Please help!I'm very confused after reading all the posts concerning a/c conductor sizing.I have 3 mitsubishi split systems that have been blowing breakers for the outside condensing units.Max fuse size is 20 amps(time delay 20 a fuses at roof top disconnect takes care of that)20a square"d" breaker at panel feeding #12 thhn to disconnect.Can I change to 30a breaker? I have read code book,hand book and many previous posts.Some say yes,some no,some say because outside unit has cooling fan in addition to compressor that I can't.Min. circuit ampacity is 16 amps.Thanks to all in advance.
A/C CONDUCTOR SIZING
- Thread starter benny
- Start date
- Status
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: A/C CONDUCTOR SIZING
My first question would be why is the breaker tripping when you've installed a 20 amp CB as required by the unit manufacturer. Could voltage drop be an issue?
In answer to your question I would say that you could use a 25 amp CB for this feeder. Since you have 20 amp time delay fuses you have met the requirement of the unit in providing protection at 20 amps. The feeder with #12 THHN could be protected at 25 amps according to article 440. Are you using an HACR Circuit breaker?
My first question would be why is the breaker tripping when you've installed a 20 amp CB as required by the unit manufacturer. Could voltage drop be an issue?
In answer to your question I would say that you could use a 25 amp CB for this feeder. Since you have 20 amp time delay fuses you have met the requirement of the unit in providing protection at 20 amps. The feeder with #12 THHN could be protected at 25 amps according to article 440. Are you using an HACR Circuit breaker?
bphgravity
Senior Member
- Location
- Florida
Re: A/C CONDUCTOR SIZING
Perhaps the units were not sized properly for the cooling needs of the building and therefore are running longer than designed or intended, hence the overload.
Perhaps the units were not sized properly for the cooling needs of the building and therefore are running longer than designed or intended, hence the overload.
milwaukeesteve
Senior Member
- Location
- Milwaukee, WI
Re: A/C CONDUCTOR SIZING
You mentioned Time delay fuses. If you have time delay fuses at the rooftop, that is you Overload, Short Circuit and your Ground Fault protection.
Time delay fuses have a built in time delay to allow for the startup of the motor with high inrush current.
The breaker inside must either be a inverse time breaker(HACR) that is size appropriate to the 'max breaker size' or calculated yourself --OR--the breaker must be a standard breaker that only provides Short circuit and Ground fault protection, but NOT your overload protection(440.22(A) sized at 175% the motorcompressor)
Your conductors are based on the 'minimum ckt amps' Using 310.16 (take into account any derating if applicable) and use the wire at the rated amperages, not the standards for 14,12 and 10GA.
You mentioned Time delay fuses. If you have time delay fuses at the rooftop, that is you Overload, Short Circuit and your Ground Fault protection.
Time delay fuses have a built in time delay to allow for the startup of the motor with high inrush current.
The breaker inside must either be a inverse time breaker(HACR) that is size appropriate to the 'max breaker size' or calculated yourself --OR--the breaker must be a standard breaker that only provides Short circuit and Ground fault protection, but NOT your overload protection(440.22(A) sized at 175% the motorcompressor)
Your conductors are based on the 'minimum ckt amps' Using 310.16 (take into account any derating if applicable) and use the wire at the rated amperages, not the standards for 14,12 and 10GA.
milwaukeesteve
Senior Member
- Location
- Milwaukee, WI
Re: A/C CONDUCTOR SIZING
If you are using a HACR breaker inside, that would be redundant protection due to the fuses.
That is neither good nor bad.
440.52 (A) states that a HACR breaker and/or time delay fuses 'shall be rated at not more than 125%'.
Even though it is redundant, it still is a HACR breaker, and providing protection, and therefore can be no more than the Max listed.
If you are using a HACR breaker inside, that would be redundant protection due to the fuses.
That is neither good nor bad.
440.52 (A) states that a HACR breaker and/or time delay fuses 'shall be rated at not more than 125%'.
Even though it is redundant, it still is a HACR breaker, and providing protection, and therefore can be no more than the Max listed.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: A/C CONDUCTOR SIZING
The manufacturer said max 20A fuse (circuit breaker is not allowed). You have supplied the branch protection of 20A fuse at the unit disconnect. You are now done with article 440.
