I currently have a 100 amp panel (square D) with no free slots. I'm looking at adding an electric heater (1000 watts, 240V) to the washroom.
I'm wondering if I can replace the 30 amp breaker for the dryer (24 amp, 3 wire + ground) with a 40 amp breaker and run wiring from the dryer receptacle to the heater. I would use 8 ga. wire for both lengths (from breaker to dryer receptacle and from dryer receptacle to heater).
Electric heaters are cheap. A 240V 1000W heater is worth about $40. You could easily spend more than that on exotic breakers and hookups. Sell it on Craigslist and get a 120V 1000W heater.
If you hardwire it, either put it on a dedicated 15A circuit, or a shared 20A circuit. A hardwired load which shares a circuit with receptacles must be less than 50% of the circuit's capacity (2400W, which is why you can't do it on a 15A 1800W circuit unless you downsize the heater.)
No. You are not allowed to put more than one receptacle on a branch circuit. NEC 210.23.
To be more precise, 210.23 calls out the exceptions where you are allowed to have multiple receptacles on a circuit, and it lists them by ampere rating. For residential, only cooking appliances for a 40-50A circuit.
Each receptacle must have the same rating as the circuit, i.e. you can't put a 30A receptacle on a 40A circuit. (NEC 210.21). The exceptions are 15A receptacles are allowed on 20A circuits... and 50A receptacles on 40A circuits (because no 40A receptacle standard exists).
The usual way to solve a "box cram" problem is a quadriplex breaker that puts two 240V 2-pole breakers in two spaces. Quadriplex may not be available in 3/4" wide breakers. But you don‘t need them to be. You can get two plain 120V duplex breakers and use them for four of your 120V circuits. That will free up two spaces for your 2-pole breaker.
Given the criterion of using the on-hand parts (240V/40A breaker and heater), sub-panel is the way to go. Put in a 240V 40A sub-panel and put in any circuits you please, does not necessarily have to include the dryer. In fact I wouldn't put the dryer in there. I'd move over as many 15-20A 120V circuits as I possibly could, to free up spaces in your main panel. You could certainly put at least six 15A circuits - I'm not familiar with the rules on panel fill. You could even put this sub-panel right next to your main panel to make it easier to move circuits.
Do not be bashful about buying a large sub-panel. Cheaping out and getting a panel with too few spaces is how you got in this pickle in the first place. You might even think about planning the sub-panel so it could become your future main panel.
It is totally OK to buy a sub-panel which is rated at a larger amperage than you are using. It is also OK if your sub-panel has a "main" breaker larger than your 40A breaker. That main breaker will simply be redundant.
Your current plan is no good for two reasons. First, you can't put a 30A receptacle on a 40A branch circuit as per 210.21(B)(3). Then, even if you wired up your dryer to a 50A plug and plugged it into a 50A receptacle, it'd still be against code as 210.23(C) prohibits non-cooking appliances on 40/50A circuits in dwelling units.
(Costs assume you already have the 8AWG wire and the 240V/1kW heater budgeted for, as well as misc parts such as j-boxes, wirenuts, etal. "Wire" is assumed to be 12/2 W/G NM-B -- that's $25 for 50' through the orange borg.)
Your options are as follows:
Cost: $125 for breaker + wire + heater
You'll need a QOT2020 breaker replacing an existing QO120 in your panel (or you can use a QOT1520 to replace a QO115, instead), as well as 12/2 wire (not hard) and a suitable heater, such as this Cadet unit from the orange borg. Pay attention to the labeling and design of your panel, though -- many QO panels reject tandem breakers altogether, and those that don't often limit the number of slots that accept them (to avoid violating the old 42 circuit rule from the NEC).
Cost: $100 for the weirdo breaker + $25 for wire
There is a 30/20A quadruplex-ish breaker available for the QO line -- it's the QO20303020. However, you'll have to have a supply house custom order the kit for you or order it online yourself (it's 2 tandem breakers and a handle tie), and again, it may not be within the UL listing of your panel as not all QO panels are listed to accept tandems, and those that are often can only accept them in certain breaker positions.
Cost: $30 or $45 for breakers, $25 for wire, $25 for subpanel, and lots more work
You'll need at least two breakers for this, namely a QO240 and a QO215. Replacing the existing QO230 is another $15, albeit not really necessary -- QOs are reliable as far as circuit breaker reliability goes. The subpanel will be a QO612L100 -- get a F suffix if you want a flushmount, S (linked) if you want surface mount, DF or DS if you want the above but with a door.
This requires you to pull the existing QO230 out of the main panel and install a QO240 in its place, run the 8AWG you have planned from the main to the sub, install the subpanel with the 8AWG hots to the main lugs and the 8AWG ground to the ground bar main lug -- you will not be pulling the bonding screw out of this sub as you'll be using wirenuts for the neutral connection instead.
The 12AWG (or 14AWG if you'd prefer) wire for the heater goes to the two hots on the QO215 (tape or paint the white wire black here to mark it as a hot), and its ground also goes to the ground bar. Finally, the two hots on the existing 10(?)AWG to the dryer that is rerouted to meet the subpanel go to the two hots on the QO230 of course, the ground from that to the ground bar, and the neutral from the dryer goes into a wirenut with the incoming neutral (nobody says you can't make the neutral and ground connections in a panel in some other way -- the bars are just there because they're convenient).
