What circuit do you actually have?

120V vs 240V garage heaters: what your circuit can actually run

Why a 15 A / 120 V receptacle tops out near 1,440 W continuous, when you need a dedicated 240 V circuit, and what breakers and wire typically mean for garage heaters.

Updated 2026-09-04

The 80% continuous-load rule

A space heater that runs for three hours or more is a continuous load in ordinary electrical practice. Branch-circuit conductors and overcurrent devices for continuous loads are typically sized so the load is not more than 80% of the circuit rating (NEC 210.19 / 210.20 and related language; local code can be stricter).

That single sentence is why garage-heater shopping goes wrong:

15 A × 120 V × 0.8 = 1,440 W. A 1,500 W heater draws 12.5 A — over the continuous ceiling — and wants to be the only load on that circuit.
20 A × 120 V × 0.8 = 1,920 W. Still a portable / utility heater, not a bay heater. Confirm the receptacle and breaker actually are 20 A; many garage outlets are 15 A on a 15 A breaker.
30 A × 240 V × 0.8 = 5,760 W. This is why the common 5,000 W ceiling unit (about 20.9 A at 240 V) is specified with a 30 A two-pole breaker.

These are planning numbers, not a substitute for the heater’s installation sheet or an electrician’s load calculation.

Approximate continuous wattage ceilings by common residential circuits
Circuit	Voltage	Continuous ceiling (80%)	What it usually buys
15 A general receptacle	120 V	~1,440 W	Milkhouse / utility / ceramic portable
20 A garage receptacle	120 V	~1,920 W	Larger portable; still spot heat
30 A two-pole dedicated	240 V	~5,760 W	5 kW Comfort Zone / Fahrenheat-class ceiling
40 A two-pole dedicated	240 V	~7,680 W	7.5 kW class unit heater
50–60 A two-pole	240 V	~9.6–11.5 kW	Larger commercial unit heaters

What 120 V can do

120 V is the outlet on the wall. It is excellent for a milkhouse heater at a bench, an oil-filled radiator in a small insulated 1-car, or keeping pipes from freezing in a mild climate. It is a poor way to heat 400–600 ft² of air. Physics does not care that the box says “garage.”

Garage receptacles on modern residential codes are GFCI-protected. A 1,500 W heater is a common nuisance-trip source, especially on a shared circuit with a freezer, opener, or charger. If the heater trips the GFCI, that is the protection doing its job or a dirty/damp environment — do not defeat the GFCI.

Details and product classes: portable heaters on a 15 A circuit.

What 240 V actually means

240 V residential heat is two hots and a ground (and sometimes a neutral, depending on the unit). It is not “twice as efficient.” Resistance heat is essentially 100% efficient at the point of use on either voltage. 240 V is how you deliver more watts without doubling the current on a 120 V conductor.

A 5,000 W heater at 240 V is about 20.9 A. The same 5,000 W at 120 V would be about 41.7 A — not something you plug into a garage outlet. That is the entire reason the Comfort Zone CZ220-class and Fahrenheat FUH54-class units are hardwired 240 V appliances.

If your panel has no spare two-pole slots, or the service is already tight, the heater conversation is a service/panel conversation first. See sizing by bay before you buy the unit.

Breakers and wire, in the language of the manuals

Manufacturer sheets for the common 5 kW ceiling units are unusually consistent:

Comfort Zone CZ220-class: 5,000 / 4,000 / 3,000 W at 208–240 V; 20.9 / 16.7 / 12.5 A; recommended 30 A breaker; 10 AWG copper.
Fahrenheat FUH54C-class: 5,000 W at 240 V is 20.9 A with a 30 A maximum fuse; 10 AWG copper minimum; aluminum wire prohibited in the manual. Jumpers can derate the element.
7.5 kW FUH-class units call for 8 AWG copper in the same manual family. That is a different circuit than the 5 kW aisle unit.

Wire size, insulation temperature rating, and run length are the electrician’s job. Do not upsize a heater onto an existing 20 A / 240 V dryer circuit “because it is 240.” Dryer circuits, EVSE, welders, and heaters are not interchangeable without checking the load, the receptacle type, and the code path.

Hardwired vs plug-in

Plug-in 240 V garage heaters exist (NEMA 6-30 and similar). They still need a correctly rated receptacle on a dedicated circuit. Most of the sub-$200 ceiling units are direct-wired: no cord, no plug, no extension cord, no relocatable power tap. The Comfort Zone CZ220 manual says that in those words.

If you are not comfortable landing 10 AWG copper on a terminal block and hanging 25–30 lb from a joist, this is a licensed electrician install. That is not a legal disclaimer for its own sake; it is how those units are designed.

Panel capacity is the hidden constraint

A 5 kW heater is a 20.9 A continuous-ish 240 V load. A 7.5 kW unit is about 31 A. Two 5 kW units are two 30 A two-pole breakers. On a 100 A service with a range, dryer, HVAC, and EV charger, that may not fit without a load calculation. We will not pretend every suburban panel has a free 30 A two-pole waiting.

If the panel cannot take the load, the useful options are: a smaller heater plus insulation, infrared at the bench, or a different heat source. Buying a 5 kW unit and feeding it from a lighting circuit is how garages burn.