20A Double Pole Switches: Heater Isolation

Quick answer

A 20A double-pole (DP) switch is the local isolator for a fixed appliance that draws up to about 20A — water heaters, immersion heaters, heated towel rails, panel heaters and similar. Double pole means it breaks both the live and the neutral, fully isolating the appliance so it can be worked on safely. Choose 20A for these smaller fixed loads (a cooker or shower needs a 45A switch instead). Many have a neon to show the circuit is live, and some include a flex outlet to hardwire the appliance. BG 20A switches start around £9.60 at Kent Traders.

What is a 20A double pole switch for?

Fixed appliances that are wired in permanently, rather than plugged in, need a local means of switching them off for safety and maintenance. A 20A double-pole switch does that for medium loads up to roughly 20A (about 4.6kW at 230V): immersion and water heaters, heated towel rails, panel and storage heaters, and similar hardwired equipment. It lets you isolate the appliance at the wall without going back to the consumer unit.

Why double pole?

Double pole (DP, sometimes marked 2-pole) means the switch breaks both the live and the neutral conductor at once. For fixed equipment that is the safe way to isolate, because it fully disconnects the appliance rather than leaving the neutral connected. A single-pole switch only breaks the live, which is fine for lighting but not the right choice for isolating a fixed heater. Many 20A DP switches also have a neon indicator that glows while the circuit is live — a useful check before anyone works on the appliance.

Which rating? 20A vs 45A vs a fused spur

Match the switch to the appliance load. These three are easy to mix up:

Device Use it for
20A double-pole switch Water/immersion heaters, towel rails, panel heaters (medium fixed loads)
45A cooker switch Electric cookers, hobs and electric showers (high loads)
13A fused connection unit (FCU/spur) Smaller appliances spurred off a ring, where a fuse is needed

With or without a flex outlet?

If the appliance cable (flex) needs to exit the wall to reach the unit, choose a 20A switch with a built-in flex outlet, or pair a plain switch with a separate flex outlet plate. A plain DP switch with no flex outlet suits installations where the appliance is wired in behind the plate or fed from a separate outlet. Check the appliance and the wiring layout before you buy.

How to choose

Decide three things: whether you need a flex outlet, whether you want a neon indicator (recommended for heaters), and the finish to match your other accessories (white moulded, brushed steel, polished chrome, black nickel, or a screwless flat plate). The rating stays 20A double pole for these medium fixed loads. For complex switching, the same function is available as a 20A grid module to build into a custom plate.

Frequently asked questions

What is a 20A double pole switch used for?

Isolating a fixed appliance that draws up to about 20A — water and immersion heaters, heated towel rails and panel heaters. It switches the appliance off locally for safety and maintenance.

Why double pole rather than single pole?

Double pole breaks both the live and the neutral, fully isolating fixed equipment. Single pole breaks only the live, which is fine for lighting but not for isolating a hardwired heater.

What is the difference between a 20A switch and a 45A cooker switch?

The rating matches the load. Use 20A for medium fixed loads like heaters and towel rails; use a 45A cooker switch for high loads like cookers, hobs and electric showers.

Do I need one with a flex outlet?

If the appliance flex must exit the wall at the switch, choose a version with a flex outlet, or add a separate flex outlet plate. Otherwise a plain switch is fine.

Do I need an electrician to fit one?

Yes. Wiring a fixed heater circuit is mains work and is notifiable under Part P of the Building Regulations in England and Wales. Use a qualified electrician.