heat pump wire size and breaker

Status
Not open for further replies.
infinity said:
Most of the stuff here has been asked before in some form so ask away. :)
Click to expand...

Well, I read most of the 72 pages.

I guess my thinking, however incorrect it might be, is that if the maximum fuse amps or maximum circuit breaker as stated on a refrigeration unit is used, then the wiring ampacity to the disconnect and the breaker at the main panel would need to be based on that maximum. So, if MOCPD is listed as 30 amp's, then the the circuit breaker should be 30 amp, and the wiring should be sized for 30 amps..

If the minimum circuit amp's. as listed on the name plate is used (call it 18.7 amps), then the branch wiring to the disco and the breaker in the main panel for the circuit could be based on that minimum circuit ampacity (18.7 amps).

I have no problem understanding having a 30 amp. breaker in the main panel with wiring sized for 30 amps going to a fused disconnect with 20 amp fuses in this case.

What I don't understand is the concept of running wiring for the circuit that meets the minimum circuit amps (18.7 amps), but protecting it with a breaker sized on the maximum circuit breaker (30 amps.). From reading the comments in the 72 page thread, the argument for using wiring for 20 amps., with a 30 breaker could be done because the unit should not have an actual run load of more than 20 amps.. Although that might be true, I can't see where the NEC allows for this, nor do I understand the desire to use a 30 amp breaker instead of a 20 amp breaker. The so called nuisance trips are usually an indication of a system problem that otherwise hasn't been identified.
 
GoldDigger said:
David, what you are missing has been discussed many times in many threads in the past, and so is taken for granted by most members.
The very short summary is that the wire is protected against overload (long term heating by current just above the wire limit) by the motor overload protection.
The OCPD is there to take care of short circuits and ground faults, and for that the higher value OCPD is sufficient.

mobile
Click to expand...

Yes, the overload device for the hermetic compressor should protect the motor. However, those devices do fuse closed occasionally, and it isn't caught until the compressor has a burn out which may ultimately trip a breaker. How high do the amps go when this happens? Is it ever high enough to destroy 20 amp wiring on a 35 amp breaker? I don't know.
 
David Goodman said:
What I don't understand is the concept of running wiring for the circuit that meets the minimum circuit amps (18.7 amps), but protecting it with a breaker sized on the maximum circuit breaker (30 amps.). From reading the comments in the 72 page thread, the argument for using wiring for 20 amps., with a 30 breaker could be done because the unit should not have an actual run load of more than 20 amps.. Although that might be true, I can't see where the NEC allows for this, nor do I understand the desire to use a 30 amp breaker instead of a 20 amp breaker. The so called nuisance trips are usually an indication of a system problem that otherwise hasn't been identified.
Click to expand...

I guess this didn't do it for you?:blink:

Little Bill said:
I don't know what thread you are referring to but if the info was correct then, it is also correct now.

To help simplify the larger breaker and smaller wire. You already know the wire size by the MCA. If you notice most will have a much larger "max OCD (breaker/fuse) then is listed in the ampacity tables in relation to wire sizes.

You don't have to go to the largest/max breaker but are usually better off because of nuisance trips due to inrush current on start up.

Someone already explained the wire is protected by the motor overloads. The "Max" ocd is sized to prevent false trips but also sized for short circuits.

This rule is only for items mentioned in 240.4(G)

Simple terms: wire is protected by motor overloads
motor is protected (short circuit) by the fuse or breaker
Where as a normal circuit, say for a light, the breaker is sized to protect the wire.
Click to expand...
 
David Goodman said:
I am good with, and understand 14 AWG based on minimum circuit ampacity of 16, but I am definitely trying to wrap my head around being able to use a 30 amp. OCPD instead of a 20 amp..

I will search this site for other longer discussions, and not trouble you any more here. No doubt, it has been thoroughly covered before.

Thank you all.
Click to expand...
One way that may help is to think of the motor overload, which is very sensitive but slow and the CB, which is less finely tuned for long term overloads are both in series with the wire.
There is a reason that the NEC only allows this special case for motors, and that is because there must *always*, per the NEC, be an overload or impedance protection or other thermal protection in the motor circuit (and the breaker can't do that function).

mobile
 
Little Bill said:
I guess this didn't do it for you?:blink:
Click to expand...

No, I am afraid not. It is still my understanding the the overload device is to protect the motor (not the wiring), and the OCP device is to protect the wiring (although 440.21 does refer to "branch circuit conductors..., and motors...").:ashamed1:

Still not seeing where the breaker can be greater than the branch circuit wiring.
 
David Goodman said:
Yes, the overload device for the hermetic compressor should protect the motor. However, those devices do fuse closed occasionally, and it isn't caught until the compressor has a burn out which may ultimately trip a breaker. How high do the amps go when this happens? Is it ever high enough to destroy 20 amp wiring on a 35 amp breaker? I don't know.
Click to expand...

