Can I Install Four Baseboard Heaters on a 20 Amp Double Pole Breaker?
You can, but only if the four heaters add up to 3,840 watts or less at 240 volts. Section 424.3(B) requires fixed electric space heating equipment to be treated as a continuous load, which means the branch circuit conductors and the overcurrent device are sized at not less than 125 percent of the total heating load. Run that backward and a 20 ampere breaker gives you 16 amperes of usable load, or 3,840 watts.
The math on a 20 amp 240 volt heater circuit
Twenty amperes times 240 volts is 4,800 watts of nameplate breaker capacity. Multiply by 80 percent, which is the inverse of the 125 percent continuous factor, and you get 3,840 watts. That is the hard ceiling for everything connected to that circuit.
Four heaters at 1,000 watts each is 4,000 watts. That exceeds the limit and it is a violation, even though the breaker will probably hold on a mild day. Four heaters at 750 watts each is 3,000 watts, which fits with room to spare. Three heaters at 1,250 watts is 3,750 watts and also fits. Baseboard is commonly sold at 250 watts per foot, so a 4 foot unit is 1,000 watts and a 6 foot unit is 1,500 watts. Two 6 foot units already puts you at 3,000 watts, and a third one blows past the limit.
Conductors, breaker, and thermostat
A 20 ampere circuit on 12 AWG copper is the standard build. Section 240.4(D)(5) caps 12 AWG copper at 20 amperes regardless of what the ampacity table says, so 12 AWG and a 20 ampere two pole breaker go together. If you need more than 3,840 watts, the next step is 10 AWG copper on a 30 ampere two pole breaker, which gives 30 times 240 times 0.8, or 5,760 watts.
The thermostat has to match. A line voltage thermostat carries the full heater current, and its rating must equal or exceed the connected load per 110.3(B) and 424.20. A double pole thermostat is required where both ungrounded conductors need to be opened, and 424.19 requires the disconnecting means to open all ungrounded conductors for a 240 volt heater. A single pole thermostat leaves one leg energized at the heater, which is legal in limited cases but is a shock hazard for the next person servicing it.
Two more rules that get missed
Section 210.11 and 220.51 govern how the heating load enters the service calculation, and fixed space heating is calculated at 100 percent of the connected load unless the optional method in 220.82 applies. Adding 3,840 watts of heat to an existing panel is 16 amperes on the service that has to be accounted for.
Section 210.52 also matters in bedrooms and living areas, because baseboard heaters cannot be installed below a receptacle outlet unless the manufacturer permits it. Most baseboard instructions explicitly prohibit a receptacle above the unit, and the receptacle spacing in 210.52(A) still has to be satisfied, so the heater layout and the receptacle layout have to be planned together rather than fought over after the drywall is up.