How to wire a dedicated storage heater circuit

Helpful video reference. The video above is from the Electrician Video Diary channel: "Problem Solving: Installing Circuit For Storage Heaters". It follows a working UK electrician through a real job, covering the common problem-solving aspects of adding storage heater circuits to an existing board. Worth watching before you start to understand the real-world complications.

1. Read the heater's data plate

The data plate on the back or base of the heater tells you the rated wattage and, on some models, the charge rate and the boost element wattage separately. Divide the total wattage by 230 to get the design current in amps: a 3.4 kW unit draws about 14.8 A, a 2 kW unit about 8.7 A.

The BS 7671 cable sizing rules require the cable to handle the full design current continuously, because the heater charges overnight for seven hours. Do not assume a smaller cable is adequate because the heater has an internal thermostat: the circuit must be rated for the maximum possible load.

2. Understand the Economy 7 supply arrangement

Storage heaters are designed to charge at night on cheap off-peak electricity, then release heat slowly through the day. Most modern properties on Economy 7 tariffs have a single consumer unit that receives a time-based signal from the meter, switching the heater circuits on only during the off-peak hours. The signal is either a ripple control relay in the meter or a programmed time-switch.

Older properties, particularly from the 1970s and 1980s, sometimes have a separate "heater board" fed by a second set of meter tails. These boards are often obsolete and not suitable for new circuits. If you have one of these, ask the DNO to remove it and set up Economy 7 time control on the main board instead.

3. Size the cable and MCB

For most domestic storage heaters up to 3.4 kW, 2.5 mm² twin and earth cable protected by a 16 A or 20 A MCB is the standard solution. Run each heater on its own radial circuit from the consumer unit: sharing one circuit between two heaters doubles the current and requires at least 4 mm² cable and a 32 A MCB.

If the cable runs under floor insulation, through roof spaces, or is clipped in bundles with other cables, apply the grouping and thermal insulation derating factors from BS 7671 Appendix 4. For a single cable clipped to a wall in open air, 2.5 mm² is adequate for any heater up to about 5 kW.

4. Plan the cable route

Storage heaters are heavy (a 3.4 kW unit can weigh 60 kg when charged) and sit against the wall, so the cable typically enters from below through the floor or behind a skirting trunking. Plan the route from the consumer unit to each heater position, keeping the cable in the prescribed safe zones where it passes through walls or ceilings: vertically from a socket or FCU, or horizontally at ceiling height.

Fix twin and earth at maximum 400 mm intervals on horizontal runs and 500 mm on vertical ones, or at whatever interval your chosen cable clips require to support the cable without sag.

5. Fit the fused connection unit

Every storage heater should have a switched fused connection unit (FCU) as a local isolation point, typically mounted on the wall at roughly skirting height or just above. Use a 20 A FCU fused at 13 A or 20 A depending on the heater's rated current. A double-pole switched FCU allows the heater to be fully isolated for servicing without returning to the consumer unit.

The FCU has two sets of terminals: supply (from the consumer unit) and load (to the heater). Keep track of which is which: the supply terminals are the ones with incoming 230 V. The fuse in the FCU protects the heater's flex, not the circuit cable, so a 13 A fuse is correct even on a 20 A circuit for most heater ratings.

6. Make the terminations

With the consumer unit main switch off and confirmed dead at the MCB terminals, connect the circuit cable at the MCB end first: brown (line) to the top of the MCB, blue (neutral) to the neutral bar with a green-and-yellow sleeved earth to the earth bar. Tighten all terminals to the torque specified in the consumer unit manufacturer's instructions.

At the FCU end, connect the incoming cable to the supply terminals and the heater's appliance flex to the load terminals. If the heater is a Class 1 appliance (metal casing), confirm the earth is continuous from the consumer unit earth bar through the circuit cable CPC, through the FCU earth terminal, and to the heater casing earth.

7. Test before switching on

Before restoring the supply, carry out the three basic tests in sequence:

• Earth continuity: Confirm that the circuit protective conductor (CPC) connects the heater casing to the earth bar at the consumer unit, with a resistance low enough to comply with BS 7671 Table 41.5 for the MCB rating.
• Insulation resistance: With the heater disconnected at the FCU or with its internal switch off, apply 500 V DC between line and earth and between neutral and earth. The reading should be at least 1 MΩ.
• Polarity: Confirm that the line conductor connects to the line terminal of the MCB and FCU, not to the neutral.

Record the test results on the Schedule of Test Results for the Electrical Installation Certificate you must issue under Part P.

8. Restore and check operation

Switch on the MCB and the FCU. The heater should begin its charging cycle at the next Economy 7 off-peak period. If the installation has a time-switch in the consumer unit, check that the off-peak hours are correctly set, typically midnight to 07:00 though this varies by DNO region and tariff.

Listen for any unusual sounds from the heater, particularly clicking or buzzing from the charge thermostat. A faint ticking as the core heats up is normal.

When to call us

New circuits from the consumer unit are Part P notifiable work in England, and the testing requirements are specific. Richard can assess the consumer unit, run the circuit, carry out the tests and issue the certificate. He also covers the full range of storage heater problems: faulty thermostats, failed charge elements and Economy 7 time-switch settings.