120V vs 240V Circuits Explained

North American residential service supplies two voltages from one set of conductors. Understanding how takes one diagram's worth of explanation and clears up most of the confusion around breakers, circuits and appliance requirements.

What is split-phase power?

The utility transformer serving your house has a secondary winding with a centre tap. That centre tap is grounded and becomes your neutral. The two ends of the winding become your two ungrounded conductors — the "legs," commonly called L1 and L2.

From that:

• Either leg to neutral = nominally 120 volts
• Leg to leg = nominally 240 volts

One service, both voltages, no conversion equipment. This is single-phase three-wire, usually called split-phase. It's why your panel has two busbars and why breaker positions alternate between them.

How it shows up in the panel

The two legs feed two busbars, and breaker positions alternate between them as you go down the panel.

A single-pole breaker connects to one leg. Its circuit runs from that leg to neutral — 120V. It occupies one position.

A double-pole breaker spans two adjacent positions and connects to both legs. Its circuit runs leg-to-leg — 240V. It occupies two positions, and both legs trip together.

That alternating arrangement is also why tandem breakers and multi-wire branch circuits behave the way they do — see tandem breakers explained, single pole vs double pole breaker and what is a multi-wire branch circuit.

Balancing. Because there are two legs, an electrician distributes circuits so the load is roughly even between them. A badly unbalanced panel puts more current on one leg's neutral path than necessary.

Why do big appliances use 240 volts?

Power = volts × amps. For a given power, doubling the voltage halves the current.

Illustrative example. A 4,800 W load:

Half the current means:

• Smaller conductors for the same power — a substantial material and labour saving on a heavy circuit
• Less voltage drop over the same run length
• Less heat in the conductors
• A practical circuit at all. Some loads would need conductors so large at 120V that the circuit isn't reasonable to build

That's the whole reason large appliances are 240V. It's an installation advantage, not a running-cost one.

Does 240V cost less to run than 120V?

A persistent myth worth killing.

Electricity is billed in kilowatt-hours — power × time. A 4,800 W load consumes 4.8 kWh per hour whether it's fed at 120V and 40A or 240V and 20A. Same energy, same bill.

What you save is conductor cost and voltage drop, not kilowatt-hours. Work out actual running costs with the electricity cost calculator.

(The one real efficiency nuance: lower current means slightly lower resistive losses in your own wiring, which is a genuine but very small effect. It's not a reason to choose 240V.)

What runs on 120V and what runs on 240V?

120V — the default for general use:

• Lighting and general receptacles
• Small appliances, electronics, computers
• Refrigerators, microwaves, dishwashers, garbage disposals
• Most portable equipment
• Gas ranges and gas dryers (the ignition and controls)

240V — heavy heating and motor loads:

• Electric ranges, ovens and cooktops
• Electric clothes dryers
• Electric water heaters
• Central air conditioners and heat pumps
• Electric furnaces and baseboard heat
• Well pumps
• Welders and large shop tools
• Level 2 EV chargers

Some 240V appliances also need 120V for controls, lights or a timer, which is why they use a four-wire connection with a neutral as well as a ground. Others are purely 240V and need only three wires. This is exactly the distinction behind NEMA 14-50 vs 6-50 outlet — see also the NEMA plug chart.

Why does my outlet read 118 volts instead of 120?

"120V" and "240V" are nominal values. Actual measured voltage varies with utility conditions and load — you'll commonly see readings somewhat above or below.

You'll also see equipment labelled 110/115/125V or 220/230/250V. These are historical or equipment-rating designations for the same nominal system. A "220V" dryer and a "240V" dryer want the same circuit.

A genuinely different reading is a symptom. If one leg reads noticeably different from the other, or voltage swings with load, that can indicate a failing service neutral — worth an urgent utility call. See flickering lights causes and who owns what on your electrical service.

Adding a 240V circuit

You can't convert a 120V receptacle by swapping the device. A 240V circuit needs:

• Two ungrounded conductors from a double-pole breaker
• Conductors sized for the load per the NEC ampacity tables
• The right conductor count — three or four depending on whether the appliance needs a neutral
• Two adjacent free positions in the panel, on opposite legs
• Panel and service capacity to support the load

Existing 120V cable typically doesn't have the conductors for it, so in practice it means a new circuit run from the panel.

Practical detail in how to install a 240V outlet and what size breaker do I need. Check capacity with the home electrical load calculator and can I add a breaker to my panel.

Continuous loads — EV charging, electric heat — are sized at 125% of running current. See the Level 2 charger breaker size calculator and dedicated circuit requirements.

Never put a 240V receptacle on 120V wiring, or vice versa. Both directions are dangerous.

The bottom line

One transformer winding with a grounded centre tap gives you 120V from either leg to neutral and 240V across both — that's split-phase, and it's why panels have two busbars with alternating positions. Big loads use 240V because halving the current allows smaller conductors and less voltage drop, not because it costs less to run; a kilowatt-hour is a kilowatt-hour. Adding a 240V circuit means a new run from a double-pole breaker, not a receptacle swap.

Check your capacity with the home electrical load calculator.