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Old 12-04-2009, 01:18 AM   #1
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wire sizing


Hi
I have heat pump unit that requires min ampacity of 29.7
Does this mean I can use a wire rated for 30 amps (10 gauge)?
The heat pump company will have already allowed for the buffer required?
Also I need max over current protection of 50 amps so I can use a 50 amp c/b?
Thanks clell

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Old 12-04-2009, 04:30 AM   #2
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wire sizing


Yes, 10 gauge should be fine. For the OCPD you'll need to go by the nameplate. If is has "max fuse size" you should use a fused disconnect. If it has "max breaker size" you can use a 50A CB for the OCP. Either way, you'll need a disconnect within line of sight.


Last edited by jerryh3; 12-04-2009 at 04:44 AM.
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Old 12-04-2009, 05:13 AM   #3
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The date plate min and max, are guides. As long as the wire and breaker/fuse are within those 2 numbers, your ok.

10 guage wire may or may not be the right size. Depending on length of run.

While the 10 guage wire will meet code. You will be better off using an 8 guage.
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Old 12-04-2009, 06:11 AM   #4
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Originally Posted by clell View Post
Hi
I have heat pump unit that requires min ampacity of 29.7
If by this you mean the "Minimum circuit ampacity" then yes, #10cu is fine.




Quote:
Originally Posted by beenthere View Post
While the 10 guage wire will meet code. You will be better off using an 8 guage.
What if the run is only 60'-70'? Then #10 is simply overkill.

A 24A load @ 240v is FINE to 100' or more.
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Old 12-04-2009, 06:26 AM   #5
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If by this you mean the "Minimum circuit ampacity" then yes, #10cu is fine.





What if the run is only 60'-70'? Then #10 is simply overkill.

A 24A load @ 240v is FINE to 100' or more.
It will meet code.

During near design conditions. The starting load can still be enough that an 8 guage would have prevented excess wear/strain on the start windings of the compressor.

While his compressor may only have a RLA of 21.3 amp. Starting amps are much more at load conditions.

Also. If he uses 10 guage. And the unit trips the breaker at times during start up. He is stuck only using a 30 amp breaker. Since a heat pump is often not a motor only load. By using an 8 guage, he will be able to increase breaker size if need be.
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Old 12-04-2009, 06:33 AM   #6
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It will meet code.

During near design conditions. The starting load can still be enough that an 8 guage would have prevented excess wear/strain on the start windings of the compressor.

While his compressor may only have a RLA of 21.3 amp. Starting amps are much more at load conditions.

Also. If he uses 10 guage. And the unit trips the breaker at times during start up. He is stuck only using a 30 amp breaker. Since a heat pump is often not a motor only load. By using an 8 guage, he will be able to increase breaker size if need be.
HVAC equipment is not locked into the #10 gage wire can only be used on a 30 amp breaker rule.

In this case a #10 can be used on a 50 amp breaker as stated on the name plate of the heat pump.
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Old 12-04-2009, 06:38 AM   #7
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HVAC equipment is not locked into the #10 gage wire can only be used on a 30 amp breaker rule.

In this case a #10 can be used on a 50 amp breaker as stated on the name plate of the heat pump.
What section of the NEC allows an electric heater to be on that large of a breaker, with a 10 guage wire.
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Old 12-04-2009, 06:45 AM   #8
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What section of the NEC allows an electric heater to be on that large of a breaker, with a 10 guage wire.
The nameplate MCA should take into account any resistive loads present in the unit. I thought we were only talking about the the outdoor unit in this case(compressor, control circuitry, maybe a crankcase heater)?

Last edited by jerryh3; 12-04-2009 at 06:47 AM.
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Old 12-04-2009, 06:50 AM   #9
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The nameplate MCA should take into account any resistive loads present in the unit. I thought we were only talking about the the outdoor unit in this case.
We are. Or at least I am.

Heat pumps often have crank case heaters.
And a crank case heater is not a motor load. So the motor only load exemption does not apply.

And a good inspector can shoot you down for using a 45 amp breaker on a 10 guage wire.
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Old 12-04-2009, 07:08 AM   #10
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We are. Or at least I am.

Heat pumps often have crank case heaters.
And a crank case heater is not a motor load. So the motor only load exemption does not apply.

And a good inspector can shoot you down for using a 45 amp breaker on a 10 guage wire.
I don't think an inspector would/should have a problem if the nameplate allowed a 50A CB. I 'm not totally familiar with HVAC equipment, but I believe any load other than the motor loads would have internal over current protection(i.e. fuses) built into the unit. This would allow higher over current protection for the entire unit but would still provide protection for the non-motor loads.
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Old 12-04-2009, 07:14 AM   #11
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Originally Posted by beenthere View Post
We are. Or at least I am.

Heat pumps often have crank case heaters.
And a crank case heater is not a motor load. So the motor only load exemption does not apply.

And a good inspector can shoot you down for using a 45 amp breaker on a 10 guage wire.

According to the OP the minimum circuit ampacity need to be 29.7 and a max breaker of 50 amps. I don't see what criteria the inspector could use as a violation when both conditions are met.
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Old 12-04-2009, 07:16 AM   #12
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I don't think an inspector would/should have a problem if the nameplate allowed a 50A CB. I 'm not totally familiar with HVAC equipment, but I believe any load other than the motor loads would have internal over current protection(i.e. fuses) built into the unit. This would allow higher over current protection for the entire unit but would still provide protection for the non-motor loads.
The crank case heater has no other protection then the units circuit breaker.
There is no other fuse, or thermal link.
And more then one has shorted to ground causing the breaker to trip.

Another draw back of using a 10 guage in an app like this. Is if he ever adds on. And needs to increase the size of the unit. He has to run a new wire anyway, since the next size up unit, would have a higher min amp requirement.
Unless he increased the units SEER rating by about 2 or more points.

In today's market. More people are adding o n to their existing home, then building/buying a new house. So future additions are something to keep in mind when doing work today.

The cost of an 8 guage wire today. Saves a lot of money over time, and often when/if an addition is added later.
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Old 12-04-2009, 07:21 AM   #13
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According to the OP the minimum circuit ampacity need to be 29.7 and a max breaker of 50 amps. I don't see what criteria the inspector could use as a violation when both conditions are met.
Thats min and max circuit protection(breaker/fuse size). Not the size wire to use.
The manufacturer can't tell you what size wire to use. They have no idea of where and how it will be installed.
They can only tell you the units amp draw(min, max fuse/breaker size to use). And what standard limitations the NEC puts on wire sizing.
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Old 12-04-2009, 07:55 AM   #14
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Thats min and max circuit protection(breaker/fuse size). Not the size wire to use.
The manufacturer can't tell you what size wire to use. They have no idea of where and how it will be installed.
They can only tell you the units amp draw(min, max fuse/breaker size to use). And what standard limitations the NEC puts on wire sizing.
I am going to disagree. The labels state "Minimum Circuit Ampacity" and "Maximum Overcurrent Protection". One tells you the required wire size, the other is the breaker size. The larger breaker size is sized to allow the startup inrush. The running amps are below the circuit ampacity.
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Old 12-04-2009, 07:59 AM   #15
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I am going to disagree. The labels state "Minimum Circuit Ampacity" and "Maximum Overcurrent Protection". One tells you the required wire size, the other is the breaker size. The larger breaker size is sized to allow the startup inrush. The running amps are below the circuit ampacity.
... and if an inspector does not allow the 10ga wire/50A OCPD setup for this heat pump, he is WRONG!

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