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I am designing the electrical for a new-construction home. Targeting NEC 2017. I have two bathrooms, each of which will have an in-wall heater. The heaters run on 240V and draw 8 amps each. I would like to run one dedicated circuit (20 amp 240) to both of these heaters. Is that allowed by code?

These units will be hardwired, and that part of the code is confusing me because it appears each hardwired appliance must have its own dedicated circuit.

  • Do the installation instructions of the heaters say they require a dedicated circuit? – JPhi1618 Jan 23 at 18:30
  • I havent purchased the heaters yet. Foundation is in, but framing wont start for a few months still. I can try to find the instructions online. – Brad Jan 23 at 18:34
  • Found them: stiebel-eltron-usa.com/sites/default/files/pdf/… But I dont see anything that says a dedicated circuit is required. It has min specs for breaker size, gauge of wire, etc. So I think its safe to assume the manufacturer doesnt require it. My question was more to whether NEC requires it. – Brad Jan 23 at 18:38
  • Something's ringing a bell about heaters being installed on one circuit providing they don't take up more than 50% of the rated circuit. So this might be a no go for you. – JACK Jan 23 at 18:44
  • @JACK, yes, but I believe that is if the circuit is shared with either outlets or lights (cant remember). The circuit I am proposing would be dedicated to just these two heaters. – Brad Jan 23 at 18:45
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No, every hardwired appliance does not need to be on its own circuit. But...

Provision power for heaters with a 125% derate

You need to study the unit's specs carefully, and note the amps or VA drawn by it (note a resistive electric heating element will have VA identical to Watts, but a fan motor may knock that off a little). Then, multiply that figure by 125% (8A -> 10A). The +125% number is how much power you must provison (allocate) on that circuit.

So for instance if you have two heaters that are 8.1 amps each. Those derate to 10.125 amps. Two of them would be 20.25 amps, which is too much for a 20A circuit. Most heater manufacturers understand this intimately, and will size their heaters to be just under a threshold number, i.e. not 8.1A :)

Also beware of heaters whose spec rating is at 125V instead of 120V. Due to Ohm's Law, a reduction in voltage causes an equal reduction in amperage. So if you have an 8A heater @125V, then you have to multiply by (120/125) to get the 120V amperage.

An interesting idea

Oftentimes, people speccing power for bathrooms and kitchens will be thinking only about the Code book (which is complicated enough), and forget that the main user of a bathroom is people who do a lot of "Beauty stuff" in the bathroom with heat appliances, often several. For instance Code allows one single 120V/20A circuit to serve all receptacles in all bathrooms, and you know the men who specced that didn't have a wife and teenage daughter! That's because Code is written by the National Fire Prevention Association not by the National Hair Care Association.

Well, look at the rules.

  • First, it is legal to have 240V and 120V loads on the same (Multi-Wire Branch) circuit.
  • Second, it is legal to have receptacles on a circuit with hardwired things, if the hardwired things draw less than 50% of rated circuit amps.
  • Third, it's legal to put receps and non-recep loads on the same circuit if they're all in the same bathroom.

So, if the heater provisions out to <=10 amps, I would be inclined to run a 4-wire circuit for each heater. Then, continue the circuit to a 2-gang box containing two GFCI receps, wired in multi-wire branch circuit fashion - sharing the neutral, and one hot per recep. Now, you can plug a curler and hair dryer in at the same time! (you'd be kissing 20A if the heater was on also, but the user could turn off the heater while using the appliances - they'll be making plenty of heat of their own.) So for about $30 extra, you've made the beauty enthusiasts in the house very happy.

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  • This answers if the circuit can handle the load, but the question is really, does NEC allow it? I am reading some language that says every hard-wired appliance must be on its own circuit. But its not clear to me if they mean each appliance individually, or that the circuit cannot be shared with outlets/lights/etc. For your interesting idea, agreed. We're running 20 amp circuits to each outlet in the bathroom (2x each). :) – Brad Jan 23 at 22:19
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    Good point, edited. – Harper - Reinstate Monica Jan 23 at 22:59
  • I'm not a big fan of MWBCs. Yeah they can save a 12-2 run, but the cost difference between 12-2 and 12-3 is so small that you aren't saving anything. Recent Home Depot price for 250' of 12-2 is $62.74, 250' of 12-3 is $120. Then you add the complexity of a MWBC, future risk of somebody putting both hots on the same leg and overloading the neutral , arc fault breakers tied with handle ties. Which arc fault breaker gets the neutral? How does that work? To me it's a lot of additional complexity with little benefit. Am I wrong? – George Anderson Jan 24 at 3:37
  • @GeorgeAnderson I respect MWBCs but I'm not putting any new ones in. Recently took a pipe with 6 hots and 2 neutrals and made it all regular circuits; in fairness one hot was a switched-hot, but still, facepalm... However there are sweet spots where MWBC makes sense, like OP. You have to follow the rules of MWBC: pigtail neutral and preferably a 2-pole breaker. So a 2-pole AFCI and you're all set (unless you're in GE, but those don't have common trip which OP would need). If some jackass converts a 2-pole to a duplex, sorry, that's on them. Also in OP's case the heater won't work. . – Harper - Reinstate Monica Jan 24 at 3:51
  • I get most of the acronyms here, but I'm slightly ashamed to ask what OP means??? – George Anderson Jan 24 at 3:55
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NEC Section 424.3(A) Branch Circuit Requirements. ....Branch circuits supplying two or more outlets for fixed electric space-heating shall be rated not more than 30 amperes....

NEC 424.3(B)Branch circuit sizing. Fixed...shall be considered continuous load.

Continuous load means load no more than 80% of circuit rating, so a 20A circuit can't be more than 16A. You should be good to go.

(The NEC definition of "outlet" is any point of connection of utilization equipment, a "receptacle" is an outlet, a outlet is not necessarily a receptacle.)

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