AC wire sizing

M

Melanconjp

Member
Location
Massachusetts
Occupation
Electrician
Can anyone help me when sizing AC wires that the breaker size is always larger than what the wire is rated for?

For example min circuit ampacity- 30 amps
Max fuse/breaker- 40 amps

Based off what I’ve learned you always size the wire for 30 amps, so #10 awg but want to put it on a 40 amps Based off breaker and not a 30a?
 
The "normal" overcurrent device for #10 would be limited to 30 amps based on 240.4(D) but 240.4(G) allows you to use the larger breaker to address the inrush on the AC compressor. The wire is actually protected from overload by the HVAC protective device.
 
augie47 said:
The "normal" overcurrent device for #10 would be limited to 30 amps based on 240.4(D) but 240.4(G) allows you to use the larger breaker to address the inrush on the AC compressor. The wire is actually protected from overload by the HVAC protective device.
Click to expand...
So the reasoning for the larger size breaker is just for the “inrush” on the compressor? And when you say hvac protective device do you mean like a time delay fuse in the disconnect?
 
Melanconjp said:
So the reasoning for the larger size breaker is just for the “inrush” on the compressor? And when you say hvac protective device do you mean like a time delay fuse in the disconnect?
Click to expand...
1. Basically, yes.
2. No, the compressor has (internal) overload protection.
 
Melanconjp said:
So the reasoning for the larger size breaker is just for the “inrush” on the compressor? And when you say hvac protective device do you mean like a time delay fuse in the disconnect?
Click to expand...
The OCPD is only providing short circuit and ground fault protection it is not protecting the conductors from an overload. Overload protection is part of the AC unit so the size of the OCPD does not have to match the conductor size. As stated the oversized OCPD allows some headroom for the starting current.
 
ok that all is making more sense , now on this cut sheet I have this is the info it gives me

Recommended fuse/breaker size - 25a
MCA- 29a
MOCP- 49a

Why would they recommend a 25 a fuse if I size the breaker from 49a shouldn’t I use a 50a breaker?
 
Melanconjp said:
ok that all is making more sense , now on this cut sheet I have this is the info it gives me

Recommended fuse/breaker size - 25a
MCA- 29a
MOCP- 49a

Why would they recommend a 25 a fuse if I size the breaker from 49a shouldn’t I use a 50a breaker?
Click to expand...
Something is wrong with those numbers. The MCA cannot be larger than the OCPD's sizes.
 
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