sizing OCP, wiring, disc for CU - verify?

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bcm

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Atlanta, Georgia
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Engineer
I haven't done this in a long time, so I'm rusty and want to make sure my thinking on this is correct. I have a unit with the following specs:
Voltage: min 187 max 253
Compressor: RLA 14.3 LRA 58.3
Outdoor Fan Motor: FLA 0.8 NEC HP 0.125 KW out 0.09
Min ckt amps 18.7
Fuse/HACR Brkr Amps 30

So, min circuit ampacity is RLA x 125% + other loads. That = (14.3 x 1.25) + 0.8 = 38.3 amps. (I assume that the motor FLA is the "other loads" I needed to add for this calculation.)
Given the calculated 38.3 amps, I need wiring sized at #8 awg. Correct?
Now, unless I'm mistaken, the disconnecting means (switch that is next to the unit outside), is required to be rated at 115% of the RLA or branch circuit selection current. So, 14.3 X 1.15 = 16.45 amps. Branch circuit selection is a 50A circuit (that #8 I calculated earlier), so I need to provide a 50A disconnect by the unit since the branch circuit selection is greater. Is that correct?

Thanks so much, folks!
 
mcclary's electrical

mcclary's electrical

Senior Member
Location
VA
bcm said:
I haven't done this in a long time, so I'm rusty and want to make sure my thinking on this is correct. I have a unit with the following specs:
Voltage: min 187 max 253
Compressor: RLA 14.3 LRA 58.3
Outdoor Fan Motor: FLA 0.8 NEC HP 0.125 KW out 0.09
Min ckt amps 18.7
Fuse/HACR Brkr Amps 30

So, min circuit ampacity is RLA x 125% + other loads. That = (14.3 x 1.25) + 0.8 = 38.3 amps. (I assume that the motor FLA is the "other loads" I needed to add for this calculation.)
Given the calculated 38.3 amps, I need wiring sized at #8 awg. Correct?
Now, unless I'm mistaken, the disconnecting means (switch that is next to the unit outside), is required to be rated at 115% of the RLA or branch circuit selection current. So, 14.3 X 1.15 = 16.45 amps. Branch circuit selection is a 50A circuit (that #8 I calculated earlier), so I need to provide a 50A disconnect by the unit since the branch circuit selection is greater. Is that correct?

Thanks so much, folks!
Click to expand...

If you typed everthing off the label correctly, you need #12 on a thirty amp ocpd.
YOu are combining the MCA with the compressor amps. You don't need to do that MCA has already firured in the125%,,,,,,all you need is #12 on a 30
 
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bcm

Member
Location
Atlanta, Georgia
Occupation
Engineer
sizing OCP, wiring, disc for CU - verify?

So if I am using the OCP device to determine the wire size (30A --> #12 awg), then how do I determine the disconnect rating for the switch I need to have next to the unit outside? I'm missing something in the logic of how we come up with the correct figures.

Thanks!
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
bcm said:
Min ckt amps 18.7
Fuse/HACR Brkr Amps 30
Click to expand...
These are the only numbers you need to look at. A #12 satisfies the first figure, and a 30a breaker (or fuses) satisfies the second.

I also believe a 30a disconnect is adequate (although they're almost all 60a these days), and it need not be a fused disconnect.
 
mcclary's electrical

mcclary's electrical

Senior Member
Location
VA
bcm said:
So if I am using the OCP device to determine the wire size (30A --> #12 awg), then how do I determine the disconnect rating for the switch I need to have next to the unit outside? I'm missing something in the logic of how we come up with the correct figures.

Thanks!
Click to expand...

I may be misunderstanding you,,,,,but you don't need a disconnect AND switch. ONLY a disconnect. And as Larry stated, they mostly come 60's anyway. If you had a 30 amp DISC, you could use it, but for under $10 you can get a 60.
 
augie47

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
bcm said:
I haven't done this in a long time, so I'm rusty and want to make sure my thinking on this is correct. I have a unit with the following specs:
Voltage: min 187 max 253
Compressor: RLA 14.3 LRA 58.3
Outdoor Fan Motor: FLA 0.8 NEC HP 0.125 KW out 0.09
Min ckt amps 18.7
Fuse/HACR Brkr Amps 30

So, min circuit ampacity is RLA x 125% + other loads. That = (14.3 x 1.25) + 0.8 = 38.3 amps. (!
Click to expand...
I think the basic error goes back to this. RLA of 14.3 x 1.25 + .08 = 18.7, the same as the nameplate shows. The factory has simply done the work for you :), but as other say, all you need know is the MCA (18.7) and the MaxOCP (30) to size your wiring/disconnect/overcurrent protection.
 
Y

yired29

Senior Member
benaround said:
The load is only 18.7a and #12 wire is used for the circuit conductors ?
Click to expand...
The branch circuit is rated based on OCPD. With a 35 amp OCPD there is a potential of 35 amps available so a 30 amp disconnect would not be large enough. The 30 amp disconnect is not listed for over 30 amps.
 
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benaround

Senior Member
Location
Arizona
yired29 said:
The branch circuit is rated based on OCPD. With a 35 amp OCPD there is a potential of 35 amps available so a 30 amp disconnect would not be large enough. The 30 amp disconnect is not listed for over 30 amps.
Click to expand...

The only thing on the circuit is the a/c compressor @ 18.7a, the disconnect is rated for

30a, the OCPD @ 35a is onlt to allow the compressor motor to start.
 
Y

yired29

Senior Member
I agree 440.31 refers us to 440.6 (A) Exception #1. 110.14 (C) 1 a - 60 degree term. #14 = 20 amps at 60 degree 310.16. # 14 would be good for a 18.7 amp load on a 30 amp OCPD. Provided proper overload protection is provided. The 30 amp OCPD is there for ground fault and short circuit protection not overload.
 
Y

yired29

Senior Member
benaround said:
The only thing on the circuit is the a/c compressor @ 18.7a, the disconnect is rated for

30a, the OCPD @ 35a is onlt to allow the compressor motor to start.
Click to expand...
If there was a ground fault or short circuit after the 30 amp disconnect it would have to handle over 35 amps and was not designed or tested to handle over 30 amps.
 
mcclary's electrical

mcclary's electrical

Senior Member
Location
VA
benaround said:
I'm not implying it, I would definitly do that on a circuit with a MCA-13.5a ,MOP-30a fuse

or HACR c.b. You would not ?
Click to expand...

I don't think I have ever see a MCA rating of 13.5 amps and an OCPD of 30.


In your example #14 would suffice for the MCA,,,,,,,but could not be put on a 30 amp breaker, therefore, #12 would be needed.
 
mcclary's electrical

mcclary's electrical

Senior Member
Location
VA
yired29 said:
I agree 440.31 refers us to 440.6 (A) Exception #1. 110.14 (C) 1 a - 60 degree term. #14 = 20 amps at 60 degree 310.16. # 14 would be good for a 18.7 amp load on a 30 amp OCPD. Provided proper overload protection is provided. The 30 amp OCPD is there for ground fault and short circuit protection not overload.
Click to expand...


Well, there you go,,,,,,how are you gonna put that on a 30?
 
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