AC wiring yet again,

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ritelec

Senior Member
Location
Jersey
Still having a hard time getting this in my thick skull.

There is an ac unit.
It requires 24 amps and an hvac breaker @ 40 amps.

The wire will be #10 on a 40 amp brk and the disconnect at the unit would have to be rated for the 40 amps (60 amp non fused) correct?


There is an ac unit.
It requires 24 amps and a fused ocp at 40 amps.

The wire will be #10 on a 40,50,100 amp breaker?? Feeding a 60 amp disc fused at 40 amps.
What would be the limit or max ocp of the breaker have to be supplying the circuit?

thank you
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Still having a hard time getting this in my thick skull.

There is an ac unit.
It requires 24 amps and an hvac breaker @ 40 amps.

The wire will be #10 on a 40 amp brk and the disconnect at the unit would have to be rated for the 40 amps (60 amp non fused) correct?


There is an ac unit.
It requires 24 amps and a fused ocp at 40 amps.

The wire will be #10 on a 40,50,100 amp breaker?? Feeding a 60 amp disc fused at 40 amps.
What would be the limit or max ocp of the breaker have to be supplying the circuit?

thank you

Exactly what is said on the unit is critical. Max OCP means fuse or CB. Max Fuse means just that. 2011 NEC 440.22(A)...not exceeding 175%...not to exceed 225% of the motor rated-load current of branch-circuit selection current, whichever is greater.

So MCA by label is 24. Max OCP is fuse of 40A.

175% of 24 = 42. I would use a 40 amp CB but under certain conditions you could use up to 225% of 24 = 54 or a 50 amp CB. But you must have 40 amp fuses at the unit disconnect.


MCA by label is 24. Max OCP is 40A. 40A CB would be used with a NF disconnect at the Unit.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
this is interesting as it relates to the fuse issue i have with hvac air handler. 20A OCD --> #10 romex --> 15A edison fuse --> 14awg plug/cord to air handler (two motors). there's 30A inrush with 14.8A running load.

is this configuration compliant to NEC specs? 20A OCD seems right, #10 romex is ok, type TL 15A fuse is questionable, but #14awg seems wrong, should be #12(min). do i have that right?
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
this is interesting as it relates to the fuse issue i have with hvac air handler. 20A OCD --> #10 romex --> 15A edison fuse --> 14awg plug/cord to air handler (two motors). there's 30A inrush with 14.8A running load.

is this configuration compliant to NEC specs? 20A OCD seems right, #10 romex is ok, type TL 15A fuse is questionable, but #14awg seems wrong, should be #12(min). do i have that right?
You cannot expect a 15 amp fuse to carry a 14.8 amp load for very long. What is the MCA?

Plug & cord to AHU is not correct, unless details are missing.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
You cannot expect a 15 amp fuse to carry a 14.8 amp load for very long. What is the MCA?

Plug & cord to AHU is not correct, unless details are missing.

i dont want to divert this thread from OP's Q. after the OP's Q is finished i will come back to this. was post #3 the answer for OP?

MCA on plate says "14A", MOCP on plate says "20A". the unit is 0.8A(running) over the the MCA. main motor ~1.5hp, small blower motor for gas heating unit says 2.3A on the motor itself, the main blower motor is buried in the unit so i cant see it. typical 1.5HP (12A) + 2.3A leaves me 0.5A over the MCA. main motor could be dragging some due to ~8yr old bearings. the 15A TL edison fuse blows about once every 6mo.
 
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david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
175% of 24 = 42. I would use a 40 amp CB but under certain conditions you could use up to 225% of 24 = 54 or a 50 amp CB.

This is not correct. The 175% or 225% would be for an Individual Motor-Compressor, and would be based upon the Rated-Load current, not the Minimum Circuit Ampacity.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
The wire will be #10 on a 40,50,100 amp breaker?? Feeding a 60 amp disc fused at 40 amps.
What would be the limit or max ocp of the breaker have to be supplying the circuit?

Per 430.62(A), the feeder OCPD would have to be 40A with the #10 wire.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Yes, major hitch being Individual Motor-Compressor. "...rated-Load current or branch-circuit selection current, whichever is greater." Isn't the MCA=BCSC?

No, the MCA does NOT equal the BCSC.

For a single motor-compressor, MCA will either be 125% of the rated-load current OR 125% of the BCSC, whichever is greater...See 440.32.
 

ritelec

Senior Member
Location
Jersey
Per 430.62(A), the feeder OCPD would have to be 40A with the #10 wire.

Feeder 10's on the 40 ?

I posted this question elsewhere and and as other posts mentioned from the breaker to the fused disconnect would be a feeder so off the 40 amp breaker would need to be 8's to the 60 amp fused disconnect with 40 amp fuses. From the fused disconnect to the unit would be 10's. So I thought I got it but maybe I didn't. ?? :- )
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Feeder 10's on the 40 ?

I posted this question elsewhere and and as other posts mentioned from the breaker to the fused disconnect would be a feeder so off the 40 amp breaker would need to be 8's to the 60 amp fused disconnect with 40 amp fuses. From the fused disconnect to the unit would be 10's. So I thought I got it but maybe I didn't. ?? :- )

The feeder conductors would only need to be #10s in order to carry the load. And with #10 feeder conductors, 430.62(A) would allow a max feeder c/b of 40A.

You can always size the feeder conductors larger than the min required, though.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
the 15A TL edison fuse blows about once every 6mo.
i spoke too soon. the fuse blew last night, about 2wks after replacing it. today is the day i make some changes to the setup.
 

kwired

Electron manager
Location
NE Nebraska
Feeder 10's on the 40 ?

I posted this question elsewhere and and as other posts mentioned from the breaker to the fused disconnect would be a feeder so off the 40 amp breaker would need to be 8's to the 60 amp fused disconnect with 40 amp fuses. From the fused disconnect to the unit would be 10's. So I thought I got it but maybe I didn't. ?? :- )
I think I see what your confusion is - art 440 has no section on feeder conductors. I'm pretty certain you are permitted to follow same rules as you follow for feeders in 430 part V, but have not yet found a section that sends you there either.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I think I see what your confusion is - art 440 has no section on feeder conductors. I'm pretty certain you are permitted to follow same rules as you follow for feeders in 430 part V, but have not yet found a section that sends you there either.

See 440.3(A).
 
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