im not sure i would want to run anything at its maximum for a long time.

good workmanship may well cover this, if you want to run a 13 amp load use a 16 or 20 amp mcb / rcbo with a 20 amp switch if needed locally.

In general the ratings of things should normally be their continuos rating.. but some standards do have other ideas. I guess you'd need to refer to BS 1363 to see precisely what it demands.

The traditional way to overheat a 13A fused connection unit was to use it to supply a3kW (at 240V) immersion heater - although the real cause was the ambient temperature in the airing cupboard rather than the current - I think BS 1363 expects something like an average of no more than 25 degrees.

That said, 13A plugs/sockets have been known to cause problems when feeding EVs - there are now special "EV" marked sockets that have a true 13A continuous rating.

   - Andy.

The british standard sets this limit, and running warm is indeed expected by design, partly due to the fuse warming up and partly the rest of it. How hot however is an interesting point. There are heating tests and accelerated ageing heat tests as part of the standard.
https://knowledge.bsigroup.com/products/13-a-plugs-socket-outlets-adaptors-and-connection-units-13-a-fused-connection-units-switched-and-unswitched-specification?version=tracked

however at over £400 you can understand why most folk probably don't have a copy to hand to check the exact figures.


Mike.

Bruno Miguel Pereira said:

Hello, Does anyone know if there are regulations stating that the 13A switch fuse spur can run Continuously at is rated current for long hours? I find this intriguing because, when a spur operates at its maximum rating, it tends to overheat, but i can't find any regulation that explicitly states this isn't allowed.

Not directly ... BUT ...Regulation 433.1 is, I think, where you need to go.

Care is needed regarding how the rating of the wiring system and accessories relates to the rating of products when calculating load currents.

For example, BS EN 60335-1 permits current-using equipment to the standard to use 5 % more power (or current) than the stated rating - meaning a device which is rated 13 A may well use 5 % more load current (13.65 A) at nominal rating plate voltage.

The utilization voltage also varies from between + 10 % (253 V) and - 11 % (204.7 V), using BS 7671 voltage drop maximum of nominal. With linear (resistive) loads, such as heaters, the load current increases with increasing voltage, and with switched-mode power loads the current increases with decreasing voltage to achieve the same power.

What this all means, is that a heating load rated 3 kW at 230 V might actually have a load current up to 15.1 A (or as little as 10.4 A). Similarly, a heating load rated 3 kW at 240 V might have a load current as much as 13.8 A (or as little as 9.6 A).

Regulation 433.1 of BS 7671 states:

Every circuit shall be designed so that a small overload of long duration is unlikely to occur.

So, as we can see, even under normal conditions of temperature, a  BS EN 60335-series product rated 13 A (2990 W) at either 230 V or 240 V perhaps should not be used for long periods from a device protected by a 13 A fuse.

Immersion heaters are often rated 3 kW at 240 V, but have the added problem that, if the connected device is in the same room as the heated cylinder, the ambient temperature will be elevated, and wiring and accessories should be derated accordingly. This is noted in Section 5.3.1 of the IET Commentary on the Wiring Regulations. This is supported by BS 1363 series ... BS 1363-4, for example, has in its stated conditions of use for relevant products (connection units, or "spurs") that the maximum ambient temperature is permitted to get to 40 deg C, BUT that the average ambient temperature over 24 hours is not to exceed 25 deg C.

Overall, where a current-using product has a rating exceeding 2.7 kW at 230 V, the rating of accessories for its connection, and any associated protective devices, should be higher than 13 A, and where elevated temperatures are expected, a much lower rating limit for products is necessary for the 13 A limit.

I would, however, at this point, state that there is nothing to prevent a manufacturer exceeding the requirements of BS 1363, and I am in no way making a statement about the suitability of any and all accessories to BS 1363 series for continuous "rated at 13 A" or "rated at 3 kW" loads ... just considering the "minimum performance" that you might need to achieve a conformant product.

Hello everyone,

Thank you for your messages. I did some further digging and came across Regulation 433.1.204 and Appendix 15 of BS 7671, which state that cables should have a minimum current-carrying capacity of 20A and should not be subjected to continuous loads exceeding this for extended periods.

I believe this information is crucial when considering the use of switched fused spurs.

Thank you all again for your feedback!

Regulation 433.1.204 and Appendix 15 of BS 7671, which state that cables should have a minimum current-carrying capacity of 20A and should not be subjected to continuous loads exceeding this for extended periods.

I think that's referring to the rating of the cables making up the ring itself - where you have two 20A cables on a 32A protective device and is trying to ensure the loads are not so unevenly distributed around the ring that one of the legs can overheat even though the total load is within the rating of the protective device.

   - Andy.

Hello,

Clause 17 of 1363-4:2023 specifies the operating temperature, and typically, the test is conducted for 8 hours with a 14A load and a 6A load, totaling 20A. :). i know the standards but thank you for your feed back. 

The answer is not simple. I have had a look at the 2018 version of BS 1363-4.

Ambient temperature -5ºC to +40ºC with an average not to exceed 25ºC.

The type testing to ensure compliance uses a 20 A supply with a 14 A load on the output and a further 6 A on the input (presumably simulating a ring, or a load further along a radial). The ambient temperature is to be 20ºC ± 5ºC. A maximum temperature rise of 52ºC is permitted after a minimum of 4 hours, but there is a caveat that an FCU is designed for a maximum cable temperature of 70ºC.

Type testing is, however, not normal use. BS 1363 states that FCUs "... shall not attain excessive temperatures in normal use." Some manufacturers exceed the minimum standards, so if you want to run a long 13 A load, I think that unless the ambient temperature is no higher than 18ºC, it would be wise to monitor the temperature, at least initially.

The answer is not simple. I have had a look at the 2018 version of BS 1363-4.

Ambient temperature -5ºC to +40ºC with an average not to exceed 25ºC.

The type testing to ensure compliance uses a 20 A supply with a 14 A load on the output and a further 6 A on the input (presumably simulating a ring, or a load further along a radial). The ambient temperature is to be 20ºC ± 5ºC. A maximum temperature rise of 52ºC is permitted after a minimum of 4 hours, but there is a caveat that an FCU is designed for a maximum cable temperature of 70ºC.

Type testing is, however, not normal use. BS 1363 states that FCUs "... shall not attain excessive temperatures in normal use." Some manufacturers exceed the minimum standards, so if you want to run a long 13 A load, I think that unless the ambient temperature is no higher than 18ºC, it would be wise to monitor the temperature, at least initially.

and realistically at 18C ambient and a unit under test  70C temperature -presumably on the outside where you can measure it, parts of the spur are quite a bit hotter inside. The plastics will age more rapidly and the whole thing may well have a short and uncomfortably hot life compared to running at half that load. Expect failure times measured  in years rather than decades.

Or for sustained loads use a DIN box and an MCB or a 20A switch and protect elsewhere.
Mike.