Condensing unit.

Status
Not open for further replies.
T

TTwizted

Member
Location
florida
My condensing unit outside reads 30 max and have a 40 amp breaker at the main and a 60amp disco. Shouldn’t it be 35 for disco because of 115% rule? And a 30 at the main?


Sent from my iPhone using Tapatalk
 
If the equipment says MOCP 30A, then you need a 30A overcurrent device somewhere in the feed to that equipment. It could be at the panel or at the disconnect. The disconnect is sized at a minimum of 115% of FLA. It can be larger than the minimum value.
 
TTwizted said:
My condensing unit outside reads 30 max and have a 40 amp breaker at the main and a 60amp disco. Shouldn’t it be 35 for disco because of 115% rule? And a 30 at the main?
Click to expand...
There must be a 30 amp (or possibly smaller) OCPD somewhere in the circuit. What is the MCA of the unit?
 
The manufacturer has done all the thinking for you. Let the nameplate be your guide. MCA to determine the conductor, MOCP to determine the size and type overcurrent device. If your unit says MAX 30 then, as dOnut states, there must be a 30 amp overcurrent device in the circuit
 
d0nut said:
If the equipment says MOCP 30A, then you need a 30A overcurrent device somewhere in the feed to that equipment. It could be at the panel or at the disconnect. The disconnect is sized at a minimum of 115% of FLA. It can be larger than the minimum value.
Click to expand...
And MCA already has 125% if compressor (or largest motor) factored into it, so in OP's case a 30 amp disconnect should be sufficient though it likely could be less if there were a size to match the actual value.
 
augie47 said:
The manufacturer has done all the thinking for you. Let the nameplate be your guide. MCA to determine the conductor, MOCP to determine the size and type overcurrent device. If your unit says MAX 30 then, as dOnut states, there must be a 30 amp overcurrent device in the circuit
Click to expand...

That’s what I thought and that would make sense. But for some reason they put a 40 amp at the main and 60 at disco. The disco is fine all it’s doing is disconnecting power correct.? But at the panel I should have a 30?!


Sent from my iPhone using Tapatalk
 
d0nut said:
If the equipment says MOCP 30A, then you need a 30A overcurrent device somewhere in the feed to that equipment. It could be at the panel or at the disconnect. The disconnect is sized at a minimum of 115% of FLA. It can be larger than the minimum value.
Click to expand...

c6596ded57a3201f7189fd0395b62805.jpg



Sent from my iPhone using Tapatalk
 
d0nut said:
If the equipment says MOCP 30A, then you need a 30A overcurrent device somewhere in the feed to that equipment. It could be at the panel or at the disconnect. The disconnect is sized at a minimum of 115% of FLA. It can be larger than the minimum value.
Click to expand...

How do they get 30a? 18.9+1.3=20.2a then
20.2x1.15=23.23a so then 23.23x1.25=29.04. Is that how?


Sent from my iPhone using Tapatalk
 
Does't matter how.

MCA -= 18a = #12 cu wire.

MOC = 30a = 30a breaker

Do you know what size the wire is leaving the panel?

If possible, I would leave the 40 in the panel and replace the 60 with a 30.
 
The manufacturers base their calculations on the fact that compressors rely on the natural cooling effect of the return refrigerant. Also, HVAC compressors are manufactured to a very high tolerance and can take considerable amount of abuse.
 
Last edited:
How they get it is compressor amps (14.1) x 1.25 +1.3amp fan =18.9 amps

But the manufacturer does all the calculations for you MOCP is the breaker 30 amp

Wire size is 18.9 amps so #12
 
TTwizted said:
How do they get 30a? 18.9+1.3=20.2a then
20.2x1.15=23.23a so then 23.23x1.25=29.04. Is that how?


Sent from my iPhone using Tapatalk
Click to expand...
The compressor and fan motor will have their own overload protection. This overload protection also inherently protects the supply conductors from overload conditions. The 30 amp MOCP is not for overload it is to assure you have high enough short circuit/ground fault protection level that it won't trip during starting.
 
Eddie702 said:
How they get it is compressor amps (14.1) x 1.25 +1.3amp fan =18.9 amps

But the manufacturer does all the calculations for you MOCP is the breaker 30 amp

Wire size is 18.9 amps so #12
Click to expand...
I can't seem to find it now, but I thought there was something in code that limited #14 wire to 15A, #12 wire to 20A and #10 wire to 30A, regardless of insulation type, temperature rating and anything else? Or was the removed from code? So the 30A breaker would need #10 wire, even though it's a motor / HVAC load that would otherwise just need #12.
 
robertd said:
I can't seem to find it now, but I thought there was something in code that limited #14 wire to 15A, #12 wire to 20A and #10 wire to 30A, regardless of insulation type, temperature rating and anything else? Or was the removed from code? So the 30A breaker would need #10 wire, even though it's a motor / HVAC load that would otherwise just need #12.
Click to expand...
240.4(D) is the Small Conductor rule you are thinking of but 240.4(G) Overcurrent Protection for Specific Conductor Applications is what allows you to oversize breakers.
 
Eddie702 said:
How they get it is compressor amps (14.1) x 1.25 +1.3amp fan =18.9 amps

But the manufacturer does all the calculations for you MOCP is the breaker 30 amp

Wire size is 18.9 amps so #12
Click to expand...
robertd said:
I can't seem to find it now, but I thought there was something in code that limited #14 wire to 15A, #12 wire to 20A and #10 wire to 30A, regardless of insulation type, temperature rating and anything else? Or was the removed from code? So the 30A breaker would need #10 wire, even though it's a motor / HVAC load that would otherwise just need #12.
Click to expand...
For an AC unit if you used a 75° C wiring method (MC cable, conduit and wire) you could use #14 AWG for a unit with an MCA of 18.9 amps and a 30 amp OCPD and be code compliant. You disregard the restrictions of 240.4(D) when using 240.4(G) and can use the ampacities in the 75° C column of 310.15(B)(16). #14 AWG at 75° C is 20 amps.
 
robertd said:
I can't seem to find it now, but I thought there was something in code that limited #14 wire to 15A, #12 wire to 20A and #10 wire to 30A, regardless of insulation type, temperature rating and anything else? Or was the removed from code? So the 30A breaker would need #10 wire, even though it's a motor / HVAC load that would otherwise just need #12.
Click to expand...
ActionDave said:
240.4(D) is the Small Conductor rule you are thinking of but 240.4(G) Overcurrent Protection for Specific Conductor Applications is what allows you to oversize breakers.
Click to expand...
As mentioned 240.4(D) is the small conductor rule. Is a general rule.

Anything mentioned in 240.4(G) pretty much bypasses art 240 and you do what it says in the article that applies. 430 and 440 applications are probably the most popular situations that you don't follow much if anything in art 240 for. Most the cases there you are providing short circuit and ground fault protection and some other device is already present via other requirements that will ultimately also provide overload protection for conductors.
 
robertd said:
I can't seem to find it now, but I thought there was something in code that limited #14 wire to 15A, #12 wire to 20A and #10 wire to 30A, regardless of insulation type, temperature rating and anything else? Or was the removed from code? So the 30A breaker would need #10 wire, even though it's a motor / HVAC load that would otherwise just need #12.
Click to expand...
motor circuits are completely different
 
Status
Not open for further replies.
Top