Heat Pump MCA

Status
Not open for further replies.
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician (retired)
I have a heat pump that has an MCA of 21.7 amps and a unit switch pre-wired with two tails for connection to the branch circuit. The branch circuit is #12 MC cable fed from a 75 degree terminal on the 2 pole circuit breaker. The unit end is connected to the wire tails with wire nuts. Is this compliant?

I say that it is but there has been some opinions that the 60 degree C ampacity of the #12 conductors must be used due to the switch terminals. IMO the switch terminals do not matter since the connection to the branch circuit is at the wire tails.

Heatpumpnameplate.jpg
 
infinity said:
I have a heat pump that has an MCA of 21.7 amps and a unit switch pre-wired with two tails for connection to the branch circuit. The branch circuit is #12 MC cable fed from a 75 degree terminal on the 2 pole circuit breaker. The unit end is connected to the wire tails with wire nuts. Is this compliant?

I say that it is but there has been some opinions that the 60 degree C ampacity of the #12 conductors must be used due to the switch terminals. IMO the switch terminals do not matter since the connection to the branch circuit is at the wire tails.

View attachment 19796
Click to expand...

MCA 21.7A

Branch circuit wire should be #10awg.

110.3(B)
Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.

Manufacturer's data label:
MCA 21.7A
 
The switch is integral to the unit and has tails connected to it to connect to the branch circuit. The discussion involves the unit switch, which may or may not have 60 degree terminals. Since the branch circuit wiring connects to the factory wires the argument is that the unit switch terminal rating does not matter.

Jamesco said:
MCA 21.7A
Click to expand...
Jamesco said:
Branch circuit wire should be #10awg.

110.3(B)
Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.

Manufacturer's data label:
MCA 21.7A
Click to expand...

The #12 branch circuit conductors in the MC cable have an ampacity of 25 amps so they're code complaint.

20180301_100708.jpg
 
infinity said:
The switch is integral to the unit and has tails connected to it to connect to the branch circuit. The discussion involves the unit switch, which may or may not have 60 degree terminals. Since the branch circuit wiring connects to the factory wires the argument is that the unit switch terminal rating does not matter.
Click to expand...

I agree. As long as the manufacturer doesnt state that 60 degree ampacity must be used.
 
infinity said:
The switch is integral to the unit and has tails connected to it to connect to the branch circuit. The discussion involves the unit switch, which may or may not have 60 degree terminals. Since the branch circuit wiring connects to the factory wires the argument is that the unit switch terminal rating does not matter.



The #12 branch circuit conductors in the MC cable have an ampacity of 25 amps so they're code complaint.

View attachment 19798
Click to expand...

Please explain how you arrived at the 25 amp rating.

240.4(D)(5) says for #12 cu use 20 amps.
I assume you used 240.4(D)(G)
From the list given where did you find the ampacity rating of 25A?

Thank you in advance,
Jim
 
infinity said:
The switch is integral to the unit and has tails connected to it to connect to the branch circuit. The discussion involves the unit switch, which may or may not have 60 degree terminals. Since the branch circuit wiring connects to the factory wires the argument is that the unit switch terminal rating does not matter.



The #12 branch circuit conductors in the MC cable have an ampacity of 25 amps so they're code complaint.

View attachment 19798
Click to expand...

infinity said:
The switch is integral to the unit and has tails connected to it to connect to the branch circuit. The discussion involves the unit switch, which may or may not have 60 degree terminals. Since the branch circuit wiring connects to the factory wires the argument is that the unit switch terminal rating does not matter.
Click to expand...

Agree.
 
Jamesco said:
Please explain how you arrived at the 25 amp rating.

240.4(D)(5) says for #12 cu use 20 amps.
I assume you used 240.4(D)(G)
From the list given where did you find the ampacity rating of 25A?

Thank you in advance,
Jim
Click to expand...

240.4(G) says to look in article 440 parts III and IV. 440.31 says to use article 310. 310 says #12 is 25 amps. In a nutshell, the small conductor sizing for #14-#10 we are so used to doesnt apply to motors and HVAC.
 
electrofelon said:
240.4(G) says to look in article 440 parts III and IV. 440.31 says to use article 310. 310 says #12 is 25 amps. In a nutshell, the small conductor sizing for #14-#10 we are so used to doesnt apply to motors and HVAC.
Click to expand...

Does 310 say for figuring the ampacity rating for #12 you can use 25 amps?
So according to 440.31 says to use 310.
Table 310.15(B)(16) #12 * * bottom of Table says refer to 240.4(D)

Where specifically in 440, Parts III, VI does it say I can use the 75 degree column for the ampacity rating of #12 wire?

What if the terminal rating on the breaker is rated for 90 degrees, can 35 amps be used for the ampacity rating for #12 wire, using 440, Parts III, VI?

In the end the AHJ will have the final say.
 
Jamesco said:
Does 310 say for figuring the ampacity rating for #12 you can use 25 amps?
Click to expand...
yes.