Now go to 215.3 for feeder overcurrent protection which sends you to 240.3. But since your air-conditioning equipment is already protected by the 20A fuses, your breaker only has to protect the conductors feeding those fuses, so table 240.3 sends you to 430.62. 430.62(A) says protection for a feeder sized to a fixed motor can not be larger than the branch protective device. But, if the feeder conductors are larger than required for the motor then 430.62(B) applies.
So, you can increase the breaker to 30A only if you increase the conductors to #10.
The manufacturer said max 20A fuse (circuit breaker is not allowed). You have supplied the branch protection of 20A fuse at the unit disconnect. You are now done with article 440.
Now go to 215.3 for feeder overcurrent protection which sends you to 240.3. But since your air-conditioning equipment is already protected by the 20A fuses, your breaker only has to protect the conductors feeding those fuses, so table 240.3 sends you to 430.62. 430.62(A) says protection for a feeder sized to a fixed motor can not be larger than the branch protective device. But, if the feeder conductors are larger than required for the motor then 430.62(B) applies.
So, you can increase the breaker to 30A only if you increase the conductors to #10.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: A/C CONDUCTOR SIZING
I would start with a 25 amp breaker and see what happens. Otherwise, as Jim said you would have to rewire the feeder with #10 AWG to utilize a 30 amp CB.
I would start with a 25 amp breaker and see what happens. Otherwise, as Jim said you would have to rewire the feeder with #10 AWG to utilize a 30 amp CB.
milwaukeesteve
Senior Member
- Location
- Milwaukee, WI
Re: A/C CONDUCTOR SIZING
I disagree with Jim only that we no longer consider 440 with the rest of the circuit. If a 20 Amp non HACR breaker is installed, it still sees the same current on the circuit as the fuses, and will not allow the unit to start.
I believe the the A/C circuit as a whole must be considered by Art 440. Especially in a series circuit, all currents are the same. That same current will be seen by both sets of protection.
That interior breaker either has to be sized according to 125% for HACR breaker, or 175% for NON HACR breaker, AND according to 440.52(A), if 175% is not sufficient for startup, then we can proceed to 225%. (I would notify inspector at this point so inspector understands why)
Also the conductors are already protected by the fuses. We don't have to protect the 'conductors' any different or special than what 440 already does.
I disagree with Jim only that we no longer consider 440 with the rest of the circuit. If a 20 Amp non HACR breaker is installed, it still sees the same current on the circuit as the fuses, and will not allow the unit to start.
I believe the the A/C circuit as a whole must be considered by Art 440. Especially in a series circuit, all currents are the same. That same current will be seen by both sets of protection.
That interior breaker either has to be sized according to 125% for HACR breaker, or 175% for NON HACR breaker, AND according to 440.52(A), if 175% is not sufficient for startup, then we can proceed to 225%. (I would notify inspector at this point so inspector understands why)
Also the conductors are already protected by the fuses. We don't have to protect the 'conductors' any different or special than what 440 already does.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: A/C CONDUCTOR SIZING
So the question is where in the circuit does Article 440 apply? I say that since there is a fused disconnect that Article 440 applies on the load side of the disconnect, and it does not apply to the line side feeder from the panel.
Well that depend on how you define the "A/C circuit". Is it a branch circuit or a feeder? If it feeds a fused disconnect than isn't the disconnect really a subpanel and the circuit feeding it a feeder? A branch circuit would be the portion of the circuit between the A/C unit and the last overcurrent device. See Article 100 for the definition of a branch circuit.
So the question is where in the circuit does Article 440 apply? I say that since there is a fused disconnect that Article 440 applies on the load side of the disconnect, and it does not apply to the line side feeder from the panel.
