# Feeder and OCP for 2 a/c units



## NolaTigaBait (Oct 19, 2008)

I have (2) a/c condensers that I need to run a feeder for...the minimum circuit ampacity is 25 and 37...From what I understand is that the MCA is the sum of the compresser plus the fan times 1.25...So, in order to find the feeder size I just add 26 and 37 , which gives me 62 amps MCA...I'm thinking that I can run #6 thhn to the trough. I guess Im more wondering about overcurrent protection...The largest branch circuit overcurrent device rating is 60 amps on the big unit....so do I add the 60 plus the mca of the smaller unit(25 amps)?....That would give me 85 amps....then next size down to 80 amp OCP????I know this is long winded bear with me:laughing:


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## jwjrw (Jan 14, 2010)

NolaTigaBait said:


> I have (2) a/c condensers that I need to run a feeder for...the minimum circuit ampacity is 25 and 37...From what I understand is that the MCA is the sum of the compresser plus the fan times 1.25...So, in order to find the feeder size I just add 26 and 37 , which gives me 62 amps MCA...I'm thinking that I can run #6 thhn to the trough. I guess Im more wondering about overcurrent protection...The largest branch circuit overcurrent device rating is 60 amps on the big unit....so do I add the 60 plus the mca of the smaller unit(25 amps)?....That would give me 85 amps....then next size down to 80 amp OCP????I know this is long winded bear with me:laughing:


Im not sure if I understand what you have but could you not use fuseable disconnects and protect each unit at the max fuse?


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## B4T (Feb 10, 2009)

I would run (1) 10/2...(30a) and (1) 8/2.. (40a) for the compressors instead of trying to combine them into (1) feeder


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## Innovative (Jan 26, 2010)

Hmmm, not real sure on this one, but will give it a try. 

I would use #4 thhn feeders with an 80 amp breaker. I believe you have to use the entire load.... as it says "you must use wire suitable to carry the entire load without overheating"
What if both units were to kick on at the same time????


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## NolaTigaBait (Oct 19, 2008)

jwjrw said:


> Im not sure if I understand what you have but could you not use fuseable disconnects and protect each unit at the max fuse?


Yeah, thats what I am doing....I'm talking about the feeder to the trough...I know what size feeder I need(#6)...I'm just sure of OCP ...I think I can go up to 80 amps...

B4t...all i need is #12 to the 25mca one and #8 to the 37 mca unit...I think it is easier just to run a feeder then tap off the feeder to the discos...


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## NolaTigaBait (Oct 19, 2008)

Innovative said:


> Hmmm, not real sure on this one, but will give it a try.
> 
> I would use #4 thhn feeders with an 80 amp breaker. I believe you have to use the entire load.... as it says "you must use wire suitable to carry the entire load without overheating"
> *What if both units were to kick on at the same time??*??


Thats what I'm trying to figure out...I think I can put the #6 on an 80 amp breaker for the feeder...Where's the Badger?


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## jwjrw (Jan 14, 2010)

NolaTigaBait said:


> Thats what I'm trying to figure out...I think I can put the #6 on an 80 amp breaker for the feeder...Where's the Badger?


70amp max on #6. Good for 65amps next size up rule to 70amp breaker


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## raider1 (Jan 22, 2007)

NolaTigaBait said:


> Thats what I'm trying to figure out...I think I can put the #6 on an 80 amp breaker for the feeder...Where's the Badger?


I'm not the Badger, but I would say that in accordance with 430.62(A) you would be OK with using an 80 amp breaker on this feeder.

Chris


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## NolaTigaBait (Oct 19, 2008)

raider1 said:


> I'm not the Badger, but I would say that in accordance with 430.62(A) you would be OK with using an 80 amp breaker on this feeder.
> 
> Chris


Thanks Chris...I know it's hard to understand, but did I use the right formula to arive at that OCP size? Oh and theres only 1 Badger:thumbsup:


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## raider1 (Jan 22, 2007)

jwjrw said:


> 70amp max on #6 I believe


When using the next size up rule in 240.4(B) I would agree, but when using the motor feeder rules you could use an 80 amp breaker.

Take a look at 430.62 for motor feeder short circuit and ground fault protection sizing.

Chris


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## jwjrw (Jan 14, 2010)

raider1 said:


> I'm not the Badger, but I would say that in accordance with 430.62(A) you would be OK with using an 80 amp breaker on this feeder.
> 
> Chris


Hmmm I'm not sure I understand 430.62(a) any help?


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## B4T (Feb 10, 2009)

What is the length of run from panel to compressors?


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## raider1 (Jan 22, 2007)

NolaTigaBait said:


> Thanks Chris...I know it's hard to understand, but did I use the right formula to arive at that OCP size? Oh and theres only 1 Badger:thumbsup:


I agree that there is only 1 Badger.

Yes you seemed to use the right formula for the feeder OCPD.

You would use the largest OCPD rating for the A/C units plus the sum of the full load current of the other A/C unit and the feeder OCPD can not exceed that value.

Chris


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## NolaTigaBait (Oct 19, 2008)

Black4Truck said:


> What is the length of run from panel to compressors?


Uh, maybe 65 feet or so....


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## raider1 (Jan 22, 2007)

jwjrw said:


> Hmmm I'm not sure I understand 430.62(a) any help?


Basically it says that the feeder OCPD can't exceed the value of the largest A/C units max breaker size plus the minimum circuit ampacity rating of the second A/C unit.

So for example with this situation the largest A/C unit max breaker size was 60 and the other A/C units minimum circuit ampacity is 25 amps the combine rating is 85 amps, so an 80 amp breaker would be acceptable.