So according to 440.31 says to use 310.
Table 310.15(B)(16) #12 * * bottom of Table says refer to 240.4(D)
Click to expand...

yes it does, but 240.4(D) does not apply to motor loads. It says right in (D)

Where specifically in 440, Parts III, VI does it say I can use the 75 degree column for the ampacity rating of #12 wire?
Click to expand...

Where does it say you cant? If the conductors are rated 75 degrees, that how it works.

What if the terminal rating on the breaker is rated for 90 degrees, can 35 amps be used for the ampacity rating for #12 wire, using 440, Parts III, VI?
Click to expand...
yes one could use 90 degree then, although I dont think there is any eqiupment with 90 degree ratings.

In the end the AHJ will have the final say.
Click to expand...

Sorry, you are off base on this one. It is well established that the small conductor restriction does not apply to motors. There is no doubt.
 
Jamesco said:
Does 310 say for figuring the ampacity rating for #12 you can use 25 amps?
So according to 440.31 says to use 310.
Table 310.15(B)(16) #12 * * bottom of Table says refer to 240.4(D)

Where specifically in 440, Parts III, VI does it say I can use the 75 degree column for the ampacity rating of #12 wire?

What if the terminal rating on the breaker is rated for 90 degrees, can 35 amps be used for the ampacity rating for #12 wire, using 440, Parts III, VI?

In the end the AHJ will have the final say.
Click to expand...
General rule is in 110.14(C). With today's equipment, and for past 20-30 years for that matter, most equipment has 75C terminations. Only time you need to use 60C ampacity is with some older equipment, NM cable, some cases with SE cable, or if you are dealing with older conductors such as type TW.

Small conductor rule in 240.4 only applies to 14-10 AWG - but as mentioned does not apply to motors or AC equipment, or anything else in 240.4(G).

Nothing in 440 mentions temperature limitations on conductors, the fact 110.14 says you can use 75C conductor on 75C terminals and that 240.4 says small conductor rule doesn't apply to this application is what allows you to use 75C conductors even if 10 AWG or smaller. Yes you could use 90 C ampacity if terminations are rated 90C -you won't find those on typical miniature circuit breakers at all, and still won't find that very often on any 600 volt or less equipment for general use.
 
kwired said:
General rule is in 110.14(C). With today's equipment, and for past 20-30 years for that matter, most equipment has 75C terminations. Only time you need to use 60C ampacity is with some older equipment, NM cable, some cases with SE cable, or if you are dealing with older conductors such as type TW.

Small conductor rule in 240.4 only applies to 14-10 AWG - but as mentioned does not apply to motors or AC equipment, or anything else in 240.4(G).

Nothing in 440 mentions temperature limitations on conductors, the fact 110.14 says you can use 75C conductor on 75C terminals and that 240.4 says small conductor rule doesn't apply to this application is what allows you to use 75C conductors even if 10 AWG or smaller. Yes you could use 90 C ampacity if terminations are rated 90C -you won't find those on typical miniature circuit breakers at all, and still won't find that very often on any 600 volt or less equipment for general use.
Click to expand...

General rule is in 110.14(C). With today's equipment, and for past 20-30 years for that matter, most equipment has 75C terminations. Only time you need to use 60C ampacity is with some older equipment, NM cable, some cases with SE cable, or if you are dealing with older conductors such as type TW.
Click to expand...

Yes, Agree 100%. No disagreement there.

90C -you won't find those on typical miniature circuit breakers at all, and still won't find that very often on any 600 volt or less equipment for general use.
Click to expand...
Agree. I only used the 90 degree rating as an example for using 30 amps for the ampere rating for #12 wire. (As it relates to Art. 440)
 
Jamesco said:
Yes, Agree 100%. No disagreement there.


Agree. I only used the 90 degree rating as an example for using 30 amps for the ampere rating for #12 wire. (As it relates to Art. 440)
Click to expand...

In this case the conductors only need to be rated for 21.7 amps with a 30 amp OCPD.
 
One common applications that may seem somewhat extreme to some is a 240 volt 3 hp motor application. NEC FLA is 17A - 125% is 21.25 A - still under 25 amp rating of 12 AWG. Short circuit/ground fault protection with an inverse time breaker can be as much as 2.5 times 17A- next standard size up allowed gives you 45 amp breaker allowed on that 12 AWG conductor, if it for some reason won't hold during starting it can be even higher setting and still be on 12 AWG conductor.
 
Yes I knew that . I thought it changed . Wasn't #12 rated for 30A a few cycles ago? That's when I thought it changed.
What about the heat strips? A nameplate with MCA at 21 amps one would have to use # 10, correct?
 
Last edited:
Buck Parrish said:
Yes I knew that . I thought it changed . Wasn't #12 rated for 30A a few cycles ago? That's when I thought it changed.
What about the heat strips? A nameplate with MCA at 21 amps one would have to use # 10, correct?
Click to expand...

There was a change, but it was for #12 &14 in the 60 degree column. 2008 has #14 and #12 60 degree at 20 and 25 respectively. 2014 has them each 5 lower. I am not sure the reason.
 
Status
Not open for further replies.
Top