10KW heat pump

[Jay Bohne]

Quick question:
Changing out a new heat pump from 9.8KW to 10KW...data plate says 9.6 KW on the heater coil...do i do the math with 10 or 9.6 to size the wire?
10000/240=41.666x125%=52.08 amps=#6awg@75degreeTHHW
9600/240=40x125%=50amps=#8awg@75degreeTHHW

Am I a knucklehead and should start doing plumbing instead of electrical? :smile:

 

[augie47]

1st off, I'd foget the math. Often the blower motor is on with the heating section so the ampacity of the heater isn't totaly realtive.
NEC 440.4 prescribes for the unit to have a nameplate providing you with the MCA and MOCP of the circuits required.
That should, IMHO, be your guide.
In regard to your calculations using 75o, don't forget NM and SE (depending on Code) have a 60o limitation.

good luck with your plumbing :smile:

 

[LarryFine]

Jay, you may be a bonehead, which means you'll fit right in around here. :grin:

I routinely supply 60a-capable circuits for 9.6/10kw furnaces, rather than see if we can squeak by with smaller.

Nobody has ever had an issue with us using #6 NM for these circuits. 55a @ 60?, and a 60a breaker.

But, yes, if the unit is marked with MCA and MOCP numbers, they reign supreme.

 

[Dennis Alwon]

Quick question:
Changing out a new heat pump from 9.8KW to 10KW...data plate says 9.6 KW on the heater coil...do i do the math with 10 or 9.6 to size the wire?
10000/240=41.666x125%=52.08 amps=#6awg@75degreeTHHW
9600/240=40x125%=50amps=#8awg@75degreeTHHW

Am I a knucklehead and should start doing plumbing instead of electrical? :smile:

Ok knucklehead:grin:-- As others have said you use the nameplate on the unit. I have never seen a so called 10 KW unit that had 10 kw of heat. They usually have 9.XXX. Meaning 9.6 or 9.8. Also the fan enters into the equation.

As of 2008 if you use SE cable you must use the 60C rating not the 75C rating that SE use to have.

Thus #6 NM or SE cable is usually okay but I have seen units that draw 56 amps in which case you would have to go to #4.

Make sure the heating guy marks the unit as to what size heater is in there. I have been red tagged because of that. If the unit is not there I would call him and ask for the mca OF THE UNIT.

 

[augie47]

Dennis, that has been my experience on heat-pumps also.
75-80% of the units I checked have a MCA slightly less than 55.
That other percentile does often present a major problem on finals when#6 NM was used. (and , if TN adopts '08, the answer of using SE will go by the wayside)

 

[Jay Bohne]

thanks guys...one more thing...i see that the #6 is generally what is needed...the 60 degree is the minimum right? what if i use thhn at 90 degree? isnt that better?

 

[LarryFine]

thanks guys...one more thing...i see that the #6 is generally what is needed...the 60 degree is the minimum right? what if i use thhn at 90 degree? isnt that better?

Of course.

 

[billsnuff]

as long as the termination temp is considered..75 deg?

 

[Jay Bohne]

nice...so that means if i use thhn-90 degree, then i could use #8? that is the whole question...

 

[Jay Bohne]

thanks for the feedback gents! yes the nameplate is 9.6K but i have a whole bunch of #8 thhn so i wanted to use it...the existing wire is too short for the relocate.

 

[Dennis Alwon]

Wait a minute. #8 thhn is only good for 50 amps and that means it cannot be part of an se or nm cable.

I would not use #8 for this install. My bet is #6 will be required. Again what does the nameplate require. I bet it is more than 50 amps. Remember #8 can only be used at the 75C because the terminations are only rated 75C. It cannot be used at 90C except for derating purposes.

 

[peter d]

What's a heat pump?

 

[Dennis Alwon]

What's a heat pump?

That's for us southerners. You northerners don't need to know that stuff...:grin:

 

[peter d]

You northerners don't need to know that stuff...:grin:

Yeah, just like you don't need to know what an oil burner is, or a snow blower.

 

[stickboy1375]

Yeah, just like you don't need to know what an oil burner is, or a snow blower.

snow plow, snow shovel, snow day, ( my favorite by the way) snow storm, etc, etc, etc...