a/c overcurrent protection

[Chev]

Wouldn't the key here be 40 amp breaker with #10 wire? Seems to me Art. 310 would have some "skin in the game" on this one.

 

[Dennis Alwon]

David, I may be missing your point here but I read both articles and I don't agree with you. I, for sure, may be totally off but I see the Branch Cir. selection current as the MCA. I have never seen a a/c unit marked branch circuit selection current. I still think they are the same.

 

[david luchini]

Wouldn't the key here be 40 amp breaker with #10 wire? Seems to me Art. 310 would have some "skin in the game" on this one.

I don't see where Art. 310 says anything about OCPD sizes?!:-?

Am I missing something?

 

[Dennis Alwon]

Wouldn't the key here be 40 amp breaker with #10 wire? Seems to me Art. 310 would have some "skin in the game" on this one.

The #10 is protected by the internal overload protector. The 35 amp breaker or 40 amp. breaker would just be for ground fault or short circuit protection.. 35 amps in this case.

 

[raider1]

No, "branch circuit selection current" and "minimum circuit ampacity" are NOT the same thing.

Read the definition of "branch circuit selection current" in the Definitions section 440.2 "The value in amperes to be used instead of the rated-load current...WHEREVER the running overload protective device permits a sustained current greater than the specified percentage of the rated-load current."

Read, also, 440.4(C): "A hermetic refrigerant motor-compressor, or equipment containing such a compressor, having a protection system that is approved for use with the motor-compressor that it protects and that permits a continuous current in excess of the specified percentage of nameplate rated-load current...shall also be marked with a branch-circuit selection current...This marking shall be provided by the equipment manufacturer and shall be on the nameplate where the rated-load current appears."

The "branch-circuit selection current" is specific to equipment that has overload protection that is greater than the specified percentage of rated load current (per 440.52), and the bcsc must be marked on the equipment nameplate by the manufacturer.

This is clearly not the same thing as the minimum circuit ampacity. In fact, per 440.32, the MCA would be 125% of the bcsc if the bcsc is larger than the rated-load current.

The manufacturers are the ones that mark the "Minimum Circuit Ampacity" rating on the nameplate of the A/C equipment.

In the 20 years I have been an electrician I have never seen a manufacture mark an A/C unit with "Branch circuit selection current" they have always indicated it by using the term "Minimum Circuit Ampacity" on the nameplate.

The problem arises with the fact that the manufacturers don't use the same term as the NEC.

Chris

 

[david luchini]

David, I may be missing your point here but I read both articles and I don't agree with you. I, for sure, may be totally off but I see the Branch Cir. selection current as the MCA. I have never seen a a/c unit marked branch circuit selection current. I still think they are the same.

Dennis, if the minimum circuit ampacity required by 440.32 is required to be 125% of the branch circuit selection current, how could they be the same thing? MCA=BCSC=1.25xBSCS? That doesn't make sense.

I have never seen a unit marked with branch circuit selection current either, but that's because I've never dealt with a unit where the overload protection was greater than the specified percentage of rated-load current, as defined in 440.2. But that doesn't mean that MCA and BCSC are the same thing.

By definition, BCSC only exists WHEREVER the running overload protective device permits a sustained current greater than the specified percentage of the rated-load current. If the unit had on overload at 130% of rated-load current (less than the percentage specified in 440.52(A)(1)) then the unit does not have a BCSC.

 

[raider1]

Wouldn't the key here be 40 amp breaker with #10 wire? Seems to me Art. 310 would have some "skin in the game" on this one.

Nothing in Article 310 deals with overcurrent protection.

Article 240 deals with overcurrent protection of conductors and 240.4(G) specifically allows us to size the overcurrent protection in accordance with Article 440 for an A/C unit. Also the small conductor rules in 240.4(D) do not apply to A/C unit branch circuit conductors.

Chris

 

[Dennis Alwon]

Dennis, if the minimum circuit ampacity required by 440.32 is required to be 125% of the branch circuit selection current, how could they be the same thing? MCA=BCSC=1.25xBSCS? That doesn't make sense.

How could what be the same thing. The mca and the branch cir. selection, IMO are the same because they are both based on 125%. The MCA is required to be 125% of the rated load not the branch cir. selection.

 

[david luchini]

The manufacturers are the ones that mark the "Minimum Circuit Ampacity" rating on the nameplate of the A/C equipment.

In the 20 years I have been an electrician I have never seen a manufacture mark an A/C unit with "Branch circuit selection current" they have always indicated it by using the term "Minimum Circuit Ampacity" on the nameplate.

The problem arises with the fact that the manufacturers don't use the same term as the NEC.

Chris

Chris, yes the manufacturer mark the MCA on the unit, but they must also mark the BCSC IF (and only if) the BCSC exists for the unit

Look at 440.4(B), there you will find that the manufacturer is required to mark the minimum supply circuit conductor ampacity (MCA) and the maximum rating of the branch-circuit short-circuit and ground-fault protective device (MOCP.) 440.4(C) requires marking of the BCSC. This is not the same as MCA.

As I mentioned before, I've never seen a unit marked with BCSC either, but that's because I've never seen a unit that met the definition for BCSC, not because BCSC is the same as MCA.

 

[roger]

Coincidently, Mike has a very close example to the question at hand here

Excerpt from link.

