Determining disconnect size for ac unit
- Thread starter cppoly
- Start date
- Status
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
115% of the mca is what I have used for a/c since that usually has the 125% built in the MCA
What is considered the branch circuit selection current? And do you have an example of why they say "whichever is greater".
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
440.2 Definitions.
Branch-Circuit Selection Current. The value in amperes
to be used instead of the rated-load current in determining
the ratings of motor branch-circuit conductors, disconnecting
means, controllers, and branch-circuit short-circuit and
ground-fault protective devices wherever the running overload
protective device permits a sustained current greater
than the specified percentage of the rated-load current. The
value of branch-circuit selection current will always be
equal to or greater than the marked rated-load current.
I copied this quote from a similar post.
Thanks.
That helps a little but I still don't know what branch circuit selection current exactly is. Could you provide an example?
I just know that it is NOT the MCA or the RLA. So why is this a separate definition if all we really care about is the RLA and MCA?
That helps a little but I still don't know what branch circuit selection current exactly is. Could you provide an example?
I just know that it is NOT the MCA or the RLA. So why is this a separate definition if all we really care about is the RLA and MCA?
- Location
- Connecticut
- Occupation
- Engineer
BCSC would be marked on a unit that has a protection system that permits sustained current of greater than 156% of the RLA.
If you had a unit with a BCSC marked on it, you would care about that rather than RLA. I've never seen a unit with a BCSC value.
Thanks!!
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Wouldn't the branch circuit selection be the overcurrent protective device since that is how the branch circuit is rated
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
My brain is fuzzy, I am not sure exactly what you are saying.
- Location
- Connecticut
- Occupation
- Engineer
The overcurrent protective device should not exceed 175% of the branch circuit selection current, or not exceed 225% of the bcsc where the smaller protective device is not sufficient for the starting current of the motor...440.22(A)
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Yes I know that. I was saying isn't that the branch circuit selection and the disconnect needed to be 115% of the overcurrent protective device?
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I know this is not the case but it seems to read that way.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
No, the disco only needs to be rated for 115% of the BCSC. The OCPD is not a factor.
For the nameplate above I could use a 30A disco with a 40A breaker.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I don't think you can use a 30 amp disconnect with a 40 amp breaker. Wouldn't the disconnect have to be rated for the 40 amps.
- Location
- Connecticut
- Occupation
- Engineer
I don't see any problem with a 30A disconnect and a 40A breaker.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
If the equipment is rated 30 amp I didn't think a 40 amp breaker was allowed. I realize the load isn't 40 amps. I think about a 30 amp fused disconnect-- you cannot use 40 amp fuses so I assumed the equipment could not have a breaker rated higher than the ampacity of the unit
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