Maximum current for a unit supplied from a 13A socket, taking supply voltages into account.

Your design need to consider the maximum voltage and also not to exceed the 13A rating with sufficient margin


Chris Chew

Is the system expected to draw full current continuously or will the current drop once the system has heated up?  3kw is the normally accepted maximum continuous load for a 13A socket and even at this level low quality plugs and sockets tend to overheat.


Best regards


Roger

The unit takes around 30 - 45 minute to get to full temperature (when set to its maximum), once at temperature the holding power is around 2kW.

I would recomend that you should design not to exceed 13A at maximum voltage. A few minutes of overcurrent is usually ok, but 45 minutes is around the time constant of typical wiring systems. This unit may be plugged into an already load extension cable and as the attached document shows some of these are not really adequate.


Best regards


Roger

Thank you all for your comments.


Tony Hoare.

I wouldn't push a BS1362 carttridge fuse beyond 13A for moe than a few minutes. Running at 'full power' for 45 Minutes will cause it to blow I suspect.

Although I have never known the Uo in the UK to be higher than 247V and and absolute maximum of 253V at +10%.


If the rated load is 3kW at 230V (13.04A) then it will be 3.3kW at 253V which equates to 13.66A that doesn't take into the rise in temperature due to the heated element and extra current

IMO, I would keep the supply lead to 2m and because you're likely to be running the load 'flat out' use a good quality extention lead if at all necessary.

I wouldn't expect the 13A fuse to blow ever on a modest overload.  The standard charts for a BS1362 13A fuse show it lasting at least 10 000 seconds at 20A.  The plug will have melted long before the fuse blows.  See http://www.pat-testing-training.net/articles/fuse-operation-characteristics.php

There are really two parts to this question,


1. It is not unusual to see units quoted as 230V, 13A 3kW, based on this I can only assume that these are mutually exclusive as the tolerance on the voltage will take this condition beyond 13A and 3kW, therefore to maintain 13A at 230V + 10% the power at 230V 3kW must be lower! Is this the case, or should products be designed around 253V, 13A, 3kW?


2. In this case the condition to exceed 13A is an absolute worst case, supply voltage at 253V (230V+10%) and all components at the top of their tolerance (the heating elements we use are +/-5% on power, which makes up more than 90% of the power usage).


Given the unit only operates at full power during its warm up phase (approx 40min) and then runs at around 2kW, and the (un)likelyhood of the voltage and element tolerences all being at the top end, I am being advised that it is acceptable to design the unit around this basis 


Max current

13.8A  (253V, Elements +5%),

13.14A (253V, Elements Nominal)

12.5A (240V, Elements Nominal).


However, I as a typically cautious engineer, would see 13A as an absolute maximum and should be based on the worst case conditions.


What would your thoughts be on the above?