How to Install Electric Baseboard Heat and a Line-Voltage Thermostat

Why this matters

Baseboard heat is resistance wire in a sheet-metal case, and it is honest about what it draws: watts over volts, every time. That makes it easy to size and easy to get wrong in the same three ways. The circuit gets sized at the heater's amps instead of 125 percent of them, so the breaker sits at its limit all winter. The heater goes under a receptacle, where a cord lies across the element. And a single-pole thermostat is used on 240 V, so the heater is never really off. This card is the install from the panel through the thermostat to the heaters, with a finish test that checks every heater against its own nameplate.

Safety first

The circuit is 240 V and the thermostat is line voltage: the full circuit voltage passes through it, unlike a low-voltage furnace thermostat. Install the breaker last, or with the panel de-energized: main off where you are working near other terminals, locked and tagged, and proved dead with live-dead-live on a CAT III or better meter, proved on a known live source before and after (NFPA 70E Article 120, 120.5 in the 2021 edition and renumbered in 2024; work practices at 29 CFR 1910.333(b)(2), and 29 CFR 1926.417 on construction work). On a replacement, a heater can be fed from either end and from a circuit you did not expect; prove every conductor in each heater's wiring compartment dead before touching it. Heaters in a room that has been running are hot to the touch for a while after the breaker goes off.

The layout

 PANEL         THERMOSTAT       HEATER A       HEATER B
 15A 2P        double-pole      1500 W         1000 W
               LINE    LOAD     240 V          240 V
 L1 ---------> L1      T1 -+--> lead 1
 L2 ---------> L2      T2 -|-+> lead 2
                           | |
                           +-|----------------> lead 1
                             +----------------> lead 2
 EGC --------> stat box ------> heater A gnd -> heater B gnd

Both heaters connect across T1 and T2, so they are in parallel: each sees the full 240 V and draws its own current. In practice the run to heater B leaves from heater A's wiring compartment, spliced there, which is electrically the same parallel connection. Never wire two 240 V heaters in series; each would see a fraction of the voltage and put out a fraction of its rated heat.

Sizing the circuit

Fixed electric space heating is a continuous load. NEC Article 424's branch-circuit sizing rule (424.4(B) in recent editions, 424.3(B) in older ones; check the edition adopted) requires, in substance, the branch circuit conductors and overcurrent device to be sized at not less than 125 percent of the total heating load. That is the governing rule for this card; the reasoning behind 125 percent in general is 'Continuous Load and the 125 Percent Rule'.

The worked room. A bedroom with a 4 ft window centered on a 12 ft exterior wall, and a second exterior wall. The heat-loss calculation (done by whoever designed the heat; for a room this size it is not the electrician's number to guess) calls for 2,500 W: a 1500 W heater under the window and a 1000 W heater on the side wall, both 240 V.

Check it from the other direction: a 15 A circuit carries continuous load up to 15 / 1.25 = 12 A, which is 12 x 240 = 2,880 W. 2,500 W fits with 380 W to spare.

The thermostat's rating. A line-voltage thermostat has its own maximum current or wattage on its label. 10.4 A must be within it. Check before buying, not after.

The heaters' wiring rating. Read each heater's instructions for the conductor temperature rating it requires at its terminals. NM-B cable has 90 C conductors with its ampacity held to the 60 C column (334.80), which is why 14 AWG NM is used here at 15 A.

The steps

1. Place the heaters before anything else. Under windows: cold glass drops a sheet of cold air down the wall, and a heater beneath it puts the warm air into that downdraft instead of across the floor. Never below a receptacle unless the heater's instructions permit it: the Informational Note to NEC 210.52 points out that listed baseboard heaters carry instructions that may not allow it, because a cord plugged in above a heater drapes across the element. Keep the clearances to drapes and furniture the instructions require.

On the worked room, heater A (1500 W) is about 6 ft long, a typical length for that wattage. Centered under the 4 ft window, it runs from 3 ft to 9 ft along the 12 ft wall. The wall's receptacle is 2 ft from the corner, so the heater clears it by 1 ft. Had the receptacle been at 5 ft, the choice was to move the receptacle or use a shorter heater, not to install under it.

2. Place the thermostat. On an interior wall about where a light switch goes, in the room's air, not above a heater or on an exterior wall where it reads the cold surface. A thermostat over a heater feels its rising heat and shuts the room off cold.

