What breaker and wire size for a water heater?
3.5 kW: 5.5 mm² THHN on a 30 A breaker. 5 kW and up: 8.0 mm² on 40 A. A heater is a continuous load, sized at 125 % of its current (16 A becomes 20 A for 3.5 kW), on its own circuit. On 2.0 mm² lighting wire it runs hot long before the breaker notices.
Anong breaker at wire ang para sa water heater?
3.5 kW: 5.5 mm² THHN sa 30 A na breaker. 5 kW pataas: 8.0 mm² sa 40 A. Ang heater ay continuous load, kaya 125 % ng current ang basehan (16 A ay nagiging 20 A sa 3.5 kW), at sariling circuit. Sa 2.0 mm² na wire ng ilaw, iinit ang wire bago pa mag-trip ang breaker.
See it run
Water heater circuit: 3.5 kW on 5.5 mm² and a 30 A breakerA build with a test sheet: every number on this page is a test you can run.
The numbers
| 3.5 kW instant heater | 15–16 A → 20 A sized; 5.5 mm², 30 A |
| 5 kW | 22–23 A → 28 A sized; 8.0 mm², 40 A |
| 6.5 kW | 29 A → 36 A sized; 8.0 mm², 40 A, or 14 mm² on 50 A |
| Storage heater 1.5 kW | 7 A: 3.5 mm², 20 A |
| 5 kW on a 15 A breaker | thermal trip within a minute |
| 3.5 kW on 2.0 mm² and a 30 A breaker | no trip; the wire is the fuse |
Questions
Why 125 %?
A heater runs for more than three hours in the code’s sense, so it counts as continuous, and a breaker is only trusted with 80 % of its rating continuously. 16 A ÷ 0.8 = 20 A.
Does it need an RCD?
Yes: a heater in a bathroom is the textbook case for a 30 mA RCD (ELCB) on its circuit. See the RCD build on the lab.
Can I use PDX?
PDX 5.5 mm² is rated 30 A (NM rating), so it works for 3.5 kW on a 30 A breaker; for 5 kW use 8.0 mm² THHN in conduit.
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