What breaker and wire for a 3.5 kW water heater?
230 V (Philippines): a 20 A breaker on 3.5 mm² THHN, for runs up to 30 m, on its own circuit.
120/240 V (US): a 20 A breaker on 12 AWG copper at 240 V, for runs up to 100 ft.
230 V: the Philippines (PEC), mm² wire
| Run (one way) | Breaker | Wire | Voltage drop | For a long run |
|---|---|---|---|---|
| 10 m | 20 A | 3.5 mm² THHN | 1.5 V (0.7 %) | fine |
| 20 m | 20 A | 3.5 mm² THHN | 3.0 V (1.3 %) | fine |
| 30 m | 20 A | 3.5 mm² THHN | 4.6 V (2.0 %) | fine |
At its limit: 19.0 A is 95 % of the 20 A breaker. Many electricians go one size up, a 30 A breaker on 5.5 mm², so it never runs near the trip point.
Running current 15.2 A at 230 V; × 1.25 = 19.0 A. Check it in the calculator.
120/240 V: the United States (NEC), AWG wire
| Run (one way) | Breaker | Wire (THHN, copper) | Voltage drop | For a long run |
|---|---|---|---|---|
| 30 ft | 20 A | 12 AWG | 1.4 V (0.6 %) | fine |
| 60 ft | 20 A | 12 AWG | 2.8 V (1.2 %) | fine |
| 100 ft | 20 A | 12 AWG | 4.7 V (2.0 %) | fine |
At its limit: 18.2 A is 91 % of the 20 A breaker. Many electricians go one size up, a 30 A breaker on 10 AWG.
3,500 W water heater at 240 V: 14.6 A running, 18.2 A at 125 %. NEC 210.19/210.20 (125 %), Table 310.16 at 75 °C, 240.4(D) for 14–10 AWG. Check it in the calculator.
Why these sizes
- The breaker protects the wire, not the appliance: the wire must carry at least what the breaker lets through.
- A load that runs for hours heats the wire and the breaker: both are sized at 125 % so they stay inside their rating.
- Long runs lose voltage in the copper: over 3 % the appliance runs hot and weak, so a longer run gets a bigger wire than the current alone asks for.
Open it on the benchWire the circuit in the lab, overload it and watch the breaker.
For planning and learning. Use the appliance's nameplate (amps, MCA and MOCP) when you have it, follow your local electrical code, and have a licensed electrician do work on a building's wiring.
In other 230 V countries (the UK, the EU, Australia), wire is also sold in mm² but the code and the breaker sizes differ: follow your local code.