What is 240V split phase? It’s how almost every US home gets power: the utility transformer delivers two 120V legs that are 180° out of phase plus a neutral, giving 120V for outlets and 240V for big appliances, as the U.S. Department of Energy’s electricity primer and the National Electrical Code describe. Understanding it matters for choosing an inverter, a generator or a home battery.
Electrical work can be dangerous: follow the manual, fuse and size every cable correctly, and use a licensed electrician for anything connected to household wiring.
How 240V Split Phase Works
The pole or pad transformer’s secondary winding is center-tapped. The center tap becomes the neutral (bonded to ground at your service). The two ends are L1 and L2.
| Measurement | Voltage |
|---|---|
| L1 to neutral | ~120V |
| L2 to neutral | ~120V |
| L1 to L2 | ~240V |
| Neutral to ground | ~0V |
It’s Not Two-Phase or Three-Phase
“Split phase” is single-phase power split at the center. Commercial buildings often get three-phase, where 120/208V service gives 208V between legs, not 240V. Some 240V appliances run slower or weaker on 208V.
Why the Neutral Current Is Small
Loads on L1 and L2 push current in opposite directions in the neutral. If both legs carry 20A, the neutral carries almost nothing. Unbalanced loads send the difference back on the neutral.
What This Means for Backup Power
- 120V-only inverters or generators can’t run 240V appliances such as well pumps, dryers or many AC units
- Stacking or split-phase inverters (e.g., 120/240V models or paired 120V units) are needed for full panels
- Per-leg limits: some inverters limit each 120V leg (for example, 6kW per leg), so balance loads
- Transformers: a 120V-only source can feed a 120/240V panel through a proper autotransformer, but 240V loads are then limited
In RVs and Vans
Most RVs are 120V only (30A service) or 120/240V on 50A service. 50A RV power is also split phase, two 120V legs.
Reading Your Panel
- Single-pole breakers (one space) = 120V circuits
- Double-pole breakers (two spaces, tied handles) = 240V circuits
Split Phase by the Numbers
| Item | Typical value |
|---|---|
| Nominal voltage | 120/ |
| Frequency | 60 Hz |
| Normal service range (ANSI C84.1, Range A) | about 114–126V per leg, 228–252V leg to leg |
| Wires into the house | L1, L2, neutral |
| Common service sizes | 100A, 150A, 200A |
| Theoretical capacity of a 200A service | 200A × 240V = 48,000W |
The 48,000W figure is just the math of the service rating. Actual load sizing follows NEC Article 220 load calculations.
How 120V and 240V Loads Connect
| Load type | Connects to | Breaker | Needs neutral? | Examples |
|---|---|---|---|---|
| 120V | L1 or L2 + neutral | Single-pole | Yes | Outlets, lights, fridge |
| 240V only | L1 + L2 | Double-pole | No | Water heater, well pump, many mini-splits, EV chargers |
| 120/ | L1 + L2 + neutral | Double-pole | Yes | Dryer, range (120V for timers, lights or motors) |
A 240V-only load still gets an equipment ground wire. It just doesn’t use a neutral.
Balancing Loads Across Both Legs
The neutral carries only the difference between the two legs. Illustrative:
| Leg 1 load | Leg 2 load | Neutral current |
|---|---|---|
| 15A | 15A | about 0A |
| 20A | 5A | about 15A |
| 20A | 0A | 20A |
| 240V dryer at 24A on both legs | Same | 0A from the dryer heating element |
Electricians spread 120V circuits across both legs in the panel for this reason. Breakers in most residential panels alternate legs as you go down each side, so adjacent full-size spaces sit on opposite legs.
Multiwire Branch Circuits
A multiwire branch circuit uses two hots on opposite legs that share one neutral, often in a 3-wire cable feeding a kitchen or a pair of circuits. Because the shared neutral carries only the difference, it works well, but the NEC requires the two hots to be disconnected together, using a 2-pole breaker or approved handle ties (Article 210.4(B)). Turning off just one breaker doesn’t fully de-energize the shared neutral path.
Split Phase and Home Batteries
For a home battery or inverter, three numbers matter:
- Output type: 120V only, or 120/240V split phase.
