Electric Boiler Calculator
Size an electric boiler by the heat loss of the house, calculate current, supply breaker, RCD and cable section, and determine whether single-phase is enough or three phases are needed.
How boiler power is selected
The right order: first calculate the heat loss of the house (heat loss calculator), then add a 10–15% margin and, if needed, the hot water load. Only then choose a boiler model from the standard power range.
The rough rule «1 kW per 10 m²» is only suitable for a preliminary estimate of average houses with 2.7 m ceilings. For accurate sizing, heat loss calculation is mandatory: in an uninsulated house, the real demand can be 2–3 times higher than the rule.
What the calculator computes
- Required boiler power: by heat loss (accurate mode) or by area and insulation quality (quick mode)
- Hot water allowance (15–25%)
- Current per phase: single-phase (230 V) or three-phase (400 V)
- Circuit breaker rating
- Supply cable section (IEC 60364-5-52, tables 52-C and B.52)
- RCD requirement and connection order (separate line from the board)
Calculation formulas
Power: P_boiler = Q_heat_loss × 1.15 + P_DHW. Without precise heat loss: P = S × q, where q is the specific power by insulation quality: 30–50 W/m² (good, modern), 60–80 W/m² (average), 100–150 W/m² (uninsulated).
Single-phase current: I = P / U = P / 230.
Three-phase current: I = P / (√3 × U) = P / 692.
Breaker: I_br ≥ 1.25 × I_design, standard range 16/20/25/32/40/50/63 A, type C.
Cable section (copper, in wall/conduit): up to 16 A — 2.5 mm²; up to 25 A — 4 mm²; up to 32 A — 6 mm²; up to 40 A — 10 mm²; up to 63 A — 16 mm² (IEC 60364-5-52, method B).
Practical rule: single-phase boilers — up to 6–7 kW (max 32 A); above 7 kW — three-phase only.
Breaker and cable selection table
| Power, kW | Mains | Current, A | Breaker | Cable (copper) | RCD |
|---|---|---|---|---|---|
| 3 kW | 1-ph 230 V | 13.0 | C16 | 3×2.5 mm² | 30 mA |
| 4.5 kW | 1-ph 230 V | 19.6 | C25 | 3×4 mm² | 30 mA |
| 6 kW | 1-ph 230 V | 26.1 | C32 | 3×6 mm² | 30 mA |
| 7 kW | 1-ph 230 V | 30.4 | C32 | 3×6 mm² | 30 mA |
| 9 kW | 3-ph 400 V | 13.0 | C16 | 5×2.5 mm² | 30 mA |
| 12 kW | 3-ph 400 V | 17.3 | C20 | 5×2.5 mm² | 30 mA |
| 15 kW | 3-ph 400 V | 21.7 | C25 | 5×4 mm² | 30 mA |
| 18 kW | 3-ph 400 V | 26.0 | C32 | 5×6 mm² | 30 mA |
| 24 kW | 3-ph 400 V | 34.7 | C40 | 5×10 mm² | 30 mA |
| 30 kW | 3-ph 400 V | 43.4 | C50 | 5×10 mm² | 30 mA |
Regulatory requirements
The electric boiler is connected by a separate line from the intake board via its own breaker and an RCD with a residual current of 30 mA (IEC 60364-4-41, HD 60364-4-41; UK: BS 7671 Section 701 for bathrooms). Power above 10–12 kW in a house usually requires a three-phase supply — check the contracted capacity.
For heating element boilers, cable with flame-retardant insulation (LSZH/FRNC type) is used. The supply cable must not pass through wet rooms. Earthing per the national system (TN-C-S in most of Europe) is mandatory.
How to use the calculator
- Select the mode: «By heat loss» or «By area».
- In heat loss mode enter the value from the heat loss calculator; in area mode — area and insulation quality.
- Indicate whether hot water is needed.
- Press «Calculate» — boiler power, current, breaker, RCD and cable section.
Common installation mistakes
- Connecting the boiler to a shared socket circuit — overload and tripping of the main breaker.
- Single-phase 10 kW boiler — 43 A will not be handled by a standard 40 A intake breaker and 6 mm² cable.
- Choosing power with a 2× margin — the boiler will cycle frequently, consumption grows, service life drops.
- Forgotten RCD — in case of element breakdown, electric shock is possible.
- Cable without flame-retardant insulation in the wall — violation of installation rules.
Electric boiler FAQ
Which electric boiler for a 100 m² house?
For an insulated 100 m² house with 2.7 m ceilings, heat loss is usually 5–7 kW. Take the boiler with 15% margin: 7.5–8 kW. Single-phase up to 7 kW, above — three-phase. The heat loss calculator gives the exact value.
Is electric heating cheaper than gas?
By tariff, gas is 3–5 times cheaper, but connection costs thousands and takes months. Electric boilers pay off on night tariffs and for houses up to 150 m². Use a dual-zone meter and a thermal storage tank.
Why must a boiler above 7 kW be three-phase?
The current of a single-phase 8 kW boiler is 35 A, requiring a 40–50 A intake and 10 mm² cable. Most homes have a single-phase supply of 25–40 A. Three-phase reduces the per-phase current by √3: 12 kW = 17.3 A per phase — standard for a 15 kW supply.
Which breaker is needed for a 9 kW electric boiler?
For a three-phase 9 kW boiler (400 V): current 13 A, breaker C16, cable 5×2.5 mm², RCD 30 mA. See the table above — the calculator gives values for other powers.
What is cascade boiler connection?
Two smaller boilers working alternately — allows precise load matching and survives the failure of one unit. Practiced for houses over 200 m² with electric heating.