Please quit telling people to oversize ocpd on AC branch and feeder circuits

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Yelram

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PA
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.
 

Yelram

Member
Location
PA
Gold digger, the max ocpd has nothing to do with conductor sizing, it is the maximum size of a circuit that can be run to the unit, it is not a suggested breaker size, it is the max, and it assumes the wire matches!
 

GoldDigger

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Gold digger, the max ocpd has nothing to do with conductor sizing, it is the maximum size of a circuit that can be run to the unit, it is not a suggested breaker size, it is the max, and it assumes the wire matches!
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.
 

david luchini

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Connecticut
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How can a 50 amp breaker protect 10 wire in a fault situation? Do you guys work on acs beyond running wires to them?

Do you understand circuit breaker time current curves?

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.

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.
 

ActionDave

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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.
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.
 

pete m.

Senior Member
Location
Ohio
How can a 50 amp breaker protect 10 wire in a fault situation?

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.

Do you guys work on acs beyond running wires to them?

No.

Have you ever seen a faulted compressor that will run and draw extra amps without a direct short?

That's what the integral overload protection is for.

What about extra resistance at terminals that will cook wire?

Really has nothing to do with your claims.

Why didnt the thermal overloads protect in these situations?

The thermal overloads aren't designed to protect against "glowing connections" or faults.

I dont think you quite undersrand the equipment, so your reading of the code is skewed and wrong.

I'm not right 100% of the time but on this one I am.

There is no reason to not match the wire size to the breaker. No benefit whatsoever.

Ok... I match the wire and the breaker to the minimum circuit ampacity. Would that be better?

Pete
 

Yelram

Member
Location
PA
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.
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?
 

iwire

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Staff member
Location
Massachusetts
Do you guys work on acs beyond running wires to them?

Often although in my case it is supermarket refrigeration compressors. Same rules apply.

Have you ever seen a faulted compressor that will run and draw extra amps without a direct short?

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.

What about extra resistance at terminals that will cook wire? Why didnt the thermal overloads protect in these situations?

Even breakers sized under your ideas will not keep high resistance connections from cooking off the wire.

I dont think you quite undersrand the equipment, so your reading of the code is skewed and wrong.

You are mistaken of our understanding of the equipment

There is no reason to not match the wire size to the breaker. No benefit whatsoever.

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.
 

GoldDigger

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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?
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?

430.22 Single Motor. Conductors that supply a single motor used in a continuous duty application shall have an ampacity
of not less than 125 percent of the motor full-load current rating, as determined by 430.6(A)(1) or not less than specified in 430.22
(A) through (G).

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.
 
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iwire

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Staff member
Location
Massachusetts
Not the case at all, the equipment will never draw 6 times the min ampacity on startup,

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.
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
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.
 

Pharon

Senior Member
Location
MA
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.

It's also usually listed on the nameplate, identified as LRA.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
According the the manufacturer, any and all conductors must be matched to the ocpd, the thermal overloads will not protect the wire in certain fault situations, the max ocpd is only telling you what the largest circuit is that can supply the unit, not what protection is required for what size wire, this refers back to article 240

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.
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Please post the section that says the ocpd on a branch circuit can be oversized for the wire.

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.
 

Pharon

Senior Member
Location
MA
Please post the section that says the ocpd on a branch circuit can be oversized for the wire.

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).
 
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