NEC Article 424 (Fixed Electric Space Heating)

Heat Strip Wire Size and Breaker

The circuit for the electric heat strips in an air handler, at 208 V and 240 V: the plate first, then the Article 424 arithmetic with the blower inside the 125%.

Field brief

The plate first if the plate prints a minimum circuit ampacity and a maximum breaker, those govern (NEC 110.3(B)). MCA · max breaker

10 kW at 240 V otherwise a 10 kW strip at 240 V draws 41.7 A, 52.1 A at 125% (NEC 424.4(B)): a 60 A breaker, #6 AWG copper (60 °C or 75 °C), plus the blower's amps. 60 A breaker

10 kW at 208 V a strip rated 10 kW at 208 V draws 48.1 A, over 48 A, so the unit subdivides its elements (424.22(B)). subdivided

Size Your Heat Strip Circuit

Enter the plate's minimum circuit ampacity and maximum breaker, or, for a unit marked only in volts and watts, the strip heat at the voltage it runs on and the blower's amps. It opens on a 10 kW strip at 240 V.

What the unit's plate prints for this circuit

A listed air handler or furnace that prints a minimum circuit ampacity and maximum breaker for each circuit is sized to those (NEC 110.3(B)). Pick the first option only for a unit marked in volts and watts or amps.

Supply voltage

The voltage the unit runs on. Enter its rating at this voltage: a strip rated at 240 V is not the same kW on 208 V.

Heat entered as

Electric heat (kW at 240 V)

Blower motor amps from the plate (if it runs on this circuit)

NEC 424.4(B) sizes the conductor at 125% of the heater and any associated motor. Left empty, the answer is for the elements only and says so.

10 kW at 240 V (41.7 A): 52.1 A at 125%, a 60 A breaker, #6 AWG copper (60 °C or 75 °C).

The conductor carries not less than 125 percent of the heater (NEC 424.4(B)); the breaker is the next standard size at or above that (240.6(A)); the conductor is then the smallest the breaker protects, with the next-size-up rule of 240.4(B) where its ampacity is not a standard rating.
Heating elements only: add the blower motor's amps from the plate if it runs on this circuit, because 424.4(B) puts any associated motor inside the 125%.
If the unit's plate prints a minimum circuit ampacity and maximum breaker, those govern (NEC 110.3(B)): use the nameplate mode.
On a circuit of 100 A or less the 60 °C size applies unless the unit is listed and identified for 75 °C conductors (NEC 110.14(C)(1)(a); such a unit is marked for them, 424.11).
Table 240.4(G), which lets a motor or air-conditioning circuit's conductor sit on a larger breaker, has no Article 424 row, so the 240.4(D) limits hold: #14 on 15 A, #12 on 20 A, #10 on 30 A copper.
Equipment grounding conductor: #10 AWG copper for a 60 A breaker (NEC 250.122).
A circuit that also supplies a compressor (a packaged heat pump) is also Article 440 (424.3): size it from its plate's minimum circuit ampacity and maximum breaker with the MCA and MOCP calculator.

NEC 424.4(B) · NEC 240.6(A) · NEC 240.4(B) · NEC 240.4(D) · NEC 250.122 · NEC 110.14(C)(1)

Sized to the plate's minimum circuit ampacity and maximum breaker where it prints them (NEC 110.3(B)); otherwise at 125% of heater and blower (424.4(B)), the 240.6(A) breaker, the smallest Table 310.16 copper that breaker protects under 240.4(B) and (D) at the 110.14(C)(1) column, and the Table 250.122 ground. See the full receipt ↓

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The Plate First, Then Article 424

A listed air handler or electric furnace prints, for each field circuit, a minimum circuit ampacity and a maximum breaker, and NEC 110.3(B) makes the listing's instructions binding: the conductor carries at least that ampacity, and the breaker is never larger than that maximum. NEC 424.28(A) requires the plate to state volts and watts or amps (and a motor's rating where it is over 1/8 hp), not a minimum circuit ampacity or maximum breaker, so for a unit marked that way the rule is 424.4(B): the branch-circuit conductors carry not less than 125 percent of the heating load and any associated motor, which is the blower when it runs on that circuit. This tool picks the breaker as the next standard size at or above that 125% figure and the smallest conductor that breaker protects (240.4, with 240.4(B)'s next size up).