- Continuous output power across both legs.
- Per-leg limits: how much any one 120V leg can supply.
Illustrative: an inverter rated 12kW total with a 6kW per-leg limit can run 6kW of 120V loads on each leg. If all 120V loads land on one leg, the inverter reaches its limit at 6kW even though half its capacity is unused. A critical loads panel lets an installer pick and balance the circuits a battery backs up. See home battery backup for power outages.
Generators and Split Phase
Many portable generators have a 120/240V twist-lock outlet (often NEMA L14-30) that provides both legs. A generator with only 120V outlets provides one leg. Connecting a generator to a home requires a transfer switch or an approved interlock. See transfer switch vs interlock.
Measuring Split Phase Safely
Only qualified people should work inside a panel. At a 240V receptacle, a CAT III meter reads about 240V between the two hot slots and about 120V from each hot to ground. See how to use a multimeter.
Split Phase vs Three Phase: 240V vs 208V
| Service | Where it’s common | Line-to-neutral | Line-to-line |
|---|---|---|---|
| 120/ | Single-family homes | 120V | 240V |
| 120/ | Apartments, condos, commercial | 120V | 208V |
| 277/ | Large commercial and industrial | 277V | 480V |
The 208V figure comes from three phases spaced 120° apart: 120V × √3 ≈ 208V. In some apartment buildings, units get two legs of a three-phase supply, so “240V” appliances actually see 208V. Many water heaters, ranges and EV chargers list a lower output at 208V, so check the nameplate. Illustrative: a heating element rated 4,500W at 240V produces about 4,500 × (208 ÷ 240)² ≈ 3,380W at 208V, because a resistor’s power scales with voltage squared.
Getting 240V From 120V Sources
| Approach | What it does | Notes |
|---|---|---|
| Split-phase inverter | Makes 120/ | The simplest path for whole-home backup |
| Two compatible 120V inverters linked for split phase | Each makes one leg, synchronized 180° apart | Only models designed for it, per the manual |
| Autotransformer | Makes a second leg from a 120V source | Lets 120V-only units feed a 120/ |
| Power station “hub” or combiner accessory | Links two units for 240V output | Only the maker’s listed accessory |
Never improvise by joining two separate 120V outlets. The legs have to be synchronized 180° apart by equipment designed for it.
50A RV Service in Numbers
A 50A RV pedestal (NEMA 14-50) supplies two 120V legs at 50A each. Illustrative: 2 × 120V × 50A = 12,000W of total capacity, versus 120V × 30A = 3,600W from a 30A TT-30 pedestal. That’s why 50A rigs can run two air conditioners while 30A rigs usually run one.
FAQ
What is 240V split phase in simple terms?
Two 120V lines in opposite phase. Use one line for 120V, both for 240V.
Can I get 240V from a 120V generator?
Not without a transformer. Most 120V-only portable units can’t run 240V loads.
Is 50A RV service 240V?
Yes, 50A RV service is 120/240V split phase, though many RVs use it as two 120V legs.
What is 240V used for?
Large loads: dryers, ranges, water heaters, well pumps, EV chargers and many heat pumps and mini-splits.
Is 240V single phase?
Yes. US residential 240V is single-phase power from a center-tapped transformer.
How many amps is 240V?
Volts don’t set amps. Amps = watts ÷ 240, so a 4,800W load draws 20A.
Sources: U.S. Department of Energy, “How Does Electricity Work?”; NFPA 70 (NEC) 2023 Articles 210, 250 and 702; ANSI C84.1; NFPA 70 (NEC) 2023 Articles 210.4 and 220. Accessed Oct 2026.
Related: EcoFlow Delta 3 Max 120V and 220V, Tadeto Circuit Breaker Finder Review
One reply on “120V vs 240V Explained: Split-Phase Power in US Homes”
[…] Normal US outlet voltage varies around the 120V nominal. ANSI C84.1 treats roughly 114–126V at the service as the normal range. A 240V outlet reads about 240V hot to hot and about 120V from each hot to ground. See 120V vs 240V split phase. […]