immersion heater wall switch

adamada

coffeehound wrote: »

As I understand it, the 3 kW figure is true for a 240 V supply. If the supply is say 230 volts, then less current flows and the power output of the same element will be about 2.8 kW instead.

no,, the 3kw heater remains constant

the calculation is written as ;;;;; i = v devided by resistance

i is amps
v is volts
resistance is the heater

so 3000w devided by 240v = 12.5 amps

if the voltage is lower, the amps will rise, the load (3kw) remains the same :beer:

keith969

adamada wrote: »

no,, the 3kw heater remains constant

the calculation is written as ;;;;; i = v devided by resistance

i is amps
v is volts
resistance is the heater

so 3000w devided by 240v = 12.5 amps

if the voltage is lower, the amps will rise, the load (3kw) remains the same :beer:

Incorrect. For a resistive load, Power = I * V. And V = I * R

So eliminating I, P = V * V / R

i.e. the power dissipated drops according to the square of the voltage across the load resistance (which is the constant, not the power).

Heedtheadvice

Risteard is factually correct - but maybe have done more to help with the original question?
Southcoastgri is factually incorrect (onm some of the later technical points) - but as a generalism does sort of give a laymans view with the advice (such as the immersion being required on a separate circuit).

As a matter of fact my recent measurements of the supply voltage are very frequently at the higher end of the allowable range (250v+ actual measurements). Possibly because this is currently summer and there will be lots of daytime generation from renewables where inverters output slightly higher voltages so as to feed back to the grid. As I write 252.3 is measured!

Different types of fuses have different characteristics so it is misleading to state that a fuse will blow at 2.5 times it's rating. That is pretty common under typical conditions for many a domestic fuse but all depends upon time, temperature and the fuse design. The fuses in domestic switches and plugs are only designed to protect the cables they feed......but appliance designers sometimes will use that to protect their equipment when really they should not. If protection of the equipment is required then it should have an integral fuse to do that job with characteristics to suit the required protection e.g. for many an electronic device or those with electronics such as modern boilers then a fast acting fuse is necessary and that is not supplied by a BS1362 fuse ( just as one example)

As the voltage increases for passive devices such as an immersion heater so will it's output power so the rated power may well be exceeded (often measured at the theoretical mean supply voltage or stated at 230V or 240V) so may consume a current higher than may be expected. 1Kw power equates to approx. 4.17Amps at 240 volts AC ( for a 57.6 resistive load) but that same load would give a power output of 1.111Kw at 253V and consume nearly 4.4Amps ( all other thing being equal) hence the reason ( as well as potentially higher currents at switch operation) to rate switches that bit higher.
So for a 3kW rated immersion heater 20A switch is suitable, cable used to do the connection rated similarly considering it will be in a hot area and the fuse rated accordingly.......and yes fed directly form the consumer unit not off the ring main which will not have been designed to cope with an immersion heated as well as other appliances that could be in use at the same time.

However, apart from the hope of clarifying understanding for anyone who reads, I am not sure at all what the later discussion does to further help Anna's original query and surely her electrician ought to be the one with the technical expertise to do a proper job?

BUT this thread does highlight the problem of posts on a forum where one has no knowledge of the veracity of statements or the expertise of the poster! I include myself here :-)

adamada

keith969 wrote: »

Incorrect. For a resistive load, Power = I * V. And V = I * R

So eliminating I, P = V * V / R

i.e. the power dissipated drops according to the square of the voltage across the load resistance (which is the constant, not the power).

eh !! :eek:

adamada

Heedtheadvice wrote: »

Risteard is factually correct - but maybe have done more to help with the original question?
Southcoastgri is factually incorrect (onm some of the later technical points) - but as a generalism does sort of give a laymans view with the advice (such as the immersion being required on a separate circuit).

As a matter of fact my recent measurements of the supply voltage are very frequently at the higher end of the allowable range (250v+ actual measurements). Possibly because this is currently summer and there will be lots of daytime generation from renewables where inverters output slightly higher voltages so as to feed back to the grid. As I write 252.3 is measured!

Different types of fuses have different characteristics so it is misleading to state that a fuse will blow at 2.5 times it's rating. That is pretty common under typical conditions for many a domestic fuse but all depends upon time, temperature and the fuse design. The fuses in domestic switches and plugs are only designed to protect the cables they feed......but appliance designers sometimes will use that to protect their equipment when really they should not. If protection of the equipment is required then it should have an integral fuse to do that job with characteristics to suit the required protection e.g. for many an electronic device or those with electronics such as modern boilers then a fast acting fuse is necessary and that is not supplied by a BS1362 fuse ( just as one example)

As the voltage increases for passive devices such as an immersion heater so will it's output power so the rated power may well be exceeded (often measured at the theoretical mean supply voltage or stated at 230V or 240V) so may consume a current higher than may be expected. 1Kw power equates to approx. 4.17Amps at 240 volts AC ( for a 57.6 resistive load) but that same load would give a power output of 1.111Kw at 253V and consume nearly 4.4Amps ( all other thing being equal) hence the reason ( as well as potentially higher currents at switch operation) to rate switches that bit higher.
So for a 3kW rated immersion heater 20A switch is suitable, cable used to do the connection rated similarly considering it will be in a hot area and the fuse rated accordingly.......and yes fed directly form the consumer unit not off the ring main which will not have been designed to cope with an immersion heated as well as other appliances that could be in use at the same time.

However, apart from the hope of clarifying understanding for anyone who reads, I am not sure at all what the later discussion does to further help Anna's original query and surely her electrician ought to be the one with the technical expertise to do a proper job?

BUT this thread does highlight the problem of posts on a forum where one has no knowledge of the veracity of statements or the expertise of the poster! I include myself here :-)

oh yeah, i just didnt want to go into detail

i think the op has has got her new switchgear so its a moot point really

im sure her sparky will know his fuse from his elbow so best leave it to them :beer:

southcoastrgi

& therefore getting back to my original comment before some decided to prove that getting that degree wasn't a total waste of my tax money, OP get a 20a switch not a fused spur & get your sparkie to make sure it's on its own circuit back to the CU & not off the ring main, which I think brings me full circle :rotfl::rotfl:

adamada

southcoastrgi wrote: »

& therefore getting back to my original comment before some decided to prove that getting that degree wasn't a total waste of my tax money, OP get a 20a switch not a fused spur & get your sparkie to make sure it's on its own circuit back to the CU & not off the ring main, which I think brings me full circle :rotfl::rotfl:

:rotfl: :T

if it is on the ring main it needs to be a fused spur,

if on own curcuit, either a spur or 20a dp switch is acceptable :beer: