Testing a 240 V Circuit & Checking a Neutral
General field practice for qualified electricians. The equipment manufacturer's instructions, your adopted code edition, and the AHJ govern where they differ.
⚠ Live testing. Use a CAT III/IV-rated meter, test the meter on a known live source before and after (live-dead-live), and wear the PPE your risk assessment calls for.
1 · Expected readings
| Measure | 120/240 V 1Ø | 208Y/120 V 3Ø (2 legs) |
|---|---|---|
| L1–L2 | ≈240 V | ≈208 V |
| L1–N, L2–N | ≈120 V each | ≈120 V each |
| L–G | ≈ same as L–N | ≈ same as L–N |
| N–G | near 0 V (a volt or two under load is common) | near 0 V |
ANSI C84.1 Range A service voltage is ±5% of nominal: 114–126 V on a 120 V base, 228–252 V on 240 V. A 240 V-rated appliance on 208 V may underperform. Check the nameplate.
2 · 240 V-only vs. 120/240 V loads
- 240 V only (water heater, many A/C condensers, well pumps): two hots and an EGC, with no neutral. Check L1–L2, L1–G, and L2–G.
- 120/240 V (range, dryer, some spas and EV chargers): two hots, a neutral, and an EGC. Every reading in the table above applies. A 4-wire receptacle needs a separate neutral and ground.
3 · Test under load
- Measure at the outlet or equipment with no load, then again with the load running.
- The drop in voltage is the voltage drop on the circuit. Compare it with the Voltage Drop calculator. More than about 3% on a branch circuit is worth investigating: look for loose terminations, undersized conductors, or a long run.
- A digital meter can read "ghost" voltage on open or floating conductors. Use a low-impedance (LoZ) setting or a solenoid tester to confirm.
4 · Checking the neutral
- Sum check: L1–N plus L2–N should equal L1–L2 (120 + 120 = 240). Each leg should stay near 120 V as loads switch on and off.
- Swinging legs (for example 140 V / 100 V that change with load) while L1–L2 stays steady mean an open or high-resistance neutral. See Identifying an Open / Backfed Neutral.
- Neutral current: clamp the neutral. On a 120/240 circuit it carries the imbalance (|IL1 − IL2|). Current on the EGC means a neutral-to-ground fault or a bond in the wrong place.
- N–G voltage that climbs steeply under load points to a loose neutral connection or a very long run.
Reference only — not engineering, legal, or code-compliance advice. Results are estimates for qualified persons and must be verified against the NEC edition adopted where the work is done (your selected edition), manufacturer instructions, and the local AHJ. Use is subject to the Terms of Use & Disclaimer. — Morningstar Electric LLC