[FONT=Arial, Helvetica, sans-serif]

Circuit Protection

[/FONT]

[FONT=Arial, Helvetica, sans-serif]The requirements for protecting the branch-circuit conductors, control apparatus, and circuits supplying hermetic refrigerant motor-compressors against short circuits and ground faults are in [440.22].[/FONT]

[FONT=Arial, Helvetica, sans-serif]The size and type of the short-circuit and ground-fault protection device for air-conditioning and refrigeration equipment are often marked on the equipment nameplate. The manufacturer calculates these ratings per [440.22] and [440.32] (Figure 440-5).[/FONT]

[FONT=Arial, Helvetica, sans-serif]If the equipment nameplate specifies "Maximum Fuse Size," use a one-time or dual-element fuse. If the nameplate specifies "HACR Circuit Breaker," use an HACR-rated circuit breaker [110.3(B)][/FONT]
[FONT=Arial, Helvetica, sans-serif].[/FONT]
[FONT=Arial, Helvetica, sans-serif]Short-circuit and ground-fault protection cannot exceed the nameplate ratings. If the equipment does not have a nameplate specifying the size and type of protection device, how do you size those devices? That depends on whether you are sizing for multiple motors or a single motor.[/FONT]

[FONT=Arial, Helvetica, sans-serif]Rating for Equipment. Where the equipment incorporates more than one hermetic refrigerant motor-compressor, or a hermetic refrigerant motor-compressor and other motors or other loads, size the equipment short-circuit and ground-fault protection with the "largest load" method. The rating of the branch-circuit short-circuit and ground-fault protective device cannot exceed the largest motor-compressor short-circuit ground-fault protection device plus the sum of the rated-load currents of the other compressors.[/FONT]

[FONT=Arial, Helvetica, sans-serif]Size the branch-circuit conductors at 125 percent of the larger motor-compressor current plus the sum of the rated-load currents of the other compressors [440.33].[/FONT]
[FONT=Arial, Helvetica, sans-serif]One Motor-Compressor. The short-circuit and ground-fault protection device for motor-compressor conductors must be capable of carrying the starting current of the motor. Also, the protection device cannot exceed 175 percent of the equipment load current rating.[/FONT]

[FONT=Arial, Helvetica, sans-serif]If the protection device sized at 175 percent is not capable of carrying the starting current of the motor-compressor, you can use the next larger protection device if it does not exceed 225 percent of the motor-compressor current rating.[/FONT]

[FONT=Arial, Helvetica, sans-serif]Test your knowledge with this question: [/FONT]
[FONT=Arial, Helvetica, sans-serif][/FONT]
[FONT=Arial, Helvetica, sans-serif]What size conductor and protection must you have for a 24A motor-compressor on a 240V circuit (Figure 440-6)?[/FONT]

1. [FONT=Arial, Helvetica, sans-serif]10 AWG, 40A[/FONT]
2. [FONT=Arial, Helvetica, sans-serif]10 AWG, 60A[/FONT]
3. [FONT=Arial, Helvetica, sans-serif]A or B[/FONT]
4. [FONT=Arial, Helvetica, sans-serif]none of these[/FONT]

[FONT=Arial, Helvetica, sans-serif]Answer: (a) 10 AWG, 40A protection.[/FONT]

[FONT=Arial, Helvetica, sans-serif]Here's how we arrive at that answer:[/FONT]

[FONT=Arial, Helvetica, sans-serif]Step 1: Size the branch-circuit conductor [Table 310.16 and 440.32].[/FONT]
[FONT=Arial, Helvetica, sans-serif]24A x 1.25 = 30A, 10 AWG, rated 30A at 60?C [110.14(C) and Table 310.16].[/FONT]

[FONT=Arial, Helvetica, sans-serif]Step 2: Size the branch-circuit protection [240.6(A) and 440.22(A)].[/FONT]
[FONT=Arial, Helvetica, sans-serif]24A x 1.75 = 42A, next size down protection = 40A.[/FONT]

[FONT=Arial, Helvetica, sans-serif]If a 40A protection device is not capable of carrying the starting current, you can size the protection device up to 225 percent of the equipment load current rating (24A x 2.25 = 54A, next size down 50A).

[/FONT]


Roger

 

[david luchini]

How could what be the same thing. The mca and the branch cir. selection, IMO are the same because they are both based on 125%. The MCA is required to be 125% of the rated load not the branch cir. selection.

No, you are not reading the section correctly. 440.32: Branch circuit conductors supplying a single motor-compressor shall have an ampacity not less than 125 percent of either the motor-compressor rated-load current or the branch-circuit selection current, whichever is greater.

 

[Dennis Alwon]

David , I see the problem. I read 440.32 differently.

440.32 Single Motor-Compressor.
Branch-circuit conductors supplying a single motor-compressor shall have an ampacity not less than 125 percent of either the motor-compressor rated-load current or the branch-circuit selection current, whichever is greater.

I read this as 125% of the rated load or------ the branch circuit selection current-- not 125% of the branch cir selection current.

 

[david luchini]

David , I see the problem. I read 440.32 differently.

I read this as 125% of the rated load or------ the branch circuit selection current-- not 125% of the branch cir selection current.

Dennis, that's interesting, I don't read it that way but you could be right.

But if I read your posts right, you believe that the BCSC is 125% of the rated-load current. In that case, 440.32 wouldn't make sense, as 125% of the rated-load current and the bcsc would be the same - "whichever is greater" no longer makes sense.

 

[LarryFine]

the minimum circuit ampacity is 24.4 and overcurrent rating is 35

This clearly tells me we can use #12 cu wire and a 35a breaker.

 

[wiredoc]

being a panel changeout,What's the insulation on your #10.60,75deg. check 310-16

 

[Dennis Alwon]

being a panel changeout,What's the insulation on your #10.60,75deg. check 310-16

What does that have to do with it. #10 is good for 30 amps minumum and the mca is 24 amps.