Heating and AC on shared circuit

ike5547

Senior Member
Location
Chico, CA
Occupation
Electrician
You have a small outbuilding and you want to install (1) 1/2 ton wall air conditioner and (1) 1200 watt wall insert heater and connect both of these to a single 120v, 15 amp circuit. You install a double throw switch that permits only one or the other of these loads to run at any given time. Never simultaneously.

Is this permissible anywhere in the NEC?

This is for a 100 sq. ft. dog house w/ 8 ft. ceiling (not my idea).
 

ike5547

Senior Member
Location
Chico, CA
Occupation
Electrician
Or how about 440.62(C)

Yep. That's the one. Answered my own question. Sorry to waste anyone's time.
 

ike5547

Senior Member
Location
Chico, CA
Occupation
Electrician
A standard 3-way switch would suffice. Or just plug and unplug each as needed.
I'll see if they want to dispense with a "center off" position and use a 3-way. A lot cheaper. Thanks.

I'm not sure that attachment plugs would qualify as an "interlock" for the purposes of 440.62(C)
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I'll see if they want to dispense with a "center off" position and use a 3-way. A lot cheaper. Thanks.
If you want a "master off" switch, wire a single-pole switch ahead of the 3-way switch.

For a 1-gang box, use a 1p/3p combo switch. Mount it horizontally for up/down toggles.
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Engineer
The respective thermostats will prevent them from running simultaneously, and if they ever do fail and turn on both the heating & cooling simultaneously, you probably want the breaker to shut them both off.
 

ike5547

Senior Member
Location
Chico, CA
Occupation
Electrician
that's what I was thinking, I don't know the code section handy but it's an article 220. My recollection of the wording is that there is no requirement that they be interlocked.
That part of the code is discussing load calculations. That's it. Not installation methods.
 

ike5547

Senior Member
Location
Chico, CA
Occupation
Electrician
The respective thermostats will prevent them from running simultaneously
The term "interlocked" is not defined in Article 100 but I would be shocked if a local inspector here would interpret it so loosely. When I think of interlocking circuits I think it's by some mechanical means.

My only question was what section of the code would permit the scenario I described, if any.
 
The term "interlocked" is not defined in Article 100 but I would be shocked if a local inspector here would interpret it so loosely. When I think of interlocking circuits I think it's by some mechanical means.

My only question was what section of the code would permit the scenario I described, if any.
220.60. my point was that they is the word "unlikely" not "impossible". To me that means they do not have to be mechanically interlocked. But of course these are undefined words so you can interpret it whatever way you want.

Note 220.60 is for feeders or services not branch circuits.

IMO a mechanical interlock looked a switch on fine and I would see no reason to count both loads.
 

Greg1707

Senior Member
Location
Alexandria, VA
Occupation
Business owner Electrical contractor
You have a small outbuilding and you want to install (1) 1/2 ton wall air conditioner and (1) 1200 watt wall insert heater and connect both of these to a single 120v, 15 amp circuit. You install a double throw switch that permits only one or the other of these loads to run at any given time. Never simultaneously.

Is this permissible anywhere in the NEC?

This is for a 100 sq. ft. dog house w/ 8 ft. ceiling (not my idea).
In my neighborhood there are thousands of apartments that have base board heaters installed directly below through the wall AC units. The heaters are hardwired and the AC units have a cord an plug. Both share the same circuit.
 

ike5547

Senior Member
Location
Chico, CA
Occupation
Electrician
In my neighborhood there are thousands of apartments that have base board heaters installed directly below through the wall AC units. The heaters are hardwired and the AC units have a cord an plug. Both share the same circuit.
Okay, thanks.
 

mikeames

Senior Member
Location
Germantown MD
Occupation
Teacher - Master Electrician - 2017 NEC
Or how about 440.62(C)

Yep. That's the one. Answered my own question. Sorry to waste anyone's time.
Yep you nailed it. I agree a standard 3-way is best. What would the value be with a center off position where nothing works? IMHO if it were my place I would prefer the 3-way over a center off. Cheaper and less confusing with no lost functionality.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
In real life, an 8,000 BTU A/C unit and a 1200 watt heater would exist fine together on a 15 amp circuit with no other loads. I don't see where there would need to be any extra switching.
Unless of course:
Summer heat a year or so ago. We have two thermostats, one for cove heating and one for A/C. Just what it is.
Office help had them both on. IDK why I thought to look, or how long it had been that way.
 
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