heat pump wire size and breaker

Status
Not open for further replies.
T

tommy

Senior Member
Location
Maryland
Heat Pump nameplate rating Max overcurrent 30 amps min. circuit 16 amps 120/208 single phase . Question Can a 35 amp breaker be installed and 10 gauge wire be ran to a fusible disconnect then to the heat pump fused at 30amp. My questionable thought is the 10 gauge wire to the fusible disconnect okay in this situation. When someone asked the 10 gauge was due to distant but wouldn't 12 gauge be good too.
 
You can run #12 NM or MC cable and protect it with a 35 amp breaker. This is perfectly acceptable by the NEC even though most electricians an inspectors do not understand that reality.

What will the actual wiring method be?
 
Thanks in advance for any help. Have not posted anything for awhile but have checked periodically. I think this forum is great
 
tommy said:
My thought has been wire size to min. overcurrent and fuse to the max . but I need to clarify each Art,
Click to expand...
Correct, but with the minor correction that the minimum is the minimum circuit ampacity, not the minimum overcurrent.
It is coincidentally the minimum size you can use for the overcurrent protection.

mobile
 
infinity said:
With MC cable you can use #14 with a 30 amp OCPD.
Click to expand...

I trust that you are correct here, but I am trying to understand how/why.

NEC 2014

Looking at Table 310.15(B)(17), 14 AWG could have allowable ampacity of 30 amps. for THWN, but that is for a single conductor in free air.

Table 310.15(B)(18) lists 14 AWG as having allowable ampacity of 34 amps. for Type Z copper with not more than three current-carrying conductors in a raceway or cable.

Please help me understand how MC with 2-14AWG THHN/THWN conductors can use a 30 amp OCPD.

I might be mis-interpreting the term "free air".
 
David Goodman said:
I trust that you are correct here, but I am trying to understand how/why.

NEC 2014

Looking at Table 310.15(B)(17), 14 AWG could have allowable ampacity of 30 amps. for THWN, but that is for a single conductor in free air.

Table 310.15(B)(18) lists 14 AWG as having allowable ampacity of 34 amps. for Type Z copper with not more than three current-carrying conductors in a raceway or cable.

Please help me understand how MC with 2-14AWG THHN/THWN conductors can use a 30 amp OCPD.

I might be mis-interpreting the term "free air".
Click to expand...

The wire only has to be sized per the minimum circuit ampacity, which in this case is 16 amps.
 
David Goodman said:
I trust that you are correct here, but I am trying to understand how/why.

NEC 2014

Looking at Table 310.15(B)(17), 14 AWG could have allowable ampacity of 30 amps. for THWN, but that is for a single conductor in free air.

Table 310.15(B)(18) lists 14 AWG as having allowable ampacity of 34 amps. for Type Z copper with not more than three current-carrying conductors in a raceway or cable.

Please help me understand how MC with 2-14AWG THHN/THWN conductors can use a 30 amp OCPD.

I might be mis-interpreting the term "free air".
Click to expand...
Look at the bottom of 310.15(B)16 and see the reference to 240.4(D) then see 240.4(G)

Roger
 
Look in T310.15(B)(16) for #14 THHN at 75 degrees the ampacity is 20 amps. You need a conductor with a minimum ampacity of 16 amps is the #14 is good.
 
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
 
GoldDigger said:
The OCPD is there to take care of short circuits and ground faults, and for that the higher value OCPD is sufficient.
Click to expand...

Right, this is key to understanding this. A short circuit or ground fault will be in the hundreds of even thousand of amps range, well above the handle rating of the circuit breaker.
 
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...

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.
 
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...

I found the 2015 thread with 72 pages of comments. It is a closed thread, but is that information on this topic still appropriate information?
 
David Goodman said:
I found the 2015 thread with 72 pages of comments. It is a closed thread, but is that information on this topic still appropriate information?
Click to expand...

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.
 
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...

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