Re: A/C CONDUCTOR SIZING
Thank you for all your comments.I believe we have solved the problem.Transformer inside outdoor unit was wired 230v.I changed to 208v connection and it has not tripped in 3 days.We normally have slightly high voltage (218v),which was allowing contactor to pull in and start the compressor.During warmer days when the elec system in area may have lower voltage than normal (208v)(where it should be anyway)contactor would not completly pull in because it was wired 230v ,causing high amp draw and blowing breaker.This is the best conclusion we can come up with,learning a lot,thanks again...
Thank you for all your comments.I believe we have solved the problem.Transformer inside outdoor unit was wired 230v.I changed to 208v connection and it has not tripped in 3 days.We normally have slightly high voltage (218v),which was allowing contactor to pull in and start the compressor.During warmer days when the elec system in area may have lower voltage than normal (208v)(where it should be anyway)contactor would not completly pull in because it was wired 230v ,causing high amp draw and blowing breaker.This is the best conclusion we can come up with,learning a lot,thanks again...
milwaukeesteve
Senior Member
- Location
- Milwaukee, WI
Re: A/C CONDUCTOR SIZING
Infinity, how can you say that a circuit changes in regards to A/C, when it is the same circuit?
Circuits (branch circuits) are derived from the Branch circuit overcurrent protection.
And since it is a series circuit, the same current seen by the fuses will be seen at the BCOP device.
That circuit has to be governed by Art 440, and not feeders, or even motors, because A/C and Refrigeration are completely different than motors and regular branch circuits. That is why they have their own section.
Infinity, how can you say that a circuit changes in regards to A/C, when it is the same circuit?
Circuits (branch circuits) are derived from the Branch circuit overcurrent protection.
And since it is a series circuit, the same current seen by the fuses will be seen at the BCOP device.
That circuit has to be governed by Art 440, and not feeders, or even motors, because A/C and Refrigeration are completely different than motors and regular branch circuits. That is why they have their own section.
- Location
- Massachusetts
Re: A/C CONDUCTOR SIZING
The conductors from the panel board to the fused disconnect are feeders.
Take a look in Article 100 at the branch circuit and feeder definitions.
The conductors from the panel board to the fused disconnect are feeders.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: A/C CONDUCTOR SIZING
Steve, the branch circuit is between the last overcurrent device and the unit, in this case between the fused disconnect and the A/C unit. The conductors between the disconnect and the panel are feeders. This is clearly outlined in the definitions of Article 100. If the unit had a non-fused disconnect than the entire circuit from the panel would be a branch circuit and would fall under article 440.
Steve, the branch circuit is between the last overcurrent device and the unit, in this case between the fused disconnect and the A/C unit. The conductors between the disconnect and the panel are feeders. This is clearly outlined in the definitions of Article 100. If the unit had a non-fused disconnect than the entire circuit from the panel would be a branch circuit and would fall under article 440.
milwaukeesteve
Senior Member
- Location
- Milwaukee, WI
Re: A/C CONDUCTOR SIZING
So, you are telling me that if Benny wants to keep the BCOP device from tripping, he needs to increase the size of the breaker to stop the tripping. But since this is a 'branch circuit' the conductors need to match the breaker. Now Benny would have to install a 35 amp non-HACR rated breaker, just to allow the motorcompressor to start. Then he would have to install #8's because of the 'feeders'. Yet on the load side of the time delay fuses, he could wire this with #14's. What kind of logic is that? It is all the same circuit.
That circuit breaker sees the same current. We don't 'feed' a distribuiton panel first. We don't have feeders.
Besides, we don't have a branch circuit here either. We have an A/C or a Refigeration circuit, and that is not subject to 210 nor 430, because we use Art 440.
So, you are telling me that if Benny wants to keep the BCOP device from tripping, he needs to increase the size of the breaker to stop the tripping. But since this is a 'branch circuit' the conductors need to match the breaker. Now Benny would have to install a 35 amp non-HACR rated breaker, just to allow the motorcompressor to start. Then he would have to install #8's because of the 'feeders'. Yet on the load side of the time delay fuses, he could wire this with #14's. What kind of logic is that? It is all the same circuit.
That circuit breaker sees the same current. We don't 'feed' a distribuiton panel first. We don't have feeders.
