sizing a disconnect ampacity

[erfitz1208]

what is the correct way to size the ampacity of conductors and disconnect for a residential a/c condensing unit??
for ex. nameplate says MCA of 26.4 ampere, MOP 45.0 amperes

is a 30 ampere #10awg circuit correct?? what about the disconnect??

thanx

 

[iwire]

MCA of 26.4 ampere.

Have to use 10 AWG here.

MOP 45.0 amperes

I would use a 45 amp or 40 amp breaker if the 45 was going to take to long to get.

The disconnect needs to be at least 115% of the nameplate.

26.4 * 1.15 = 30.36 Maybe you could round down but I would go with a 60 amp switch.

 

[erfitz1208]

thanks

thanks

30 amp breaker would be cheapest. would that be acceptable?

u mentioned a 40 amp breaker. is that because of the LRA being over 70 a on startup would cause nuisance tripping??
thanks again

 

[Dennis Alwon]

30 amp breaker would be cheapest. would that be acceptable?

u mentioned a 40 amp breaker. is that because of the LRA being over 70 a on startup would cause nuisance tripping??
thanks again

In time the 30 may be an issue. I always use the max allowed. Remember the a/c unit is protected by overload protection so putting a 45 amp breaker on a #10 wire is perfectly safe.

 

[erfitz1208]

ok

ok

thanks for the replys.

 

[infinity]

Have to use 10 AWG here.





I would use a 45 amp or 40 amp breaker if the 45 was going to take to long to get.

The disconnect needs to be at least 115% of the nameplate.

26.4 * 1.15 = 30.36 Maybe you could round down but I would go with a 60 amp switch.

Bob,

Do we size the disconnect based on 115% of the MCA? The MCA already contains an additional 25% added to the largest load.

 

[iwire]

Bob,

Do we size the disconnect based on 115% of the MCA? The MCA already contains an additional 25% added to the largest load.

Honestly I am not sure, I based my answer on 440.12(B) but I am finding that section a tough read. I am finding 440.12(B)(1) and (2) to be conflicting

 

[infinity]

Honestly I am not sure, I based my answer on 440.12(B) but I am finding that section a tough read. I am finding 440.12(B)(1) and (2) to be conflicting

I'm not sure either but was thinking in terms of a motor where the disconnect would be sized at 115% of the FLA and the conductors sized at 125% of the FLA. If an AC unit already has 125% built into the MCA I'm wondering if we need to add anything additional for the disconnect sizing?

 

[erfitz1208]

i found the anwser to that question in my schl book.

states- nameplate MCA=(RLC x 1.25) +other loads... fan motor, etc.

RLC= rated-load current

 

[nakulak]

i found the anwser to that question in my schl book.

states- nameplate MCA=(RLC x 1.25) +other loads... fan motor, etc.

RLC= rated-load current

where does your school book say that is written in the code book ?

 

[erfitz1208]

it refers to NEC 440.2, and UL Standard 1995,
with ex. a/c nameplate from Shiver Manufacturing Co.

i