AC unit feeder and branch circuits NEC 430

[Eddie702]

It has become an abbreviation

 

[infinity]

Not exactly the OP's question but it is germane to the discussion. Since the circuit breaker is only providing short circuit and ground fault protection why is there a limit as to the maximum? Won't a 60 amp CB trip on a ground fault just as quickly as a 30 amp CB or is there a difference in the trip curves as the breakers get larger in size? Would the conductor size have anything to do with limiting the MaxOCPD to a certain size?

 

[jap]

One might think that an overload would trip immediately at a certain amp setting.

Maybe after a while, but, I've worked in many feed mills and motor controls and can tell you it's not an exact science.

Especially the old solder type heaters.

If you've ever been around motor starters with open overloads you can see that it takes some time for the actual heat to build up to trip them out.

I don't think one would want to stretch the size of the SSGFP too much.

Jap>

 

[jim dungar]

Not exactly the OP's question but it is germane to the discussion. Since the circuit breaker is only providing short circuit and ground fault protection why is there a limit as to the maximum? Won't a 60 amp CB trip on a ground fault just as quickly as a 30 amp CB or is there a difference in the trip curves as the breakers get larger in size? Would the conductor size have anything to do with limiting the MaxOCPD to a certain size?

Typically the magnetic trip element is based on the frame size (largest ampacity) of the breaker, regardless of its handle rating. However, this does not always hold true for breakers less than 100A where there may be several different mechanisms.

 

[wwhitney]

Not exactly the OP's question but it is germane to the discussion. Since the circuit breaker is only providing short circuit and ground fault protection why is there a limit as to the maximum?

Having overload protection at the load end of the conductors does open a vulnerability for high resistance faults in the supply that would draw currents in the thermal trip range of the OCPD. No upside to making that vulnerability any greater than necessary.

Cheers, Wayne

 

[tortuga]

This subject has been beat almost to death before, but there is one distinction I would like everyone's opinion on.

There is a 12AWG Romex, running 240V (on black and white conductors) from the inside load distribution center on a two pole 30A breaker, feeding the required outside disconnecting means, which happens to be fused with 30A time delay fuses. The circuit feeds a single AC unit in a residential setting.

I'd say if it were a tray cable or MC yes but not NM cable, per 334.80 NM is limited to 60C, so the OCPD would need to be 20A.

 

[wwhitney]

per 334.80 NM is limited to 60C, so the OCPD would need to be 20A.

Negative for motor/HVAC applications. If the MCA is 20A or less, and the MOCP is 30A or greater, then you can use #12 NM on a 30A OCPD, barring severe ampacity adjustment and correction being required.

Cheers, Wayne

 

[winnie]

(Agreeing and expanding post 27) The point being that 334.80 limits NM to 60C, and thus limits the ampacity of #12 NM to 20A, but that in this specific case a conductor that ampacity is permitted to be protected by a breaker of higher trip rating.

Now if the HVAC unit said 'MCA 21A MOCP 30A' you would not be able to use #12 NM, exactly because of the limit pointed out by @tortuga .

-Jon

 

[tortuga]

I could see that 240.4(G) -> article 440 part III for the 12/2 NM from the disconnect to the unit, but not for the feeder from the panel to the disconnect.
EDIT nevermind brain fart it would only be a feeder if the disconnect is fused.

 

[wwhitney]

I could see that 240.4(G) -> article 440 part III for the 12/2 NM from the disconnect to the unit, but not for the feeder from the panel to the disconnect.
EDIT nevermind brain fart it would only be a feeder if the disconnect is fused.

The feeder/branch circuit distinction is immaterial to this question. The 240.4(G) references cover both the feeder and branch circuit sections of Articles 430 and 440. And the conductor and OCPD sizing rules are identical for a branch circuit serving just one motor/HVAC appliance and a feeder serving just one motor/HVAC appliance.

Cheers, Wayne

 

[tortuga]

I see @Augi covered it in post #11 sorry for the noise.
i guess that explains why those ductless mini-splits run a 14/3 tap to the indoor units without any additional OC protection.

 

[infinity]

Typically the magnetic trip element is based on the frame size (largest ampacity) of the breaker, regardless of its handle rating. However, this does not always hold true for breakers less than 100A where there may be several different mechanisms.

So I believe that what you're saying is that if the MaxOCPD used is slightly larger than value listed on the nameplate there really is no negative consequence. This is because the larger device will still provide the GF and short circuit protection.

 

[jim dungar]

Most likely, yes. But not necessarily the case at frame break points like 225A up sized to 250A.

 

[mtnelect]

How about adding some wiggle room to your next project ? It shouldn't coast that much.

 

[Crash117]

Had a situation on this exact thread last week. New hvac outdoor heat pump condenser. MCA of 29 amps and min breaker of 35 and max of 50. Used 10 gauge Romex 50 amp breaker and the inspection failed. He noted 10 can never be over 30 breaker or fuse but didn’t site code section. Wish I had been able to be there. Would have had the book ready and tried to work him through it.

 

[infinity]

Had a situation on this exact thread last week. New hvac outdoor heat pump condenser. MCA of 29 amps and min breaker of 35 and max of 50. Used 10 gauge Romex 50 amp breaker and the inspection failed. He noted 10 can never be over 30 breaker or fuse but didn’t site code section. Wish I had been able to be there. Would have had the book ready and tried to work him through it.

That inspector needs a refresher course. What you've described is Inspector 101.