Baseboard heater breaker size calculator

How the breaker and the circuits are worked

A baseboard heater is a resistor, so its current follows from its watts and the voltage across it. The conductors are sized at 125 percent of that current, which is the same as loading the breaker to 80 percent, and the breaker is the next standard size up. Everything else on the page, from watts per circuit to heaters per circuit, comes from those two steps.

Current

I = W x V / Vr^2For a heater on its rated voltage this is W / V. A 750 W heater at 240 V draws 3.13 A, and the five of the default example draw 15.63 A. Vr is the voltage on the nameplate and V the supply.

240 V heater on 208 V

W208 = W240 x (208 / 240)^2 = 0.751 x W240The resistance of the element stays the same, so watts fall with the square of the voltage. A 1,500 W heater makes 1,126.7 W on 208 V, and the Marley/Qmark catalog prints 1,128 W. On 120 V it would make 25 percent, 375 W.

Conductor ampacity

A = 1.25 x INEC 424.4(B) sizes the branch-circuit conductors for fixed electric space heating at no less than 125 percent of the load. The default example needs 19.53 A of conductor.

Breaker

smallest standard size at or above 1.25 x I, not below 15 AA heater runs three hours or more, a continuous load by the NEC Article 100 definition, and NEC 2017 424.3(B) says so by name. NEC 210.20(A) rates the breaker on a continuous load at no less than 125 percent of it. Standard sizes are from NEC Table 240.6(A). The default example needs a 20 A breaker.

Watts per circuit

W max = breaker x V / 1.25At 240 V a 15, 20, 25 and 30 A circuit carries 2,880, 3,840, 4,800 and 5,760 W. At 120 V it carries half as much, and at 208 V 3,328 W on 20 A.

Heaters per circuit

n = whole number of (0.8 x breaker / I)Heaters cannot be split, so the count rounds down. 5 heaters of 750 W fit on 20 A at 240 V, 3 of 1,000 W and 2 of 1,500 W.

Circuits

circuit rating of 30 A or less when it feeds two or more heatersNEC 2017 424.3(A) and 2020 424.4(A) limit a circuit that feeds more than one heater to 30 A. Eight 1,000 W heaters at 240 V need 2 circuits of 30 A, or 3 of 20 A.

Eight jobs through the same formula

The first three rows are the heater lists in a Mike Holt forum thread, and the page gives the same circuits. The others show what 120 V, a 208 V supply or a bigger load does to the breaker.

Watts set the circuit, not the number of heaters. On 20 A at 240 V, 5 heaters of 750 W fit, 3 of 1,000 W, 2 of 1,500 W and 1 of 2,000 W. Three 2,000 W heaters need 2 circuits, because one circuit would need 35 A and a circuit with more than one heater is held to 30 A.

Standards and data sources

• NEC 2023, Article 424 (424.4), Table 240.6(A), Table 310.16 and 240.4(D) - the 125 percent conductor rule of 424.4(B), the 125 percent breaker rule of 210.20(A) for a continuous load, the rule that one circuit may feed a heater of any rating it is rated for, the 30 A limit for a circuit that feeds two or more heaters in the 2017 and 2020 editions, the standard breaker sizes and the small-conductor limits. The 2023 wording of the 30 A sentence was not confirmed.
• Mike Holt forum threads on baseboard circuits, 2013 and 2016 - the 3,840 W limit of a 20 A circuit and 2,880 W of a 15 A circuit at 240 V, and the five 750 W heater example this page opens on. The same forum shows the 424.3(A) and 424.4(A) text as members pasted it.
• Cadet Electric Baseboard Heater Owner's Guide and King Electric, Sizing the Heater Circuit - the heater makers' limit of 80 percent of the breaker or the thermostat rating, whichever is lower, and King's table of heater current and breaker, for example 4,000 W at 240 V on a 25 A breaker.
• Marley/Qmark baseboard heater catalog, 2500 series - watts printed at 240 and 208 V, which the 208 V derating matches to within 0.2 percent, and the 22 A line-voltage thermostats.

