Wiring up a 3kW immersion heater in my garden office — is a 6mm² cable run overkill?

Finally got round to sorting proper hot water in my garden office and I'm debating the cable sizing. The office is about 14 metres from the house consumer unit, and I want to run a dedicated circuit for a 3kW immersion heater (standard 13A draw). I've already got a 32A MCB spare in the CU and was thinking 6mm² T&E for the run, mostly clipped direct under the floorboards and through the cavity. Some people are saying 4mm² is fine for 13A but I always feel happier going a size up, especially with the length of the run.

The main reason I'm leaning toward 6mm² is the voltage drop. By my rough calculations on a 14m run at 13A, even 4mm² should technically come in under the 3% threshold, but it's tight. 6mm² gives a fair bit more headroom and the cable cost difference is maybe £25 for the whole roll, which feels like a no-brainer to me.

Anyone else run a dedicated immersion circuit to a garden office or outbuilding? Curious whether you went T&E or SWA for the outside section — I've got about 4 metres that'll need to go underground across the garden path. Was planning to sleeve the SWA in conduit for that bit but not sure if that's doubling up unnecessarily.

MPPTFan | 847 posts

@SparkySailor 6mm² is absolutely the right call for that run, not overkill at all. At 14 metres you'll have minimal voltage drop, and a 3kW immersion at 230V is drawing roughly 13 amps continuously — that's a sustained load, not an occasional spike. 6mm² gives you comfortable headroom and keeps you well within regs.

Couple of things worth checking: make sure your CU has a suitable 20A MCB spare way, and given it's a garden office (essentially a separate structure), you'll want RCD protection if you haven't got it already — Part P applies here. Also worth confirming the cable route; if any of it's buried, you'll need armoured or proper ducting.

Honestly the extra cost of 6mm² over 4mm² on a short run like that is peanuts compared to doing it twice! 👍

PanelBuff | 312 posts

@SparkySailor worth also thinking about volt drop over that 14m run — even with 6mm² you'll want to keep it within the 3% tolerance BS 7671 allows for a final circuit. At 13A continuous (3kW at 230V) it stacks up quicker than people expect.

Also — is this off-grid or grid-tied? On my narrowboat I use a Victron MultiPlus and a 3kW immersion would absolutely flatten my batteries if I wasn't careful with the assistant settings. If you're running from solar/battery, a Immersun or similar divert controller makes far more sense than a straight circuit anyway.

Either way, 6mm² is not overkill for that load and distance. Don't cheap out.