Besides, we don't have a branch circuit here either. We have an A/C or a Refigeration circuit, and that is not subject to 210 nor 430, because we use Art 440.
milwaukeesteve
Senior Member
- Location
- Milwaukee, WI
Re: A/C CONDUCTOR SIZING
Actually, the more I think about it, how are you supposed to do this your way?
You couldn't put a HACR breaker, because you say that it is just a brach circuit.
You can't size the BCOP device the same as the time delay fuses. Then the breaker would trip on startup/inrush.
What then is my percentage for sizing my Branch Circuit Overcurrent Protection device?
You can't use 440, since you say it is a BC.
You can't use 430, again, since you say it is a BC.
What percentages or multipliers are you going to use to find that circuit?
We would probably be feeding a 30 Amp fused disconnect, maybe that's the way to size it. But what if that doesn't start the compressor?
Actually, the more I think about it, how are you supposed to do this your way?
You couldn't put a HACR breaker, because you say that it is just a brach circuit.
You can't size the BCOP device the same as the time delay fuses. Then the breaker would trip on startup/inrush.
What then is my percentage for sizing my Branch Circuit Overcurrent Protection device?
You can't use 440, since you say it is a BC.
You can't use 430, again, since you say it is a BC.
What percentages or multipliers are you going to use to find that circuit?
We would probably be feeding a 30 Amp fused disconnect, maybe that's the way to size it. But what if that doesn't start the compressor?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: A/C CONDUCTOR SIZING
Steve, I see your point but I don't see where the code supports your arguement. The inclusion of the fuses at the A/C unit changes the definition of the circuit supplying it. These are from Article 100:
Branch Circuit -The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).
Outlet -A point on the wiring system at which current is taken to supply utilization equipment.
Feeder -All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.
So the conductors between the fuses and the A/C unit are branch circuit conductors by definition.
The conductors feeding the fuses are feeder conductors by definition. Article 440 would apply only to the branch circuit conductors.
Steve, I see your point but I don't see where the code supports your arguement. The inclusion of the fuses at the A/C unit changes the definition of the circuit supplying it. These are from Article 100:
Branch Circuit -The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).
Outlet -A point on the wiring system at which current is taken to supply utilization equipment.
Feeder -All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.
So the conductors between the fuses and the A/C unit are branch circuit conductors by definition.
The conductors feeding the fuses are feeder conductors by definition. Article 440 would apply only to the branch circuit conductors.
- Location
- Massachusetts
Re: A/C CONDUCTOR SIZING
Much of the talk on this forum is about the strict reading of the NEC while you want to talk about the design of the job. Thats great but IMO you should make it clear when you are talking about design and not code requirements.
As an example, think of a HVAC unit that has a labeled minimum circuit ampacity of 19.9 amps and a Maximum fuse size of 35 amps.
We could install a 60 amp fused disconnect with 35 amp fuses and the feeder breaker could be 20 amps.
IMO that would meet the minimum NEC requirements.
That said, in no way do I think that is a good design or one that would make for a happy customer.
Remember the label on the unit refers to a maximum over current protection. Any over current protection up to that rating is code compliant.
That is a design issue, not a safety issue.
Much of the talk on this forum is about the strict reading of the NEC while you want to talk about the design of the job. Thats great but IMO you should make it clear when you are talking about design and not code requirements.
As an example, think of a HVAC unit that has a labeled minimum circuit ampacity of 19.9 amps and a Maximum fuse size of 35 amps.
We could install a 60 amp fused disconnect with 35 amp fuses and the feeder breaker could be 20 amps.
IMO that would meet the minimum NEC requirements.
That said, in no way do I think that is a good design or one that would make for a happy customer.
Remember the label on the unit refers to a maximum over current protection. Any over current protection up to that rating is code compliant.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: A/C CONDUCTOR SIZING
So what your saying that the conductors from the 20 amp breaker to the fused disconnect would have to be #12 but the conductors on the load side of the fuses could be #14?
- Location
- Massachusetts
Re: A/C CONDUCTOR SIZING
I do not think 240.4(D) applies here. (440.35)
Would they have to be 12 AWG?
I do not think 240.4(D) applies here. (440.35)
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