A heat strip circuit is not an air-conditioning circuit. Table 240.4(G), which lets a condenser's conductor sit on a larger breaker, has no Article 424 row, so NEC 240.4(D) holds #10 copper to 30 A, #12 to 20 A and #14 to 15 A here. A circuit that also supplies a compressor (a packaged heat pump) is also Article 440 (424.3); size it with the MCA and MOCP calculator for it.

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receipt

Where these heat strip circuits come from

The breaker, conductor and ground on this page are computed by the locked heat-circuit engine from the NEC 2023 sections below, and the unit's own plate comes first wherever it prints a minimum circuit ampacity and a maximum breaker. These figures live in one locked module rather than being retyped page by page, so every place this number appears is the same number. Every release runs this site's own checks against that locked module before it ships, and the source is cited above so you can hold the number to the document yourself.

NEC 110.3(B): A listed air handler or electric furnace prints, for each field circuit, a minimum circuit ampacity and a maximum breaker, and 110.3(B) makes the listing's instructions binding. So where the plate prints that pair this tool sizes to it: the conductor carries at least the minimum circuit ampacity, the breaker is never above the maximum, and each field circuit is sized from its own pair.
NEC 424.4(B): NFPA 70-2023, p. 70-323: "The branch-circuit conductor(s) ampacity shall not be less than 125 percent of the load of the fixed electric space-heating equipment and any associated motor(s)." It sizes the conductors, and its load includes the blower where the blower runs on the same circuit. It is the rule for a unit marked only in volts and watts or volts and amperes, which is what 424.28(A) has the plate state. The 125 percent is taken on the exact load and rounded only for display, never shown at or below a breaker rating the exact figure passes.
NEC 240.6(A): The breaker is the smallest standard rating at or above the 125 percent figure. 424.4(B) itself says nothing about the breaker: covering 125 percent is this tool's own conservative choice, the one 210.20(A) asks of a load that runs three hours or more.
NEC 240.4(B) and 240.4(D): The conductor is the smallest the breaker protects. The 240.4(D) limits hold #14, #12 and #10 copper to 15, 20 and 30 A, and above them a conductor whose ampacity is not a standard rating may sit on the next standard breaker above it on an individual circuit (240.4(B)). Table 240.4(G), which lets a motor or air-conditioning circuit's conductor sit on a larger breaker, has no Article 424 row (NFPA 70-2023, p. 70-124), so that relief never reaches a heat strip circuit.
NEC Table 310.16 and 110.14(C)(1): Copper ampacity in both the 60 °C and 75 °C columns, and the page gives both sizes. Which one governs is 110.14(C)(1). On a circuit of 100 A or less the 60 °C size applies unless the unit is listed and identified for 75 °C conductors (NEC 110.14(C)(1)(a); such a unit is marked for them, 424.11). Over 100 A the 75 °C column applies (NEC 110.14(C)(1)(b)).
NEC 424.22(B): NFPA 70-2023, p. 70-324: "Resistance-type heating elements in electric space-heating equipment shall be protected at not more than 60 amperes. Equipment rated more than 48 amperes and employing such elements shall have the heating elements subdivided, and each subdivided load shall not exceed 48 amperes." The 48 A line is read against the equipment's load, heater plus blower, never against the 125 percent figure. Over 48 A of equipment load, NEC 424.22(B) has the unit's heating elements subdivided at 48 A or less each, protected at 60 A or less by supplementary devices in, on or supplied with the unit (424.22(C)). Whether the unit then takes one field circuit or several is its listing: wire what the plate prints (110.3(B)).
NEC Table 250.122: Equipment grounding conductor, sized from the breaker and never from the conductor, read from the same locked table the wire-size calculator reads.
What this tool refuses: It sizes at 208 V or 240 V, the voltage the unit runs on, and never converts a rating between them: a strip rated at 240 V run on 208 V is not the 240 V row, so the plate's rating at 208 V is the one to enter. It refuses a load whose 125 percent figure is past a 400 A breaker, which is a feeder design rather than one circuit; that limit is this tool's own, not a table's. It refuses a plate whose minimum circuit ampacity and maximum breaker have no standard breaker between them. A circuit that also supplies a compressor, as in a packaged heat pump, is also Article 440 (424.3), and is sized from its plate with the MCA and MOCP calculator, not here.

