10kw Heat strip amp cals and the NEC

[mattlikesbikes]

Hi,

I am hoping y'all can help me figure out a he said/he said issue. I am having a heat pump installed at my house and when I picked the unit the AC guy told me to tell the electrician to wire for a 10kw system. The electrician goes ahead and runs a #8 line. The unit shows up and is stamped that it needs 60amps, which is a #6 line. AC guy says they have not seen a 50amp 10kw system in a long time. Electrician says that a 60 amp system is a 15kw system in secret.

I'm an engineer and know enough about electricity to hire a professional, but when I look at the NEC guidance and the tables I have trouble figuring out where my professional came up with only running a #8 for a 10kw system. When I do the math I get 52.5 amps without the blower (10,000/240 *125% = 52.5). Already the peak is above 50 amps and you don't have 2-4 amps for a blower, which clearly kick you to a 60 amp #6. At the very least isn't the continuous load on a 10kw system over 80% of 50 amps (42 amps) and thus by NEC you have to go up to 60 amp?

Ultimately I think that my electrician is just using old and maybe fuzzy math. He is running the #6 as we speak, but I've got to figure out if this is an electrician I want doing my next remodel (we just did floor 1 of our 2 story duplex), want to recommend, etc.

 

[Dennis Alwon]

I have never seen a 10 kw that doesn't require a #6 or in some cases a #4 copper conductor. If you are using nm then the wire is rated 60C and a #6 is good for 55 amps. Some of the units have a MCA of 56 or 57 amps.

 

[Dennis Alwon]

Be careful as usually a 10kw furnace only has 9,6kw of heat but even so a 60 is needed. Check the nameplate on the heaters. If they installed a 7.**kw heater in there it will be fine.

 

[augie47]

Two or three things you need to keep in mind.
As Dennis states, "10kw" heat strips are often actually 9.6 kw.
424.3 states that fixed electric space heating equipment SHALL be considered a continuous load, so with no other loads considered you have 9.6/240 x 1.25 or 50 amps.
When installing NM cable, 334.80 dictates that the load ampacity shall be calculated at the 60? rating, or for #8, 40 amps, so, as stated, you start out in a hole before even thinking of additional loads.