 430.52 (A-C)?
You need a conductor that has an ampacity of 34 amps or more after derating.
If I'm understanding you installation, with only 8' on the roof and an 80' run derating might not even be required.
 

h1h2h3

Member
Location
MA
I'm really sorry, I looked in both of those sections and all i found was information regarding overcurrent protection. I see nothing about branch circuit selection or use of the 75 degree column where number 10 was concerned.
 

Smart $

Esteemed Member
Location
Ohio
I'm really sorry, I looked in both of those sections and all i found was information regarding overcurrent protection. I see nothing about branch circuit selection or use of the 75 degree column where number 10 was concerned.
Article 240 only concerns overcurrent protection... and only for instances which are not covered in other articles noted in 240.3 [or 240.4(G)]. Overcurrent protection and conductor selection for air-conditioning equipment is covered in Article 440.
 

h1h2h3

Member
Location
MA
You need a conductor that has an ampacity of 34 amps or more after derating.
If I'm understanding you installation, with only 8' on the roof and an 80' run derating might not even be required.
It's not required, however, I'm not 100% positive its 80 ft. Its close to it and I'm not paying for the stock, so for piece of mind and an assured quality installation. I'd rather derate. My calculations show that a number 8 THHN after derating will be fine for the enitire install. Without the deration, I'm still confused as to how the number 10 will suffice.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Engineer
Most HVAC units are rated for 75C terminations.
#10 THHN at 75C is good for 35A.
240.4(D) does not apply to motors or HVAC units, so #14/12/10 conductors can be breakered above their "customary" limits.

Thermal derating would be done from the 90C column for THHN (40A for #10, 55A for #8). The end result needs to be greater or equal to the HVAC MCA nameplate value. Because of the circuit length and rooftop heat, I would not be pushing wire sizes and think the #8 choice is a good one for this application.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Just forget for a moment that the conductor size for 34 amp MCA is #10.

Let's say you needed a conductor for an MCA of 65 amps, the terminal and conductors are 75? C. You would go to Table 310.15(B)(16), look down the 75? C column and find a conductor, THHN #6.

The same logic would apply if the MCA were 34 amps (#10) or even 24 amps (#12). Since this is under Article 440 you can disregard the ampacity limitations of 240.4(D).
 
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