Hvac

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RRJ

RRJ

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atlanta georgia
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Hvac is its own animal. 240.4G, 440.31-35 for conductor sizing, but in simple terms look at the nameplate. If max breaker is 30 amps then 30 amps breaker is fine, if fuses are 25amp that’s fine and if FLA total is 17.5 amps then 14 AWG is good for up to 20 amps based on 310.16.


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LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
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Henrico County, VA
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Electrical Contractor
infinity said:
The conductors between the 30 amp OCPD at the panel and the 25 amp OCPD at the disconnect can be by definition (OCPD at each end) a feeder. That means that they need to be a minimum of 30 amps but if the 25 amp OCPD is supplementary (Article 100 definition post #16) then the branch circuit begins at the panel and the conductors only need to be sized according to the MCA of 17.2 amps.
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"But, how do it know?!" :unsure:
 
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wwhitney

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hbiss said:
No, his original post stated that it was 17.2A.
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Infinity meant that the MOCP is 25A. Which we did find out in post #6.

BTW, why does 430.63 use the phrase "not less than" and why doesn't that allow arbitrarily large OCPD? I assume that 430.62 is only for motor loads, so that it doesn't apply to a feeder supplying a motor and other loads.

Cheers, Wayne
 
infinity

infinity

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wwhitney said:
Infinity meant that the MOCP is 25A. Which we did find out in post #6.

BTW, why does 430.63 use the phrase "not less than" and why doesn't that allow arbitrarily large OCPD? I assume that 430.62 is only for motor loads, so that it doesn't apply to a feeder supplying a motor and other loads.

Cheers, Wayne
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You're correct, thanks I'll fix it.
 
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wwhitney

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So for an HVAC unit with a 17.2A MCA and a 25A MOCP, the branch circuit protected at 25A can be #14 Cu in a 75C wiring method. Here's an argument that the feeder protected at 30A can still be #14 Cu:

Add a planned 2A non-continuous load for the feeder. Then 430.24 says that the required conductor ampacity is 19.2A, still under the 20A 75C ampacity of #14 Cu. 430.63 says that the feeder OCPD has to be at least 27A. So we round up to 30A, for a #14 Cu feeder protected at 30A.

[430.63 seems weird to me.]

Cheers, Wayne
 
RRJ

RRJ

Senior Member
Location
atlanta georgia
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Electrician
LarryFine said:
I would run #12 for almost any residential A/C, even if code allows #14, in case a replacement needs it.
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Agree. I even run # 10 if some hack later on decides to install a different equipment that’s not Hvac just because it has a 30 amp.


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J

jap

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Electrician
RRJ said:
Agree. I even run # 10 if some hack later on decides to install a different equipment that’s not Hvac just because it has a 30 amp.


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I'd run #10 to a 30 amp disconnect and all the way to the unit.

Unlike some "hacks" that would run #14. :)

JAP>
 
RRJ

RRJ

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atlanta georgia
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Electrician
jap said:
I'd run #10 to a 30 amp disconnect and all the way to the unit.

Unlike some "hacks" that would run #14. :)

JAP>
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#14 is legally aceptable because your equipment won’t pull more than 20 amps. It’s just like wiring any Motor.


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J

jap

Senior Member
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Electrician
RRJ said:
#14 is legally aceptable because your equipment won’t pull more than 20 amps. It’s just like wiring any Motor.


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Yes it is, but, it's not a "hack" for someone to assume that a 30 amp disconnect would be wired with 30 amp rated conductors...... just sayin.

JAP>
 
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W

wwhitney

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Berkeley, CA
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Retired
wwhitney said:
I'm not following this emphasis on whether the conductors are feeders. If the OCPD does not exceed the MOCP, then a feeder supplying only one motor load/piece of HVAC equipment would be sized the same as a branch circuit, per my understanding of 430.62 (and as is only reasonable).
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OK, I've pored over Articles 240 and 430 some and now realize the above is in error. My current understanding:

240.21 requires conductors to to be protected against overcurrent at their point of supply except under specified circumstances. [And the Article 100 definition of overcurrent covers both overload and SC/GF.] 240.21(F) says "Motor-feeder and branch-circuit conductors shall be permitted to be protected against overcurrent in accordance with 430.28 and 430.53, respectively." 430.53 covers the case of branch circuits, but 430.28 covers only "Feeder Taps," not the general case of a motor feeder.

The upshot is that a motor feeder must have its overload protection at its point of supply unless it qualifies under 430.28 or under one of the sections of 240.21. Unlike a motor circuit branch circuit, where the overload protection need not be at the point of supply, only the SC/GF protection needs to be.

So if the MOCP in the OP's example were 30A, then it would be necessary to treat the 25A fuses as supplementary protection in order to say that the branch circuit starts at the 30A breaker, to allow #14 conductors on the 30A breaker, with overload protection provided integral to the HVAC equipment. And as the MOCP is actually only 25A, that is not possible, the conductors from the 30A breaker to the 25A fuses are definitely a feeder, and must have their overload protection at their point of supply, or meet one of the requirements of 430.28 or one of the sections of 240.21.

Getting back to the OP, if the conductors from the 30A OCPD to the 25A fuses meet one of the tap rules (say the 25' rule), then they could be #14s, rather than #10s.

Cheers, Wayne
 
RRJ

RRJ

Senior Member
Location
atlanta georgia
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Electrician
wwhitney said:
OK, I've pored over Articles 240 and 430 some and now realize the above is in error. My current understanding:

240.21 requires conductors to to be protected against overcurrent at their point of supply except under specified circumstances. [And the Article 100 definition of overcurrent covers both overload and SC/GF.] 240.21(F) says "Motor-feeder and branch-circuit conductors shall be permitted to be protected against overcurrent in accordance with 430.28 and 430.53, respectively." 430.53 covers the case of branch circuits, but 430.28 covers only "Feeder Taps," not the general case of a motor feeder.

The upshot is that a motor feeder must have its overload protection at its point of supply unless it qualifies under 430.28 or under one of the sections of 240.21. Unlike a motor circuit branch circuit, where the overload protection need not be at the point of supply, only the SC/GF protection needs to be.

So if the MOCP in the OP's example were 30A, then it would be necessary to treat the 25A fuses as supplementary protection in order to say that the branch circuit starts at the 30A breaker, to allow #14 conductors on the 30A breaker, with overload protection provided integral to the HVAC equipment. And as the MOCP is actually only 25A, that is not possible, the conductors from the 30A breaker to the 25A fuses are definitely a feeder, and must have their overload protection at their point of supply, or meet one of the requirements of 430.28 or one of the sections of 240.21.

Getting back to the OP, if the conductors from the 30A OCPD to the 25A fuses meet one of the tap rules (say the 25' rule), then they could be #14s, rather than #10s.

Cheers, Wayne
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The sole intent of the 30 amp branch circuit is to feed the hvac system then it falls under 240.4G.


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Little Bill

Little Bill

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Tennessee NEC:2017
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RRJ said:
The sole intent of the 30 amp branch circuit is to feed the hvac system then it falls under 240.4G.


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The 30A breaker effectively "kills" 240.4(G) for the entire run since the MOCP size is 25A. 240.4(G) would only apply from the fused disconnect to the unit.
 
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