Chris


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## jwjrw (Jan 14, 2010)

raider1 said:


> I agree that there is only 1 Badger.
> 
> Yes you seemed to use the right formula for the feeder OCPD.
> 
> ...


So how is it that #6 could be used on an 80amp breaker? I assume you use the 90deg column (75a) and next size up breaker? If not Im really lost. Motors are not my strong point YET!


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## jwjrw (Jan 14, 2010)

raider1 said:


> Basically it says that the feeder OCPD can't exceed the value of the largest A/C units max breaker size plus the minimum circuit ampacity rating of the second A/C unit.
> 
> So for example with this situation the largest A/C unit max breaker size was 60 and the other A/C units minimum circuit ampacity is 25 amps the combine rating is 85 amps, so an 80 amp breaker would be acceptable.
> 
> Chris


 
OK I now understand that part. But the #6 at 80amps?


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## NolaTigaBait (Oct 19, 2008)

jwjrw said:


> OK I now understand that part. But the #6 at 80amps?


The breaker is for ground faults, the wire is protected internally(by the a/c unit), therefore, you can use larger size breakers than normal for compresser circuits...


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## raider1 (Jan 22, 2007)

jwjrw said:


> So how is it that #6 could be used on an 80amp breaker? I assume you use the 90deg column (75a) and next size up breaker? If not Im really lost. Motors are not my strong point YET!


Nope, not using the 90 degree column.

You have to remember that with motors the short circuit and ground fault protection device can be much larger than a normal circuit because the overload protection is a separate device.

An A/C compressor will have an overload device at the A/C unit itself that will protect the conductors and A/C unit from overload. The breaker is just providing protection from a short circuit and a ground fault so the rules in Article 430 permit this breaker to be sized larger.

Chris


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## NolaTigaBait (Oct 19, 2008)

raider1 said:


> Nope, not using the 90 degree column.
> 
> You have to remember that with motors the short circuit and ground fault protection device can be much larger than a normal circuit because the overload protection is a separate device.
> 
> ...


You explain things much better:laughing:


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## raider1 (Jan 22, 2007)

NolaTigaBait said:


> You explain things much better:laughing:


That comes from being an instructor.

I though you did a good job describing it yourself.

Chris


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## jwjrw (Jan 14, 2010)

raider1 said:


> Nope, not using the 90 degree column.
> 
> You have to remember that with motors the short circuit and ground fault protection device can be much larger than a normal circuit because the overload protection is a separate device.
> 
> ...


Thank you Chris. You are a big help. I'm still learning the finer points of motors!


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## raider1 (Jan 22, 2007)

jwjrw said:


> Thank you Chris. You are a big help. I'm still learning the finer points of motors!


Your very welcome.

Article 430 and motors in general can be difficult to understand.

Chris


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## DaveZeki (Dec 10, 2009)

raider1 said:


> Nope, not using the 90 degree column.
> 
> You have to remember that with motors the short circuit and ground fault protection device can be much larger than a normal circuit because the overload protection is a separate device.
> 
> ...



Smart guy!!!

Remember guys.....a motor circuit is an even better protected circuit than a reg. branch circuit serving outlets or light (for example)


In a motor circuit you hace TWO forms of protection:

1.) Overcurrent (Circuit Breaker)
2.) Overload (I.E. Heaters)


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## Magnettica (Jan 23, 2007)

The OCPD for the *feeder* is determined by the largest A/C OCPD plus the full-load current of the other A/C condensing unit. 

The feeder conductor size is determined by the sum of both of the A/C full-load currents plus 125% of the largest condenser of the two.


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## Magnettica (Jan 23, 2007)

For the record... I believe you need #4 copper (FLA = 71.25)

And an 80 circuit breaker for the feeder. (largest A/C OCPD is 60 amps + 25 = 85) 

Branches up, feeders down.


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## NolaTigaBait (Oct 19, 2008)

Magnettica said:


> For the record... I believe you need #4 copper (FLA = 71.25)
> 
> And an 80 circuit breaker for the feeder. (largest A/C OCPD is 60 amps + 25 = 85)
> 
> Branches up, feeders down.


I disagree, because the MCA of the condensers already has 125% factored into it...on both units...so i think its actually less than 62 amps...


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## Magnettica (Jan 23, 2007)

NolaTigaBait said:


> I disagree, because the MCA of the condensers already has 125% factored into it...on both units...so i think its actually less than 62 amps...


The c/u should have an FLA rating on it but 440.6 might negate that.


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## NolaTigaBait (Oct 19, 2008)

Magnettica said:


> The c/u should have an FLA rating on it but 440.6 might negate that.


I'm going to have to look at the nameplate again. I use the MCA b/c it usually hasa bigger cushion built into it.


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## raider1 (Jan 22, 2007)

NolaTigaBait said:


> I disagree, because the MCA of the condensers already has 125% factored into it...on both units...so i think its actually less than 62 amps...


I agree with this.



Magnettica said:


> The c/u should have an FLA rating on it but 440.6 might negate that.


The exception #1 to 440.6(A) allows the use of the branch circuit selection current rating marked on the nameplate in lieu of the FLA for sizing the branch circuit conductors.

Also 440.7 for the highest rated motor for application of 430.62(A) has an exception that allows the use of the branch circuit selection current marked on the nameplate to be used to determine the higest rated motor instead of the FLA.

Chris


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## Magnettica (Jan 23, 2007)

raider1 said:


> I agree with this.
> 
> 
> 
> ...


Showoff! 

You're right, Mike... as you were sir. :whistling2:


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