AC Equipment Sizing

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darkov

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Location
Scottsdale, AZ
I have a question. If we use manufacturers name plate for sizing AC disconnect and wire size, for instance ampecity is 30.1 and Circuit breaker is max. 50, so we'll install 10 AWG wire and 50 Amp. breaker. My question is : is there a min. size breaker, can I install 30 or 40 Amp. breaker?
Thanks,

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Little Bill

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Location
Tennessee NEC:2017
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Semi-Retired Electrician
I have a question. If we use manufacturers name plate for sizing AC disconnect and wire size, for instance ampecity is 30.1 and Circuit breaker is max. 50, so we'll install 10 AWG wire and 50 Amp. breaker. My question is : is there a min. size breaker, can I install 30 or 40 Amp. breaker?
Thanks,

Sent from my SGH-M919 using Tapatalk

Yes you can size the breaker smaller. The 50A is the max size. However if you go much smaller the breaker may not hold on start up due to inrush current.
 

Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
Thank you

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You're welcome.

I see you are in Phoenix AZ. I just got back from there a couple of weeks ago.
The next time I hear someone here say "but it's a dry heat" I'm going to slap them!:rant:

It was 102° there when we landed. Went swimming at 10 pm and it was still near 90°!
 

johnmeto

Banned
Location
US
60-amp breaker will work

60-amp breaker will work

you'll need not only a 60-amp breaker but also #4 NM-B from the panel to the range/oven. Best do the work now - you'll have to do it sooner or later.
 

Little Bill

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you'll need not only a 60-amp breaker but also #4 NM-B from the panel to the range/oven. Best do the work now - you'll have to do it sooner or later.

I think you are replying to the wrong thread as this one was about HVAC equipment, not for a range/oven.
 

infinity

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Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I have a question. If we use manufacturers name plate for sizing AC disconnect and wire size, for instance ampecity is 30.1 and Circuit breaker is max. 50, so we'll install 10 AWG wire and 50 Amp. breaker. My question is : is there a min. size breaker, can I install 30 or 40 Amp. breaker?
Thanks,

The circuit can be sized according to the MCA as the name implies it's the minimum circuit ampacity. A 20 amp MCA with a 20 amp conductor and a 20amp OCPD would be compliant.
 
The disconnect ampere rating shall be at least 115% of the nameplate rated-load current or branch circuit selection current, whichever is greater. (Nec 440.12 (1) )

115% of 30.1= 34.615A
So...35A minimum disconnect

For wire size determine derating from table 310.5 (I'm assuming less than 4 conductors, so we'll assume 100% then go to table 310.16 60 DEGREE COLUMN (110.14(C) Based on nameplate 8awg. is acceptable. Therefore a 40A breaker is acceptable. Always reference (Article 440-AIR CONDITIONING AND REFRIGERATING EQUIPMENT) as there are specific codes pertaining to this type of electrical work.

Disconnect = 35 Amp
Wire Size = 8 Awg.
Breaker Size = 40 Amps
 
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infinity

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Location
New Jersey
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Journeyman Electrician
The disconnect ampere rating shall be at least 115% of the nameplate rated-load current or branch circuit selection current, whichever is greater. (Nec 440.12 (1) )

115% of 30.1= 34.615A
So...35A minimum disconnect

For wire size determine derating from table 310.5 (I'm assuming less than 4 conductors, so we'll assume 100% then go to table 310.16 60 DEGREE COLUMN (110.14(C) Based on nameplate 8awg. is acceptable. Therefore a 40A breaker is acceptable. Always reference (Article 440-AIR CONDITIONING AND REFRIGERATING EQUIPMENT) as there are specific codes pertaining to this type of electrical work.

Disconnect = 35 Amp
Wire Size = 8 Awg.
Breaker Size = 40 Amps

The 30.1 number is the MCA which already has the 25% factor added to the full load current so there is no need to add an additional 15%.
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
I have a question. If we use manufacturers name plate for sizing AC disconnect and wire size, for instance ampecity is 30.1 and Circuit breaker is max. 50,

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The way the OP worded his post, I would put money on the info being MCA & MOCP.
 

growler

Senior Member
Location
Atlanta,GA
The disconnect ampere rating shall be at least 115% of the nameplate rated-load current or branch circuit selection current, whichever is greater. (Nec 440.12 (1) )

115% of 30.1= 34.615A
So...35A minimum disconnect

For wire size determine derating from table 310.5 (I'm assuming less than 4 conductors, so we'll assume 100% then go to table 310.16 60 DEGREE COLUMN (110.14(C) Based on nameplate 8awg. is acceptable. Therefore a 40A breaker is acceptable. Always reference (Article 440-AIR CONDITIONING AND REFRIGERATING EQUIPMENT) as there are specific codes pertaining to this type of electrical work.

Disconnect = 35 Amp
Wire Size = 8 Awg.
Breaker Size = 40 Amps

I don't understand where you are getting your numbers.

The disconnect is going to be a 60 amp because that's the next size up from 30 amp.

Wire size may be #8 if they calculate voltage drop and it a long distance from the panel but is not required (may be speced). #10 is allowed as a minimum.

But why a breaker or fuse size of 40? If you put a 40 amp breaker there is a good chance it won't hold. The manufacturer has alread said that you can use a 50 amp breaker and not void the warranty so why not use a 50.

If it's an RTU (roof top unit ) it proabably will require fuses and it will state the max fuse size.

They already know that's what you are going to use.
 

darkov

Member
Location
Scottsdale, AZ
Yes, 30.1 and 50 are on name plate attached to the AC.
Use #10 awg cu. at 75 C
Are there any rules on min breaker size. We know max. Is 50 per name plate.
 

GoldDigger

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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Yes, 30.1 and 50 are on name plate attached to the AC.
Use #10 awg cu. at 75 C
Are there any rules on min breaker size. We know max. Is 50 per name plate.
The minimum breaker size is the smallest standard size above 30.1 if you consider the breaker as limiting the circuit ampacity.
But, as mentioned, a 40 would likely give nuisance trips.
 

growler

Senior Member
Location
Atlanta,GA
Yes, 30.1 and 50 are on name plate attached to the AC.
Use #10 awg cu. at 75 C
Are there any rules on min breaker size. We know max. Is 50 per name plate.

No minimum breaker size ( as stated by little Bill). But the NEC is concerned with safety and not design. You can install a 20 amp breaker and it will be safe but may not work.
 

darkov

Member
Location
Scottsdale, AZ
No minimum breaker size ( as stated by little Bill). But the NEC is concerned with safety and not design. You can install a 20 amp breaker and it will be safe but may not work.
I agree with little bill, smaller breaker may not hold at start up, but this is not point of this post. Point is are there any rules for this? And in this case I agree with growler, NEC is not for design.

Thank you all.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I agree with little bill, smaller breaker may not hold at start up, but this is not point of this post. Point is are there any rules for this?

The rule is that the breaker must be capable of carrying the starting current of the motor or the equipment.

It's not really a minimum "size" rule, but a minimum performance rule.
 
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