There is always a chance that anything will fail to operate as designed. Since that is the case the question is how high, how strong, how fail safe do you want to go. There has to be a line drawn somewhere. The NEC has drawn a line it's just that not everybody is aware of some of the details.

You can't get your head around a 30A breaker on anything smaller than 10AWG wire, and so on, because you have never thought it was allowed. 240.4(D) and on to 240.4(G) and on to 430 and 440 allows a lager breaker on a smaller wire for motor circuits because it is only there for short circuit and ground fault protection.

It has been this way for as long as most of us has been alive, and if there were a problem it would have been addressed already. In particular to A/C MCA and MOCP guidelines, the manufactures have done all the R&D needed to put a safe product on the market and there is no practical reason to go beyond their guidelines.
 
David Goodman said:
Still not seeing where the breaker can be greater than the branch circuit wiring.
Click to expand...
It would be easier it the NEC just had a sentence that said, "Motors are different." but it doesn't.

Instead 240.4(D) says, "Unless specifically permitted in 240.4(E) or (G). 240.4(G)"

240.4(G) says, "Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G).

Then you read the table and it takes you to 440, 430, and a few other places where you can use smaller wires than usually allowed like 630.
 
ActionDave said:
It would be easier it the NEC just had a sentence that said, "Motors are different." but it doesn't.

Instead 240.4(D) says, "Unless specifically permitted in 240.4(E) or (G). 240.4(G)"

240.4(G) says, "Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G).

Then you read the table and it takes you to 440, 430, and a few other places where you can use smaller wires than usually allowed like 630.
Click to expand...


I am digging deep into 440. That is actually where I started. Either I can't find the exception, or I simply can't understand the terminology of the point that allows it. I'll read 240.4(D) and (G) several more times tomorrow. Maybe it will click for me.

I am going to give it a rest.

Again, thank you all for trying to guide me in the right direction.
 
David Goodman said:
I am digging deep into 440. That is actually where I started. Either I can't find the exception, or I simply can't understand the terminology of the point that allows it. I'll read 240.4(D) and (G) several more times tomorrow. Maybe it will click for me.

I am going to give it a rest.

Again, thank you all for trying to guide me in the right direction.
Click to expand...

Take a look at 430.51 and 52.
 
ActionDave said:
There is always a chance that anything will fail to operate as designed. Since that is the case the question is how high, how strong, how fail safe do you want to go. There has to be a line drawn somewhere. The NEC has drawn a line it's just that not everybody is aware of some of the details.

You can't get your head around a 30A breaker on anything smaller than 10AWG wire, and so on, because you have never thought it was allowed. 240.4(D) and on to 240.4(G) and on to 430 and 440 allows a lager breaker on a smaller wire for motor circuits because it is only there for short circuit and ground fault protection.

It has been this way for as long as most of us has been alive, and if there were a problem it would have been addressed already. In particular to A/C MCA and MOCP guidelines, the manufactures have done all the R&D needed to put a safe product on the market and there is no practical reason to go beyond their guidelines.
Click to expand...

It has been quite an epiphany for me to read in the NEC that in fact smaller conductors are allowed in certain situations.

I doubt that in the work I do, I will attempt to take advantage of those exceptions. However, if I come across existing wiring that appears to be too small for the application, I will be able to reference the NEC sections mentioned here to determine if the wiring is suitable.

Thank you for tolerating my discussion on someone else's thread.
 
Last edited:
David Goodman said:
It has been quite an epiphany for me to read in the NEC that in fact smaller conductors are allowed in certain situations.

I doubt that in the work I do, I will attempt to take advantage of those exceptions. However, if I come across existing wiring that appears to be too small for the application, I will be able to reference the NEC sections mentioned here to determine if the wiring is suitable.

Thank you for tolerating my discussion on someone else's thread.
Click to expand...
In the case of motors or hermatic compressors - the motor overload protection device not only protects the motor from overload, but inherently protects the conductors. Those circuits need a higher short circuit and ground fault protection to allow for the surge of current that flows when starting the motor without tripping the SC/GF protection device.

You said sometimes motor overloads fail - which may be true, but they fail to protect the motor in that case as well. Ultimately a ground fault usually develops and the branch circuit device will then trip, still protecting life and property from overheated conductors, if the breaker fails as well we just have a streak of bad luck going on I guess.
 
tommy said:
Where is that code Art.
Click to expand...

