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In the UK, are there higher current limits for short hard-wired appliance power cords than for in-wall wiring?

I see for example that a 4mm2 wire requires a 32A circuit breaker. Say you have a 38A appliance. If you use 6mm cable from the Consumer Unit to a junction box near the appliance, can you use 4mm wire in a short cable, in open air, from the junction box to the appliance? Is an open-air hard wired appliance power cord rated for higher current in the UK than the same-sized wire installed in the building structure?

The background to this question is:

  • This document for a cooker that consumes 7700W but supports only 4mm wire for single-phase installation.
  • This answer, where I say you have to reduce the cooker's max power if you have only one phase, because of the documented 4mm2 limit, but I wonder if that's wrong?
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  • A US-based corollary: I must use #14AWG (min) for in-wall wiring on a 15A circuit, however a luminaire manufacturer can use #16, #18, or even #20 wire inside the device if it meets UL testing, because it's been tested safe with this wiring. I'd presume that it's the same for UK appliances gaining a Kitemark - it's been tested as such, therefore it's allowed. (Yes, I realize the wire size changes are the opposite direction, but I think the theory holds.)
    – FreeMan
    Nov 7, 2023 at 12:57
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    @freeman As far as I can tell, in the US, manufactured extension cords and integral power cords are often smaller gauge than would be required for given current in-wall. But user-installed power cords (eg dryers) and hard-wired appliances all go by the same current ratings as in-wall cable. You don't use smaller wire for the last 6 feet. It's similar in the UK, but there all appliance cords are protected by a fuse in the wall plug. Maybe that is the answer to my question? I dont' know (hence the question) ... is a cooker installed with a fused connector?
    – jay613
    Nov 7, 2023 at 15:23
  • Not disagreeing with you, just trying to think about it. It could be the device-level fuse on every device that allows for this...
    – FreeMan
    Nov 7, 2023 at 15:55

2 Answers 2

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A cable hanging in free air can handle more current than one buried in a wall, and a lot more current than one bundled with other cables under a pile of loft insulation.

BS7671 has a whole load of tables for different cables under different conditions. A good electrician should have a copy, but most people don't because it's expensive and unreadable to the lay person.

If the appliance manufacturer says to use a 4mm cord, then that's what you can do.

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  • The appliance manual says to use at most a 4mm cable, and it has software settings to limit its max current draw to suit the installation. It does not say or imply that a 4mm cable can be used to deliver its full maximum power (7620W). If a 4mm cable from the CU and through walls is extended right to the appliance, it cannot (or must not) do that.
    – jay613
    Nov 3, 2023 at 1:17
  • Are you saying that an electrician following BS7671 could install a 2 meter 4mm cord from the 6mm cable in the wall to the cooker, put a 36A breaker on it, and remain in compliance with UK regulations? Is that how a homeowner is expected to interpret the manual, by extrapolating all that? Is that in fact normal practice for installing cookers in the UK, IE that the appliance cannot accommodate wires big enough to carry full load to it through the walls? If that's normal, it seems ridiculous.
    – jay613
    Nov 3, 2023 at 1:19
  • @jay613 (The OP said a 32A breaker. I have never seen a 36A one) Yes. The rules in the UK are designed to get away with using as little copper as we can possibly get away with, because it's expensive. A 4mm cable in free air can handle 32A, so you're good to go.
    – Simon B
    Nov 3, 2023 at 13:48
  • Right but the thing uses 35A.
    – jay613
    Nov 3, 2023 at 17:19
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What's of concern here is:

  1. Voltage drop
  2. Heating in the cable

These are a function of cable cross-sectional area and length. Heating is a function of the current going through any length of a particular thickness of cable, the cable's construction and its thermal situation (embedded in fibreglass insulation, clipped to a wall or hanging in free air). Voltage drop is a function of thickness and length - the cable may be fine thermally, but you may need a thicker (lower resistance) cable for a long run because too much voltage would be lost along the run.

As mentioned, BS7671 and the 'on site guide' have tables as to the different derating factors, and there are online calculators.

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