The ampacities, standard ratings and grounding conductor sizes are NEC 2023 table values, looked up by the engine; the 125 percent figure, the breaker and the conductor are computed from the load you enter. The listed unit's plate governs over this arithmetic (110.3(B)), and whether the unit is listed for 75 °C conductors, for a single-point connection or for more than one field circuit is printed on that plate, not known here. The installer and the authority having jurisdiction have the final say.

Intry Verified

The breaker and copper conductor sizes in the tables and the calculator on this page are computed by the heat-strip engine: the amps from the kW rating at its voltage, 125 percent of the heater and any blower on the circuit (NEC 424.4(B)), the next standard rating (240.6(A)), the smallest conductor that breaker protects (240.4, with 240.4(B)'s next size up and 240.4(D)'s small-conductor limits) from the 60 °C and 75 °C columns of NEC Table 310.16, and the 48 A subdivision line of 424.22(B). Where the plate prints a minimum circuit ampacity and a maximum breaker, those govern (110.3(B)), and the installer is responsible for the conditions of use. Those figures are computed by a locked engine straight from the source it cites, not hand-typed, and that computation re-runs inside a deterministic test on every deploy. Source-checked. Locked. This is our own deterministic gate, not a third-party audit. What Intry Verified means

Heat Strips at 208 V (elements only)

kW rated at 208 V, copper. Amps = watts / 208 V; x1.25 (NEC 424.4(B)); breaker the next standard size (240.6(A)). Add the blower's amps before the 125%.
Strip Heat	Load Amps	At 125%	Breaker	Copper 60 °C (governs to 100 A unless marked)	Copper 75 °C (governs over 100 A)
2.5 kW	12.02 A	15.02 A	20 A	#12 AWG	#12 AWG
3 kW	14.4 A	18 A	20 A	#12 AWG	#12 AWG
4 kW	19.2 A	24 A	25 A	#10 AWG	#10 AWG
5 kW	24.04 A	30.05 A	35 A	#8 AWG	#8 AWG
6 kW	28.8 A	36.1 A	40 A	#8 AWG	#8 AWG
7.5 kW	36.1 A	45.1 A	50 A	#6 AWG	#8 AWG
8 kW	38.5 A	48.1 A	50 A	#6 AWG	#8 AWG
9 kW	43.3 A	54.1 A	60 A	#6 AWG	#6 AWG
10 kW *	48.1 A	60.1 A	70 A	#4 AWG	#6 AWG
12.5 kW *	60.1 A	75.1 A	80 A	#3 AWG	#4 AWG
15 kW *	72.1 A	90.1 A	100 A	#2 AWG	#3 AWG
18 kW *	86.5 A	108.2 A	110 A	#1 AWG	#2 AWG
20 kW *	96.2 A	120.2 A	125 A	1/0 AWG	#1 AWG
25 kW *	120.2 A	150.2 A	175 A	3/0 AWG	2/0 AWG

* Over 48 A of equipment load, NEC 424.22(B) has the unit's heating elements subdivided at 48 A or less each, protected at 60 A or less by supplementary devices in, on or supplied with the unit (424.22(C)). Whether the unit then takes one field circuit or several is its listing: wire what the plate prints (110.3(B)). The single-circuit figures on that row apply only where the unit is listed for a single-point connection. The governing column is in bold: On a circuit of 100 A or less the 60 °C size applies unless the unit is listed and identified for 75 °C conductors (NEC 110.14(C)(1)(a); such a unit is marked for them, 424.11). Over 100 A the 75 °C column applies (NEC 110.14(C)(1)(b)).

Heat Strips at 240 V (elements only)