The basic rule in 240.4 (D) is to protect a wire at its ampacity
240.(G) allows overcurrent protection for specific conductors as referenced in Table 240.4(G), for HVAC you are sent to ARt 440 Parts III and IV
Those sections allow you to use a larger OCPD size than you are used to.
For HVAC, you install a conductor that meets the minimum circuit ampacity, in this case 16 amps, which can be a 14 AWG which is rated 25 amps in table 310.15(B)(16)
Many electricians don't understand this so end up using a larger wire than required
 
David Goodman said:
Well, I read most of the 72 pages.
What I don't understand is the concept of running wiring for the circuit that meets the minimum circuit amps (18.7 amps), but protecting it with a breaker sized on the maximum circuit breaker (30 amps.). From reading the comments in the 72 page thread, the argument for using wiring for 20 amps., with a 30 breaker could be done because the unit should not have an actual run load of more than 20 amps.. Although that might be true, I can't see where the NEC allows for this, nor do I understand the desire to use a 30 amp breaker instead of a 20 amp breaker. The so called nuisance trips are usually an indication of a system problem that otherwise hasn't been identified.
Click to expand...

I have instructed an overcurrent and conductors class many many times, but before I did I read through the sections in 240 and 310, and worked out how the rules work.
Here is what I tell my classes " The breaker protects the wire, the overload protects the motor". I find for motors the concept of a 12 AWG wire on a 40 amp OCPD easier to understand.
The code calls the motor branch circuit the "motor branch-circuit short-circuit and ground-fault protection", that long description is very important.
A motor draws up to 6X the nameplate amps on starting, so the motor branch circuit breaker is sized larger to allow it to start, but the breaker protects against short-circuit and ground-faults, while the overloads protect against slow, long time overloads, such as a bad bearing.
"the breaker protects the wire, the overloads protect the motor". Thats all you need to know.

HVAC rules are similar, but since the motor runs in a refrigerant, the motor runs cooler.
 
David Goodman said:
I am digging deep into 440. That is actually where I started. Either I can't find the exception, or I simply can't understand the terminology of the point that allows it. I'll read 240.4(D) and (G) several more times tomorrow. Maybe it will click for me.

I am going to give it a rest.

Again, thank you all for trying to guide me in the right direction.
Click to expand...

Its not an exception, its a basic rule. I have done motor work for a very long time, never had a overload fail....
 
tom baker said:
Its not an exception, its a basic rule. I have done motor work for a very long time, never had a overload fail....
Click to expand...
If anything they fail open more then they fail closed.

Unlike a controller contact you are not as likely to close them into a fault and cause the contact to weld closed, and if they do end up closing into a fault - the compressor was very likely toast anyway.
 
tom baker said:
I have instructed an overcurrent and conductors class many many times, but before I did I read through the sections in 240 and 310, and worked out how the rules work.
Here is what I tell my classes " The breaker protects the wire, the overload protects the motor". I find for motors the concept of a 12 AWG wire on a 40 amp OCPD easier to understand.
The code calls the motor branch circuit the "motor branch-circuit short-circuit and ground-fault protection", that long description is very important.
A motor draws up to 6X the nameplate amps on starting, so the motor branch circuit breaker is sized larger to allow it to start, but the breaker protects against short-circuit and ground-faults, while the overloads protect against slow, long time overloads, such as a bad bearing.
"the breaker protects the wire, the overloads protect the motor". Thats all you need to know.

HVAC rules are similar, but since the motor runs in a refrigerant, the motor runs cooler.
Click to expand...


It is starting to come together for me. Like I said at the start, I trusted that information being provided was correct. I needed to understand it. Different people's comments are helping me shuffle it around.
 
tom baker said:
" The breaker protects the wire, the overload protects the motor". I find for motors the concept of a 12 AWG wire on a 40 amp OCPD easier to understand.

"the breaker protects the wire, the overloads protect the motor". Thats all you need to know.

HVAC rules are similar, but since the motor runs in a refrigerant, the motor runs cooler.
Click to expand...

Tom, how does a 40 amp breaker protect a #12 conductor from being damaged by an overload?
 
infinity said:
Tom, how does a 40 amp breaker protect a #12 conductor from being damaged by an overload?
Click to expand...
+1

the overload primary mission is to protect the motor, but it protects the conductor as well. Properly sized overload for a motor with full load of 17 amps protects the conductor and motor up to 17 amps, well maybe 125% of that - but the conductor minimum ampacity is also 125% of the full load amps most of the time.

That 17 amp motor can be fed with 12 AWG @ 75C rating. Short circuit and ground fault protection can easily be 40 amps, and if it won't hold during starting it can be increased more. If the device is an instantaneous trip breaker it could easily be a 125 amp device - still on the 12 AWG conductor.

OP topic is regarding art 440 applications and you would not likely see an instantaneous breaker in those applications - but shows how much higher the short circuit and ground fault setting can be over the overload setting on a particular motor circuit conductor.
 
Status
Not open for further replies.
Top