Instant Electric Water Heater, AC, and Induction Hob Wire Gauge & Breaker Sizing Chart
Dedicated copper cable cross-section (mm²), B/C-curve MCB, and 30 mA RCD sizing chart for 7–11 kW instant water heaters, 9000–24000 BTU air conditioners, ovens, and induction hobs.
Instant electric water heaters, air conditioners, and induction hobs cause most melted wall outlets and nuisance breaker trips in residential boards. Because a standard 230 V socket circuit (`3×2.5 mm²`) is rated for 16 A (`~3.68 kW`) max, high-draw loads like a 7–7.5 kW instant shower heater (`~32 A`) or 7.2 kW induction hob must never tap into a socket ring—they require a dedicated copper radial circuit.
> Related Category: Browse heaters and protection devices in Electric Instant Water Heaters, Electric Heaters, Miniature Circuit Breakers, and Residual Current Relays.
Appliance current draw, copper wire gauge (mm²), and MCB chart
For 230 V single-phase resistive loads, `I = P (W) ÷ 230 V` (`7000 W ÷ 230 V = 30.4 A`). Assuming flush-in-conduit installation (IEC 60364-5-52 Reference Method B, run ≤20–25 m):
| Appliance & rated power | Typical load current | Dedicated copper cable | Recommended MCB & RCD | Termination method |
|---|---|---|---|---|
| Instant water heater (7.0 – 7.5 kW, 230 V) | 30.4 – 32.6 A | 3×4 mm² (≤15 m) or ideal 3×6 mm² | 32 A or 40 A (Curve B) + 30 mA RCD | Hardwired terminal / 40 A DP isolator (no 16 A plug!) |
| Single-phase instant heater (9 – 11 kW) | 39 – 48 A | 3×10 mm² copper | 50 A (Curve B) + 30 mA RCD | Hardwired dedicated feeder |
| Three-phase instant heater (18 – 21 kW, 400 V) | 26 – 30 A / phase | 5×4 mm² or 5×6 mm² copper | 3×32 A (Curve B) + 4P 30 mA RCD | 3P+N+PE hardwired block |
| Storage water heater / boiler (1.8 – 2.2 kW) | 7.8 – 9.6 A | 3×2.5 mm² dedicated radial | 16 A (Curve B) + 30 mA RCD | Dedicated isolator / socket |
| 9,000 – 12,000 BTU inverter split AC | 4.0 – 6.5 A (`~0.9–1.4 kW`) | 3×2.5 mm² dedicated radial | 16 A (Curve C) | Outdoor/indoor unit terminal |
| 18,000 – 24,000 BTU split AC | 8.0 – 13.0 A (`~1.8–2.9 kW`) | 3×2.5 mm² (or 3×4 mm² long run) | 16 A or 20 A (Curve C) | Dedicated panel MCB |
| Built-in electric oven (2.5 – 3.5 kW) | 11 – 15.2 A | 3×2.5 mm² dedicated radial | 16 A (Curve B/C) | Dedicated oven circuit |
| Induction hob (6.8 – 7.4 kW) | 30–32 A (1P) or 2×16 A (2P+N) | 3×6 mm² (1P) or 5×2.5 mm² (3P panel) | 32 A (1P) or 3×16 A (multi-phase) | Hardwired cooker outlet plate |
For full derating and voltage-drop formulas, see our Cable Current-Carrying Capacity & Sizing Guide and B, C, and D Tripping Curve Guide.
Four mandatory safety rules for electric instant water heaters
- Never use a 16 A plug and socket: At stage 3 (~7 kW = 30–32 A), a 16 A domestic socket rapidly overheats and carbonizes. Pull a continuous NYM damp-area cable or conduit wire directly from the panel to a double-pole isolator and the heater's internal terminal block.
- 30 mA RCD life protection is mandatory: Protect every bathroom water heater behind a 30 mA residual current device (see RCD Wiring Diagram & Tripping Causes).
- Continuous protective earth (`PE`): Bond the green-yellow conductor directly to the main panel earth bar—never use metallic water pipes as the sole protective earth.
- Never oversize the MCB above the installed wire's ampacity: Use a 40 A B-curve MCB only on a 3×6 mm² circuit; if an existing wall conduit holds `3×4 mm²`, cap the breaker at 32 A to protect the cable insulation.
BTU/h cooling capacity vs electrical kW input on air conditioners
A 12,000 BTU/h (`~3.5 kW` thermal cooling) inverter air conditioner does not draw 3.5 kW of electricity: thanks to heat-pump COP/EER values of 3.2–4.0, its electrical input is only 0.9 – 1.3 kW (`4 – 6 A`). Always select a C-curve MCB (`5–10 × In` magnetic trip) to ride through compressor and inverter DC-bus inrush.
Frequently asked questions
Can a 7 kW instant water heater run on 2.5 mm² wire?
No. Stage 2 and Stage 3 (~22–32 A) severely overload `2.5 mm²` PVC insulation, softening jackets inside junction boxes and creating a serious fire hazard. Pull a dedicated `3×6 mm²` (minimum `3×4 mm²` on short runs) feeder.
Why does my shower breaker trip after 5–8 minutes of hot water?
Delayed tripping indicates thermal bimetal overload—either the MCB rating (e.g., 25 A on a 32 A heater) is undersized for Stage 3, or a loose terminal screw is conducting heat directly into the breaker's thermal strip.
How can a 7.2 kW induction hob connect to either single-phase or three-phase?
Most 7.2 kW hobs split their heating zones across two internal 3.6 kW modules (`L1` and `L2`). On a three-phase board they balance across two phases (`5×2.5 mm²`, `16 A` per phase); on single-phase (`230 V`), a factory brass link bridges `L1+L2` onto a dedicated 32 A / 3×6 mm² feeder.
Why use Curve B on heaters and Curve C on air conditioners?
Resistive heating elements have zero magnetic inrush, so Curve B (`3–5 × In`) gives faster fault disconnection on bathroom runs; AC compressors have inductive/capacitive startup surges requiring Curve C (`5–10 × In`).