How can a 50 amp breaker protect 10 wire in a fault situation? Do you guys work on acs beyond running wires to them? Have you ever seen a faulted compressor that will run and draw extra amps without a direct short? What about extra resistance at terminals that will cook wire? Why didnt the thermal overloads protect in these situations? I dont think you quite undersrand the equipment, so your reading of the code is skewed and wrong. There is no reason to not match the wire size to the breaker. No benefit whatsoever.
Please quit telling people to oversize ocpd on AC branch and feeder circuits
- Thread starter Yelram
- Start date
- Status
You're right, my bad. It's 430.51.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
That nay be what you and some manufacturer's tech rep think, but it is not what the NEC says, as supported both by commentary and by AHJs.
- Location
- Connecticut
- Occupation
- Engineer
Do you understand circuit breaker time current curves?
Again, thermal overloads aren't supposed to protect against faults. There is no reason to match the wire size to the breaker. It is clear that you have little understanding of the NEC.
- Occupation
- Licensed Electrician
I don't think you understand the equipment. The larger over current device is there so the unit doesn't trip from inrush on start up. That can be six times the running current. There is no need to size a wire six times bigger than the normal running current when the starting current lasts only a movement.
Typically, the 50 amp breaker will open under fault conditions based upon the magnitude of current in the circuit during the fault in a time quick enough to avoid damage to the insulation on the smaller conductor.
No.
That's what the integral overload protection is for.
Really has nothing to do with your claims.
The thermal overloads aren't designed to protect against "glowing connections" or faults.
I'm not right 100% of the time but on this one I am.
Ok... I match the wire and the breaker to the minimum circuit ampacity. Would that be better?
Pete
Not the case at all, the equipment will never draw 6 times the min ampacity on startup, do you have any experience with hvac equipment to back that statement up?
- Location
- Massachusetts
Often although in my case it is supermarket refrigeration compressors. Same rules apply.
Of course and you must have also noticed that they may cycle, that is due to the built in self resetting overloads that protect the compressor motor and conductors feeding it from starting a fire.
Even breakers sized under your ideas will not keep high resistance connections from cooking off the wire.
You are mistaken of our understanding of the equipment
Wire costs money, that is the benefit of being able to use a breaker large enough to deal with the inrush of start up and a conductor sized for the running load.
The people responding to this thread know what they are talking about and I urge you to listen and learn.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Have you ever used a peak-catching digital ammeter on your AC at startup time? Otherwise you would never know what the starting current is.
Unless the unit contains an integral soft start circuit, the starting current will be high.
PS: Have you looked at 430.22?
No mention of the breaker or other OCPD size. The OCPD size is specified in other sections.
And the MCA listed by the manufacturer is typically 125% of the FLA if there are no significant non-motor loads.
Last edited:
- Occupation
- Licensed Electrician
Yes I do. And a pretty good understanding of motors.
- Location
- Massachusetts
The inrush cuurent the moment the compressor contactor closes will in fact be 6 to 8 times the compressor motors FLA. This is because you have to both energize the windings and deal with a locked rotor condition. (Power on motor not yet spinning is a locked rotor. )
You can't see this inrush with a standard amp clamp but trust us, it is happening.
Yelram, you are fighting a loosing battle here. I have no idea what individual at what manufacturer gave you that info but they have basically sent you into a gun fight armed with a butter knife.
There are tons of folks here on the Forum with extensive knowledge of most all sections of the Code and I don't recall one knowledgeable member who supports you argument.
The Code is very clear on how to select the branch circuit conductor and the branch circuit short-circuit/Ground fault Protective device.
You will have to come up with more than "they said" and "I feel" to sway this group one inch on matters that are well covered by the NEC.
There are tons of folks here on the Forum with extensive knowledge of most all sections of the Code and I don't recall one knowledgeable member who supports you argument.
The Code is very clear on how to select the branch circuit conductor and the branch circuit short-circuit/Ground fault Protective device.
You will have to come up with more than "they said" and "I feel" to sway this group one inch on matters that are well covered by the NEC.
It's also usually listed on the nameplate, identified as LRA.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I don't want to pile on but 10 people have told you that you're incorrect and provided various code articles that you're simply not understanding.
AC units nameplates provide the MCA and the MaxOCPD, use the MCA to size the conductor, use any OCPD between the MCA and the MaxOCPD to provide short circuit and ground fault protection. That's it, it's not unsafe and if a manufacturer is telling you differently then they need some help understanding the NEC.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
On the nameplate of a bare motor, yes, but less often on the nameplate of an appliance where you would see MCA and MOCPD instead.
240.4 which refers you to 240.4(G) which lists Art 440.
Therein lies the permission to exceed the condcutor ampacity with the OCPD.
430.31 tells you how to size for overload and 430.52(B) tells you that you need to size your short circuit and ground fault need to be sized to handle the STARTING CURRENT of the motor (i.e. the LRA).
- Status