protective device selection

[Sbc]

Hello all,

A quick question... when selecting a protective device should you use the nominal voltage or the measured voltage?

example
9.8k / Uo = 42.6A select 50A mcb
9.8 / measured voltage 245V = 40A select 40A mcb

Thanks in advance

 

[Paul.M]

Measured for me. Btw some manufactures make a 45A MCB if that is any help.

 

[HandySparks]

I'm assuming that what you're talking about is protecting a shower. In which case, the 9.8kW will be specified at a particular supply voltage. If you increase the supply voltage you will INCREASE the current and the power, so you calculation is incorrect. What's fixed is the heater resistance (or nearly so), not the power.

Generally best just to use the nominal figures.

 

[Knobhead]

For you,re 9.8KW @ 230V the resistance would be 5.4Ώ

So for you're 245V supply it works out at 45.5A assuming a linier coefficient of resistance/temperature. Which you can't.

 

[Sbc]

Handysparks, Yes its a shower. 10mm cable. Manufacturers instruction say 45A breaker except cant get wylex 45A that easily. Shall i go with 50A breaker then?

Tony, what is "linier coefficient of resistance/temperature" can you explain further please?

 

[malcolmsanford]

Unless things have drastically changed in design, you are suppose to design everything at Uo

Most showers though are rated still at 240v which does make our life a little complicated but if you design to 230v then you are erring on the side of caution by saying that your load is going to be higher.

You never though design to RMS .....................

 

[LeckyBill]

I would stick it on a 50a

Tony, what is "linier coefficient of resistance/temperature" can you explain further please?

No idea and I don't even know what Linear coefficient of resistance/temperature is either, don't think it's relevant

 

[malcolmsanford]

Linear coefficient is really to do with thermal expansion and no on a domestic shower circuit I don't think there is a lot to worry about ..............Think our Tony was slightly pulling the leg

 

[Sbc]

thanks for all the contributions fellas.

 

[HandySparks]

Handysparks, Yes its a shower. 10mm cable. Manufacturers instruction say 45A breaker except cant get wylex 45A that easily. Shall i go with 50A breaker then?

Tony, what is "linier coefficient of resistance/temperature" can you explain further please?

If you're using 10mm² T&E then, providing your correction factors don't drag the current carrying capacity down too far, a 50A breaker will be fine.

I think Tony meant to say that, if the resistance of the heater remains constant with current/power, then the current will be... This is probably reasonable, as the element is surrounded by water, so it's temperature, and therefore resistance, isn't going to vary much with current. Unlike a filament lamp, where the resistance changes hugely with current and filament temperature.

 

[Sbc]

yes, the current carrying cap was on my mind. its partly clipped direct, then along the floor void and up through the stud to the 45A switch on the landing wall not much to derate the cable. oh, now is that a problem 45A rated switch/50A breaker?
thanks

 

[malcolmsanford]

yes, the current carrying cap was on my mind. its partly clipped direct, then along the floor void and up through the stud to the 45A switch on the landing wall not much to derate the cable. oh, now is that a problem 45A rated switch/50A breaker?
thanks

No..................

 

[Sbc]

Thanks fellas.

 

[Geoffsd]

9.8k / Uo = 42.6A select 50A mcb
9.8 / measured voltage 245V = 40A select 40A mcb

Does the manufacturer quote 9.8kW @ 240V ?

If so, then that is equivalent to 8.99kW @ 230V.

8990 / 230 = 39.1A - these are the figures that should be used in calculations

 

[Sbc]

its a triton shower, 2 sizes, it states 9.8k - 45A & 8.5k - 40A. Its currently being protected by a 40A bs1361 cartridge fuse, again another reason why i pondered the voltage in the calc

 

[Geoffsd]

its a triton shower, 2 sizes, it states 9.8k - 45A & 8.5k - 40A. Its currently being protected by a 40A bs1361 cartridge fuse, again another reason why i pondered the voltage in the calc

Those are the recommended OPDs to use for different showers not the amperage of the units.
The MIs should be followed.

I'm not talking about two sizes but two voltages.
You cannot quote a wattage without stating the voltage.

My previous post is correct for a shower quoted by the manufacturer as 9.8kW @ 240V.