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

[Yelram]

I cannot understand how anyone could take article 440 out of context to mean that as long as you size the conductor for min ampacity, you can use the max ocpd. I called a manufacturer, and they said all circuits must have an ocpd that is correctly sized to the max ampacity of the conductor, not the min ampacity of the appliance. The code even specifies that it 440 is in addition to 240.
440.21 General. The provisions of Part III specify devices intended to protect the branch-circuit conductors, control apparatus, and motors in circuits supplying hermetic refrigerant motor-compressors against overcurrent due to short circuits and ground faults They are in addition to or amendatory of the provisions of Article 240.
You are going to cause house fires! Noone in the HVAC trade would agree with this

 

[tom baker]

"Please quit telling people to oversize ocpd on AC branch and feeder circuits"
Who is doing the telling?
My comment on HVAC is if the minimum circuit ampacity is 21 amps, many electricians will use 10 AWG THHN

 

[Yelram]

If the panel on the aplliance says max ocpd 50 amps, do you put a 30 amp breaker on, or a 50?

 

[ActionDave]

It's the manufacturers that put the MCA and MOCP on the units. If they want bigger wires all they have to do is spec a bigger MCA.

 

[pete m.]

I cannot understand how anyone could take article 440 out of context to mean that as long as you size the conductor for min ampacity, you can use the max ocpd. I called a manufacturer, and they said all circuits must have an ocpd that is correctly sized to the max ampacity of the conductor, not the min ampacity of the appliance. The code even specifies that it 440 is in addition to 240.
440.21 General. The provisions of Part III specify devices intended to protect the branch-circuit conductors, control apparatus, and motors in circuits supplying hermetic refrigerant motor-compressors against overcurrent due to short circuits and ground faults They are in addition to or amendatory of the provisions of Article 240.
You are going to cause house fires! Noone in the HVAC trade would agree with this

There are many things that can cause a fire in an electrical circuit but sizing conductors to the minimum circuit ampacity and the OCPD to the maximum listed on the nameplate isn't one of them.

It has been done like this on too many installations to even count and not one of them (that I personally know of) has had a fire due to this practice.

Pete

 

[iwire]

I cannot understand how anyone could take article 440 out of context to mean that as long as you size the conductor for min ampacity, you can use the max ocpd.

We tell people that because it is 100% true.

If the unit says MCA 15 MOCP 30 we can use 14 AWG with a 30 amp breaker.

The overload units in the AC unit will protect the conductors from overload and the 30 amp breakers will protect the conductors from short circuits.

You are going to cause house fires! Noone in the HVAC trade would agree with this

Both those points are absolutely false.

 

[iwire]

If the panel on the aplliance says max ocpd 50 amps, do you put a 30 amp breaker on, or a 50?

50

And the conductor size might be 10 or 12AWG depending on the units MCA.

I understand you are concerned and that is a good thing but you really do not understand the rules for this.

 

[Yelram]

According to the manufacturer you are wrong. They said if you use the max ocpd, the wire should be sized according to 240. The code says that as well.(A) Overload Protection. The motor-compressor shall be provided with overload protection selected as specified in 440.52(A). Both the controller and motor overload protective device shall be identified for installation with the short-circuit and ground-fault protective device for the branch circuit to which the equipment is connected.

Noone does it like you are saying, noone

 

[Dennis Alwon]

I think you are misunderstanding article 440. Perhaps you should quote what article you are talking about but as others stated you can size youir conductor to the min cir.amp. and still use the maximun overcurrent protective device listed on the unit. Iwire mentioned that the overload protection in the unit will handle the overload. The breaker is sized to handle ground fault and short circuit protection only.

 

[ActionDave]

If the panel on the aplliance says max ocpd 50 amps, do you put a 30 amp breaker on, or a 50?

50A. I just did it last week with 10 AWG wire.

 

[Dennis Alwon]

According to the manufacturer you are wrong. They said if you use the max ocpd, the wire should be sized according to 240.

Well the manufacturer is wrong. I have been told all kinds of things from those guys in the past. Sizing the overcurrent protective device still satisfies 240 because the overload protection built in to the unit covers that.

 

[iwire]

According to the manufacturer you are wrong. They said if you use the max ocpd, the wire should be sized according to 240.

Yes we start in article 240 and we get to 240.4(G).

240.4(G) Overcurrent Protection for Specific Conductor Ap-plications. Overcurrent protection for the specific conduc-tors shall be permitted to be provided as referenced in Table 240.4(G).

 

[Pharon]

The overload units in the AC unit will protect the conductors from overload and the 30 amp breakers will protect the conductors from short circuits.

Here's the Code section to back up iwire's comment:

430.72(B)(2) Branch-Circuit Overcurrent Protective Device.
Conductors shall be permitted to be protected by the motor branch circuit short-circuit and ground-fault protective device and shall require only short-circuit and ground-fault protection.

 

[Yelram]

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

 

[roger]

Noone does it like you are saying, noone

Uhhhh,I know many people that do and I don't see a trail of burning houses either.

Roger

 

[Yelram]

Pharon, I dont know what youre reading, but thay does not say what you think it says

 

[Pharon]

Pharon, I dont know what youre reading, but thay does not say what you think it says

It does, actually. But I'm more curious to hear what you think it says.

 

[david luchini]

According the the manufacturer, any and all conductors must be matched to the ocpd,

What manufacturer is saying that? I've never come across one that did.

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

The thermal overloads aren't supposed to protect the wire in fault conditions, it is supposed to protect the wire in overload conditions. The short-circuit and ground-fault protection is supposed to protect the wire in fault conditions.

 

[david luchini]

It does, actually. But I'm more curious to hear what you think it says.

430.72(B)(2) isn't the correct reference. That is for motor control circuits.

 

[GoldDigger]

What manufacturer is saying that? I've never come across one that did.



The thermal overloads aren't supposed to protect the wire in fault conditions, it is supposed to protect the wire in overload conditions. The short-circuit and ground-fault protection is supposed to protect the wire in fault conditions.

And as long as there are no other loads on the circuit (a requirement for this configuration), the likelihood of s fault outside the appliance causing a current that is above MCA but below MOCP is vanishingly small.
If the appliance itself can cause an overload, the manufacturer should not publish the larger number for MOCP.
In different circumstances, where there is no MCA/MOCP label, the Code permits a motor breaker to be rated substantially above the motor FLA without requiring an increase in the wire size. Despite the mismatch in ratings, the conductor is effectively protected by the combination of breaker and motor overload device.