Minimum Ampacity Question - Electrician Talk - Professional Electrical Contractors Forum

NolaTigaBait · 04-15-2009, 06:26 PM

i have 2 condensers that i am moving to a new location(new homeruns)...unit #1 mca is 20.4 and unit #2's mca is 25.9... max fuse size is 35 for #1 and 45 for unit #2
My question is: can I run one #6-2(nmb) to a 4 circuit outdoor panel and fuse it at 70 amps????
Also, i can use a 40 amp breaker and a 50 amp breaker for each unit correct, b/c they are the next common size up

electricista · 04-15-2009, 07:27 PM

Quote:

Originally Posted by NolaTigaBait

i have 2 condensers that i am moving to a new location(new homeruns)...unit #1 mca is 20.4 and unit #2's mca is 25.9... max fuse size is 35 for #1 and 45 for unit #2

My question is: can I run one #6-2(nmb) to a 4 circuit outdoor panel and fuse it at 70 amps????

6/2 nm is only good for 55 amps and if your load is less than 55 amps you can protect it at 60 amps. This should work with your units.

Quote:

Also, i can use a 40 amp breaker and a 50 amp breaker for each unit correct, b/c they are the next common size up

No you cannot. Max fuse is 35 and 45 and then are standard size breakers art. 240.6. You must not go above the max size

480sparky · 04-15-2009, 07:29 PM

Do they state "Max. Fuse Size" or "Max. Breaker Size"? If it states Max Fuse Size, then you must use fuses to protect it.

robnj772 · 04-15-2009, 07:39 PM

Why not just run a 10-2 and and 8-2 out there,put a 30 and a 40 amp breaker in the panel inside and use two A/c disconnects?

Magnettica · 04-15-2009, 07:40 PM

6-2 NM-B is rated at 55 amps using the 60º column (310.16). So the overcurrent protection for the feeder would need to be 60 amps (240.4 and 240.6).

The name plate rating of the condensing units must be used, 440.6.

I think you'll be ok like that but I am looking at 215.2 (B)(2) and not sure if the 125% rule applies here or not.

electricista · 04-15-2009, 07:42 PM

Quote:

Originally Posted by robnj772

Why not just run a 10-2 and and 8-2 out there,put a 30 and a 40 amp breaker in the panel inside and use two A/c disconnects?

Why #10. You could run a 15 amp wire for the first unit 20.4 and a #10 to the second unit

electricista · 04-15-2009, 07:45 PM

Quote:

Originally Posted by Magnettica

I think you'll be ok like that but I am looking at 215.2 (B)(2) and not sure if the 125% rule applies here or not.

I think not simply because the transformer in the unit is part of the utilization equipment. I do not think that is the intent of this art. but I have been wrong many times before

Magnettica · 04-15-2009, 07:53 PM

Quote:

Originally Posted by electricista

I think not simply because the transformer in the unit is part of the utilization equipment. I do not think that is the intent of this art. but I have been wrong many times before

And that section is for OVER 600 volts, not less then.

electricista · 04-15-2009, 07:55 PM

Quote:

Originally Posted by Magnettica

And that section is for OVER 600 volts, not less then.

Yeah I missed that too.

NolaTigaBait · 04-15-2009, 07:58 PM

Quote:

Originally Posted by 480sparky

Do they state "Max. Fuse Size" or "Max. Breaker Size"? If it states Max Fuse Size, then you must use fuses to protect it.

i dont remember, but i thought that new breakers were made to withstand starting current, so i wouldn't need a fused disconnect

electricista · 04-15-2009, 08:00 PM

Quote:

Originally Posted by NolaTigaBait

i dont remember, but i thought that new breakers were made to withstand starting current, so i wouldn't need a fused disconnect

If the unit says max fuse you must use fuses however I have not seen an hvac unit with "fuses only" in many many years. All the new ones usually have both max fuse or circuit breaker written on it

NolaTigaBait · 04-15-2009, 08:02 PM

Quote:

Originally Posted by robnj772

Why not just run a 10-2 and and 8-2 out there,put a 30 and a 40 amp breaker in the panel inside and use two A/c disconnects?

well, my first thought was to run a #14 b/c 20.4 amps(i guess i can drop the decimal) and a #10 for the other and use a 60 amp fused disconnect and fuse them at the max fuse sizes...it was just a thought, i dodnt know if you could add teh the two units mca and the max fuse breaker and use one feeder

480sparky · 04-15-2009, 08:03 PM

Quote:

Originally Posted by electricista

If the unit says max fuse you must use fuses however I have not seen an hvac unit with "fuses only" in many many years. All the new ones usually have both max fuse or circuit breaker written on it

Some times, EEs will specify units with fuse-only protection. They also will design an oversized circuit to the unit for voltage drop. Protect the circuit with a larger breaker, and the unit with fuses.

Magnettica · 04-15-2009, 08:04 PM

Quote:

Originally Posted by NolaTigaBait

well, my first thought was to run a #14 b/c 20.4 amps(i guess i can drop the decimal) and a #10 for the other and use a 60 amp fused disconnect and fuse them at the max fuse sizes...it was just a thought, i dodnt know if you could add teh the two units mca and the max fuse breaker and use one feeder

You'll need to use a circuit breaker to protect the feeder conductors supplying the panel and circuit breakers to protect the conductors supplying the condensing units.

NolaTigaBait · 04-15-2009, 08:04 PM

Quote:

Originally Posted by electricista

If the unit says max fuse you must use fuses however I have not seen an hvac unit with "fuses only" in many many years. All the new ones usually have both max fuse or circuit breaker written on it

they are pretty new(5 years) if were to guess

NolaTigaBait · 04-15-2009, 08:05 PM

Quote:

Originally Posted by Magnettica

You'll need to use a circuit breaker to protect the feeder conductors supplying the panel and circuit breakers to protect the conductors supplying the condensing units.

yes, i know this... my question wasn't whether i should use breakers or fuses, but if i can add the ampacities and run one feeder

electricista · 04-15-2009, 08:05 PM

Quote:

Originally Posted by 480sparky

Some times, EEs will specify units with fuse-only protection. They also will design an oversized circuit to the unit for voltage drop. Protect the circuit with a larger breaker, and the unit with fuses.

Well if the EE specs it that is one thing but it is not the same as being on the nameplate. I am curious- have you seen any recently that are marked fuse only. In resi work I have not seen them.

electricista · 04-15-2009, 08:07 PM

Quote:

Originally Posted by NolaTigaBait

yes, i know this... my question wasn't whether i should use breakers or fuses, but if i can add the ampacities and run one feeder

Yes you can run a feeder to a panel and then feed the units with the proper OCP

NolaTigaBait · 04-15-2009, 08:10 PM

Quote:

Originally Posted by electricista

Yes you can run a feeder to a panel and then feed the units with the proper OCP

and thats my question....ocp would have to be 60????...i know its a number 6 and its good for 55 amps, but, where would the max fuse size come into play here??

Magnettica · 04-15-2009, 08:10 PM

Quote:

Originally Posted by electricista

Well if the EE specs it that is one thing but it is not the same as being on the nameplate. I am curious- have you seen any recently that are marked fuse only. In resi work I have not seen them.

Been awhile since I've seen new cu's that required fuses only.

On occasion, a smaller unit (ampacity wise) replaces an old one and instead of changing the circuit breaker from say 40 amps to 25 amps, I'll just install 25 amp fuses. This is specifically a good reason to always have fused disconnects on the truck. You know, sometimes it's just easier to replace the disconnect then to go messing around inside an old, and outdated FPE panel and have a callback later that night. Been there, done that.