kW rated at 240 V, copper. Amps = watts / 240 V; x1.25 (NEC 424.4(B)); breaker the next standard size (240.6(A)). Add the blower's amps before the 125%.
Strip Heat	Load Amps	At 125%	Breaker	Copper 60 °C (governs to 100 A unless marked)	Copper 75 °C (governs over 100 A)
2.5 kW	10.4 A	13 A	15 A	#14 AWG	#14 AWG
3 kW	12.5 A	15.6 A	20 A	#12 AWG	#12 AWG
4 kW	16.7 A	20.8 A	25 A	#10 AWG	#10 AWG
5 kW	20.8 A	26 A	30 A	#10 AWG	#10 AWG
6 kW	25 A	31.3 A	35 A	#8 AWG	#8 AWG
7.5 kW	31.3 A	39.1 A	40 A	#8 AWG	#8 AWG
8 kW	33.3 A	41.7 A	45 A	#6 AWG	#8 AWG
9 kW	37.5 A	46.9 A	50 A	#6 AWG	#8 AWG
10 kW	41.7 A	52.1 A	60 A	#6 AWG	#6 AWG
12.5 kW *	52.1 A	65.1 A	70 A	#4 AWG	#4 AWG
15 kW *	62.5 A	78.1 A	80 A	#3 AWG	#4 AWG
18 kW *	75 A	93.8 A	100 A	#2 AWG	#3 AWG
20 kW *	83.3 A	104.2 A	110 A	#1 AWG	#2 AWG
25 kW *	104.2 A	130.2 A	150 A	2/0 AWG	1/0 AWG

Frequently Asked Questions

What size wire and breaker for 10 kW heat strips?

If the air handler's plate prints a minimum circuit ampacity and maximum breaker, size to those (NEC 110.3(B)). For a unit marked only in volts and watts: a 10 kW strip at 240 V draws 41.7 A, and NEC 424.4(B) sizes the conductor at not less than 125 percent, 52.1 A; the next standard breaker is 60 A, and the smallest conductor it protects is #6 AWG copper (60 °C or 75 °C), before the blower. Add the blower motor's amps inside the 125% when it runs on the same circuit.

Why is a 208 V heat strip circuit different from 240 V?

Because the kW on the plate is a rating at a stated voltage, and the amps are that rating divided by the voltage. A 5 kW rating at 240 V is 20.8 A (26 A at 125%, a 30 A breaker); a 5 kW rating at 208 V is 24.04 A (30.05 A at 125%, over 30, so a 35 A breaker with #8 AWG copper (60 °C or 75 °C)). Use the rating the plate prints for 208 V; a strip rated at 240 V run on 208 V is not the 240 V row.

Is the blower included in the heat strip circuit?

Where the blower runs on the same circuit, yes: NEC 424.4(B) sizes the branch-circuit conductors at not less than 125 percent of the heating load and any associated motor. Read the blower motor's amps from the plate and add them to the heater amps before the 125%. On a listed unit, the plate's minimum circuit ampacity for that circuit is the figure to use (110.3(B)).

What happens to heat strips over 48 A?

Over 48 A of equipment load, NEC 424.22(B) has the unit's heating elements subdivided at 48 A or less each, protected at 60 A or less by supplementary devices in, on or supplied with the unit (424.22(C)). Whether the unit then takes one field circuit or several is its listing: wire what the plate prints (110.3(B)). A strip rated 10 kW at 208 V (48.1 A) is over that line; one rated 10 kW at 240 V (41.7 A) is not.

Can I use #10 wire on a 35 or 40 A heat strip breaker?

Not on a heating circuit. Table 240.4(G) lets an air-conditioning circuit's conductor sit on the plate's larger maximum breaker, but that table has no Article 424 row, so on heat strips NEC 240.4(D) holds #10 copper to 30 A, #12 to 20 A and #14 to 15 A. If the plate allows a larger breaker, the calculator gives the conductor that breaker needs.

Does a heat strip air handler need its own disconnect?

NEC 424.19 requires a means that disconnects the heater, the motor controller and any supplementary overcurrent devices from all ungrounded conductors at once, rated at least 125 percent of the motor and heater load and capable of being locked open. Where the unit has factory element fuses or breakers (424.19(A)) that means is within sight of them; with a blower over 1/8 hp it may also serve the motor controller where it is in sight from both and complies with Part IX of Article 430, or it may be out of sight from the motor controller where the motor and heater have a single unit switch that complies with 422.34 (424.19(A)(2)). A branch breaker that is merely lockable is the disconnect only for equipment with no supplementary devices and no motor over 1/8 hp (424.19(B)(1)). A unit fed by more than one circuit has its disconnects grouped and identified.

Don’t stop at the number.

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Heat Strip Wire Size and Breaker

Size Your Heat Strip Circuit

The Plate First, Then Article 424

Heat Strips at 208 V (elements only)

Heat Strips at 240 V (elements only)

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