Limitations of this calculator

• This page gives preliminary numbers for estimating and for checking a quote. A licensed electrician sizes and installs the circuit under the permit and the inspection of the local authority having jurisdiction (AHJ). Nothing here is an approval of an installation.
• The heater's nameplate and installation instructions come first. They can set a smaller circuit, a particular wire, or a thermostat rating that is lower than the breaker allows, and listed equipment is installed according to its listing.
• The breaker is sized at 125 percent of the heater load because NEC 210.20(A) requires it for a continuous load. A heater is one by the Article 100 definition of three hours or more, and the 2017 edition (424.3(B)) says so by name; the later wording of that sentence was not confirmed. The exception is a breaker and panel listed for 100 percent operation, which residential panel breakers are not, so this page does not use it.
• A circuit that feeds two or more heaters is held to 30 A, as in the 2017 edition (424.3(A), a list of 15, 20, 25 or 30 A) and the 2020 edition (424.4(A), not over 30 A). The 2023 text of 424.4(A), as a member pasted it into a forum, shows only the sentence that lets an individual circuit feed any rating, so whether the 30 A sentence moved or was dropped was not confirmed. This page keeps 30 A, the more cautious reading. Check the edition your AHJ adopted.
• A 240 V heater on a 208 V supply is derated by the square of the voltage ratio. Real elements change resistance a little with temperature, so a maker's printed 208 V watts can differ by a fraction of a percent. A heater made for 208 V is better entered at 208 V with the nameplate watts.
• Wire sizes are for copper and aluminum at 86°F with two current-carrying conductors and 90°C (194°F) insulation, for a circuit loaded to its limit. Cable type, a hot attic, more conductors in the conduit and the length of the run change them. Use the AC wire size calculator for terminal ratings and corrections, and the voltage drop calculator for a long run.
• The page works single phase at 120, 208 and 240 V, up to two heater sizes, and heaters from 100 to 4,000 W each. Three phase, 277 V and 480 V commercial heaters, hydronic baseboard and cord-and-plug heaters are not covered.
• The page does not say how many watts a room needs, and it does not apply the demand factors that a service or feeder calculation may use. Size the heat from the room's load, and have the service checked by an electrician.
• A line-voltage thermostat or relay in the circuit has its own amp rating, and the lower of that and the breaker limit governs. Low-voltage controls and smart thermostats that switch a heater through a relay are not worked here.
• The tables are from the 2023 edition of the NEC where it was read. Your state or city may enforce another edition or amend it.

Common mistakes with baseboard heater circuits

1. Sizing the breaker at the running current. A 1,500 W heater at 120 V draws 12.50 A, which looks fine on a 15 A breaker. At 125 percent it is 15.63 A, so the breaker is 20 A. The same heater at 240 V draws 6.25 A and takes a 15 A double-pole breaker.
2. Counting watts at the wrong voltage. A 20 A circuit carries 3,840 W at 240 V and only 1,920 W at 120 V. A plan that fits 5 heaters of 750 W on a 240 V circuit fits 2 on a 120 V circuit of the same size.
3. Counting heaters instead of watts. Three heaters to a circuit works for 750 W heaters and fails for 2,000 W ones. Three 2,000 W heaters draw 25.00 A at 240 V and would need 35 A, over the 30 A limit for a circuit with more than one heater. They need 2 circuits.
4. Expecting full heat from a 240 V heater on 208 V. A 2,000 W, 240 V heater makes 1,502.2 W on a 208 V supply, about 75 percent. The current is lower too, but the room gets less heat. Size the heater count for the 208 V watts, or buy 208 V heaters.
5. Raising the breaker to stop nuisance trips. A breaker that trips under a steady heater load may be carrying too much, since the limit is 80 percent of its rating. A 25 A breaker on 12 AWG copper is not allowed, because NEC 240.4(D) caps 12 AWG at 20 A. A 25 A circuit needs 10 AWG copper, a 20 A circuit 12 AWG and a 15 A circuit 14 AWG. Move a heater to another circuit, or size the circuit up from the start.
6. Wiring a 240 V heater to a 120 V circuit. A 240 V element on 120 V gets half the voltage and makes a quarter of its watts: a 1,500 W heater gives 375 W. The room stays cold and the load looks small. Read the voltage on the nameplate before sizing anything.

Common questions about baseboard heater breakers

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Used on the same job

• Room addition BTU calculator How much heat the room needs, before the heaters and the circuit.
• AC wire size calculator The conductor for the breaker this page gives, with corrections.
• Voltage drop calculator Whether the wire is big enough for the length of the run.