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Max amps on a 240v UK socket? Model 3

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It’s limited to 10amps rather than 13amps on the assumption that a lot of household wiring may not be able to handle a sustained maximum load for the ~10 hours or so it would be asked to. 30seconds for a kettle is fine, but heat buildup over 10 hours maybe not.
 
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It's primarily limited because the design of a BS1363 plug, and the design of the integral fuse, are not rated for 13 A continuous use. The maximum continuous rating of a 13 A plug and fuse is really only 10 A, 13 A is a maximum rating. The main issue is heat generated by the fuse when run at 13 A, which then tends to overheat the line terminal of the plug, as the fuse is in intimate thermal contact with that pin. The heat then passes down the line plug pin and heats up the sprung brass contacts in the outlet, causing them to reduce their contact pressure with the pin. This then increases the contact resistance at this point and generates more heat, making the problem worse.

If looking at burned outlets, it's almost always the line terminal that's heat damaged, like this random photo grabbed from the web by way of illustration:

Temporary+Socket+Outlet+Picture.JPG


Household wiring is usually fine, as a single 2.5mm² T&E cable is normally rated at 27 A (assuming it's not running through insulation, etc) and there are, in effect, two parallel runs of this cable in the loop that forms the ring final, so the cable rating is somewhere around 54 A, ignoring all the other loads that may be on that ring. The protection device on a ring final like this will normally be either a 30 A fuse or a 32 A MCB/RCBO, so there's a fair bit of leeway in terms of protection device versus cable capacity, usually.
 
It's primarily limited because the design of a BS1363 plug, and the design of the integral fuse, are not rated for 13 A continuous use. The maximum continuous rating of a 13 A plug and fuse is really only 10 A, 13 A is a maximum rating. The main issue is heat generated by the fuse when run at 13 A, which then tends to overheat the line terminal of the plug, as the fuse is in intimate thermal contact with that pin. The heat then passes down the line plug pin and heats up the sprung brass contacts in the outlet, causing them to reduce their contact pressure with the pin. This then increases the contact resistance at this point and generates more heat, making the problem worse.

Very interesting - thanks. I had previously assumed that the robust appearance of UK plugs made them better than the rather weedy alternatives in other countries ... but those other countries don't normally include a fuse so I suppose that's how they manage it. Wouldn't it be better if the UK had an alternative fuseless plug design for use in high load applications? I realise there is/was the 15A round pin design but I was thinking of one based on the 13A design without a fuse in the plug itself but adding a heavy duty inline fuse for the charger (or whatever other piece of equipment).
 
Very interesting - thanks. I had previously assumed that the robust appearance of UK plugs made them better than the rather weedy alternatives in other countries ... but those other countries don't normally include a fuse so I suppose that's how they manage it. Wouldn't it be better if the UK had an alternative fuseless plug design for use in high load applications? I realise there is/was the 15A round pin design but I was thinking of one based on the 13A design without a fuse in the plug itself but adding a heavy duty inline fuse for the charger (or whatever other piece of equipment).

I believe that this problem all stems from the switch to ring finals in the 1950's, a decision made because of a copper shortage at the time. A new set of regs were being drafted up, which included new house wiring standards, as well as a new design of plug and outlet. The changes in design were driven in part, by a fairly high accident rate, as it was commonplace for relatively powerful appliances to be wired to circuits that were not adequate for the power, and for adapters to be used that didn't feed through the required protective earth connection (the most common being light fitting adapters used to power things like irons - my mother used one of these regularly).

The old system we had of round, three pin outlets and plugs had different sized outlets for each current rating (commonly 3 A, 6 A, 10 A and 15 A, IIRC). The circuits were radials, so adequately protected by fuses in the distribution board. When we switched to ring finals, we had a circuit in the house that had potentially lots of outlets, and that had a current capacity that was much greater than a single outlet (a 30 A fuse would be protecting a ring final with 13 A outlets).

This meant that there had to be an additional way introduced to protect the flexible cable from the plug to the appliance, as it wasn't reasonable to expect appliances to be fitted with cable that could tolerate ~30 A (appliances would have needed ~4mm² flex - very heavy stuff). The solution was to fit a fuse in the plug, initially only rated at 13 A, as that allowed the use of thinner (typically 1.5mm²) flex.

The downside is that the fuse then ran hot at high load, as described above.

The solution, IMHO, is to get rid of ring final circuits, and do as most other countries do, and install radial circuits, with lower rated protection devices in the consumer unit. We could then adopt a more robust outlet and plug design, perhaps like the Schuko design that's fairly common in Europe.
 
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The solution, IMHO, is to get rid of ring final circuits, and do as most other countries do, and install radial circuits, with lower rated protection devices in the consumer unit. We could then adopt a more robust outlet and plug design, perhaps like the Schuko design that's fairly common in Europe.

..so is there any technical reason why we couldn't have our present 13A plug design but without a fuse so it doesn't get hot (and put the fuse in the equipment itself or inline)? This would presumably allow us to use the full 13A for continuous applications.
 
..so is there any technical reason why we couldn't have our present 13A plug design but without a fuse so it doesn't get hot (and put the fuse in the equipment itself or inline)? This would presumably allow us to use the full 13A for continuous applications.

Unless we reduce the over current protection rating of the ring final (normally 32 A for a modern installation) then no, there's no way to remove the fuse in the plug. That fuse is there to protect the relatively thin flex to any appliance, as without it a pretty high current could flow through the flex with the only protection being the 32 A circuit breaker on the ring final.

Putting the fuse in the equipment wouldn't protect the flex, unfortunately, as a fault in the flex (which is quite likely as a consequence of physical damage) could then cause a high current to flow through it, and the fuse in the appliance couldn't protect against that.

The advantage of radial wiring is that the circuits are protected by lower current circuit breakers, typically 16 A, so everything downstream of that 16 A breaker is protected, including the flex to the appliance. The disadvantage of radial power circuits is that you have more of them in the consumer unit. It's not uncommon to find eight or ten power circuits in houses in Europe that use this sort of system, rather than the one or two that is most common in the UK.



Does the car report the current draw from the main or the current after AC>DC conversion 'cos I guess there will be losses?

The car seems to report the current drawn from the supply, before the onboard charger. This current may be around 5% to 10% greater than the power actually charging the car, at a rough guess.
 
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Putting the fuse in the equipment wouldn't protect the flex, unfortunately, as a fault in the flex (which is quite likely as a consequence of physical damage) could then cause a high current to flow through it, and the fuse in the appliance couldn't protect against that.

got it now ... it would need the fully 32A capable flex .. which, as you say, is a bit heavy duty ... despite in non-fault conditions not needing that capacity.