[bernasj]

I have an electric unit heater given to a friend of mine who does not mess with things electrical. He wishes to install this in his garage. Although the nameplate is obliterated, I was able with optical aid to pull a model no. from it. It is A Qmark Model MUH072. No serial, no voltages, no current specs. It is wired 240 1Phase. Finding info online puts this unit at 31.3 amps at 240v. Wiring it in my garage shows that the unit is drawing 28.5 amps/leg with the fan motor on also. My question is, since I expect this unit to be considered continuous, If I run #10 thhn, and a 30 amp 2 pole am I doing things incorrectly due to NEC. 240.4 (D) (7). Thanks for any help.

 

[seharper]

31.3 amps on the nameplate? That is a VERY deliberate choice by the builder.

Since we are provisioning a heater, we must provision power for 125% of the actual load.

125% of 31.3 amps is *calculate figure headscratch* 39.125 amps.

We must now round UP to the next available size of breaker. Oh look, 40A is a standard size. How nicely that worked out! (not by accident. The builder anticipated this).

So you must provision 40A to this heater, and you use #8 wire. The breaker is 40A. Open and shut case.

As far as 240.4(D), you're stuck with that in any case. You cannot find any wires or terminations on the market capable of 40A on any wire smaller than #8. "But #10 is allowed [email protected] according to 310.15(B)(16)!!" Great kid, but that requires 90C terminations, and where will you find those? (Nowhere.)

Also, if you're looking at a proper 310.15(B)(16) it will list the wire types to which this rating applies. Yours isn't on the list. NM-B and UF-B are only allowed to use the 90C column for derate calculations such as 310.15(B)(3)(A) - you are still stuck at 60C with them.

 

[bernasj]

Thank you Sir.

 

[bernasj]

I just wanted to come back and thank seharper again for the information. As I dived into the Nec. concerning wire sizing, ampacity, breakers, fixed electric space heating and related nec. suggestions, I had so many light bulbs illuminate that I don't need to operate lights when I go out to the garage.