3. Choose a double-pole thermostat. A single-pole line-voltage thermostat opens one leg. On 240 V the other leg still reaches the heater, so the element is at 120 V to ground with the dial at its lowest. A double-pole thermostat with a marked off position opens both legs. NEC 424.20 as adopted sets the conditions under which such a thermostat may also serve as the heater's disconnecting means; they include the marked off position, opening all ungrounded conductors, and not re-energizing automatically from off. Do not assume a thermostat meets them: check the label. On this job the disconnect is the panel breaker, which has a lockable handle accessory installed (NEC 424.19 as adopted permits a breaker capable of being locked open), so the thermostat is a control only.

4. Run the cable. 14/2 NM with ground from the panel to the thermostat box, and from the thermostat to heater A, and from heater A to heater B. The white in each 2-wire cable is used as an ungrounded conductor for 240 V and re-identified at every termination with tape or marking (see related: 'Reidentifying a White Conductor in Cable'). The box fill at the thermostat box is two 14/2 cables, the grounds and the thermostat yoke; count it before choosing the box.

5. Terminate. At the thermostat, LINE terminals to the panel cable, LOAD terminals to the heater cable. Swap them and an electronic thermostat may not power its own electronics, because many draw their supply from the LINE side; follow the terminal markings, not the wire colors. At heater A, the incoming leads to the heater leads, spliced together with the cable continuing to heater B, all in heater A's wiring compartment with a cable clamp at the knockout. EGC to every heater's ground screw and to the thermostat's ground. Torque the panel breaker's terminals to the value marked on it with a calibrated tool (NEC 110.14(D)).

6. Mount the heaters level, on the wall, at the height the instructions give, usually with a gap at the floor so air enters at the bottom. Covers and compartment plates back on.

The finish test: every heater against its own nameplate

Resistance heaters draw current in proportion to voltage: a 1500 W, 240 V element is 240 x 240 / 1500 = 38.4 ohms, and the 1000 W one is 240 x 240 / 1000 = 57.6 ohms. At the voltage actually measured, each should draw voltage divided by its resistance.

1. Before energizing. With the heater leads landed and the thermostat off, put an ohmmeter across T1 and T2 at the thermostat's load side, breaker locked off. Two heaters in parallel: 38.4 x 57.6 / (38.4 + 57.6) = 23.0 ohms expected. On this job: 23.4 ohms, within 2 percent of expected. An open reading means a missed splice; near zero means a short.
2. Restore. Stand to the side of the panel as the breaker closes. Turn the thermostat to full call.
3. Measure voltage at the thermostat with its cover off, probes only: 244 V on this job.
4. Clamp each heater. Remove heater A's compartment cover and clamp one lead, then the conductor continuing to heater B. This is energized diagnostic work: the PPE your shop's 70E assessment sets, the clamp jaw around a single insulated conductor, no tools on terminals.

Both within reading accuracy of expected. A heater drawing roughly half its expected current is usually wired in series with something, or is a 240 V element on 120 V. One drawing nothing has an open element or a high-limit that has tripped.

1. Turn the dial to off. Current on both heaters goes to zero. Covers back on.

What changes the answer

• A third heater. Adding a 1500 W heater makes 4,000 W: 4000 / 240 = 16.7 A, x 1.25 = 20.8 A. That exceeds a 20 A circuit, so the choices are a 25 A circuit on 10 AWG (12 AWG is held to 20 A by 240.4(D)) with a thermostat rated for 16.7 A, or two circuits. Splitting into two is usually better, because one thermostat per room gives control per room.
• 120 V heaters. Some small heaters are 120 V. The same 125 percent rule applies to their current, and a single-pole thermostat breaking the hot is then normal, since the neutral is not switched.

References

• NFPA 70, National Electrical Code, 2023 edition (2020 where numbering moved) as adopted, local amendments checked: 110.14(D), 210.52 Informational Note, 240.4(D), 334.80, Article 424 branch-circuit sizing (424.4(B) in recent editions, 424.3(B) in older), 424.19, 424.20
• NFPA 70E, current edition, Article 120; 29 CFR 1910.333(b)(2); 29 CFR 1926.417 for construction work
• See related: Continuous Load and the 125 Percent Rule
• See related: Reidentifying a White Conductor in Cable
• Heater and thermostat manufacturer installation instructions (placement, clearances, ratings)