Immersion Heater in 1.5mm flex

[tomodo]

Hi guys rewiring an immersion heater from a double pole 20amp switch, circuit is wired in 2.5mm t+e up to DP switch. What are peoples thoughts on using 1.5mm flex the regs state that this is capable of carrying u to 16 amps ,would prefer to use 2.5mm flex but can not find a 20amp dp switch with a flex outlet to take a 2.5mm flex ...

circuit is on 16amp breaker and flex run is about 1 metre.

Thanks

 

[davesparks]

1.5 is the normal size to use, 2.5 is normally too big for both the 20A DP switch and the immersion heater.

 

[Simonslimline]

Cant you use a Double pole switched fuse connection unit instead of the 20A double pole?

 

[davesparks]

Cant you use a Double pole switched fuse connection unit instead of the 20A double pole?

Why? That wouldn't achieve anything other than inconvenience when the fuse becomes tired from running at its rating for long periods of time.

 

[andyb]

1.5 butyl, always used it.

 

[Simonslimline]

Why? That wouldn't achieve anything other than inconvenience when the fuse becomes tired from running at its rating for long periods of time.

Ok fair point.

Regarding the 2.5mm2 & flex, would 433.3.1 (ii) not come in to play here?

 

[7029 dave]

1.5m butyl is plenty big enough.

 

[Darkwood]

Yep the butyl 1.5mm is the best choice as has been stated, it has a greater heat tolerance and won't break down at the Imm' heater terminals due to the heat.

 

[tomodo]

What is reg 433.3.1? haven't got Reg book on me atm ...

Thanks for the replys

 

[Richard Burns]

What is reg 433.3.1? haven't got Reg book on me atm ...

Thanks for the replys

It is whether you need to have overload protection.
In this case the immersion can be normal operation or full fault current, nothing in between, so overload does not happen and does not need to be protected against.

 

[GEOFF S]

Hi


1.5 is pushing it if its a 3kw element, best to use 2.5mm cable.

Cheers

 

[Leesparkykent]

Hi


1.5 is pushing it if its a 3kw element, best to use 2.5mm cable.

Cheers

Why's it pushing it?

 

[davesparks]

Hi


1.5 is pushing it if its a 3kw element, best to use 2.5mm cable.

Cheers

How on earth is that pushing it? 13A through a cable rated for a constant current of 16A is not pushing it.

 

[7029 dave]

Hi


1.5 is pushing it if its a 3kw element, best to use 2.5mm cable.

Cheers

Which most are,why's that then.?

 

[Amp David]

Which reg relates to 'pushing it'?

 

[GEOFF S]

Which reg relates to 'pushing it'?

Hi

No reg relates to pushing it, i personally dont like running cables near to their maximum, some heat resistant cables have a ccc less than 16 amps some more, also i like to derate due to ambient temp. So 2.5mm runs cooler, less volt drop, better thermal constaints etc. I never said 1.5mm couldnt be used, it depends upon the cable and other factors.

You all have your preferences and veiws, some a little emphatic, i have mine.

Cheers

 

[davesparks]

Hi

No reg relates to pushing it, i personally dont like running cables near to their maximum, some heat resistant cables have a ccc less than 16 amps some more, also i like to derate due to ambient temp. So 2.5mm runs cooler, less volt drop, better thermal constaints etc. I never said 1.5mm couldnt be used, it depends upon the cable and other factors.

You all have your preferences and veiws, some a little emphatic, i have mine.

Cheers

The tabulated values in the regs book are not the maximum for the cables, they are already worked out to avoid running the cables at their maximum!
All you do by applying the 'I don't like running near the max' approach is to end up wasting money.
Plus the ratings are already at 30degrees so to apply a factor for normal room temperature in this country would actually increase the ccc.

Plus of course the terminals on an immersion heater and cable grip are designed for 1.5 so using bigger cable often leads to broken cable clamps and poor terminations

 

[GEOFF S]

The tabulated values in the regs book are not the maximum for the cables, they are already worked out to avoid running the cables at their maximum!

All you do by applying the 'I don't like running near the max' approach is to end up wasting money.
Plus the ratings are already at 30degrees so to apply a factor for normal room temperature in this country would actually increase the ccc.

Plus of course the terminals on an immersion heater and cable grip are designed for 1.5 so using bigger cable often leads to broken cable clamps and poor terminations

Hi

Which tabulated values are they dave? i think you will find they are max values, reference a table please.

Well Dave the life span of a cable reduces the higher the temp, waste money, i dont think so, i bet the I2R losses would easily pay for the bigger cable.

Those little cupboards get toasty with a big copper radiator an ambient temp of 35 C is prudent dont you think?

Your clearly using inferior elements, go for quality.

Cheers

 

[Midwest]

Hi

Which tabulated values are they dave? i think you will find they are max values, reference a table please.

Well Dave the life span of a cable reduces the higher the temp, waste money, i dont think so, i bet the I2R losses would easily pay for the bigger cable.

Those little cupboards get toasty with a big copper radiator an ambient temp of 35 C is prudent dont you think?

Your clearly using inferior elements, go for quality.

Cheers

Looking at table 4F3A, which states current-carrying capacity (no mention of minimum or maximum). I note the current carrying capacity of flexible cables (single phase & non armoured) for 1.5mm & 2.5mm to be 16amp & 25amp respectively. The table does not submit an ambient temperature for those particular figures. However, does start to give rating factors for ambient temperatures from 35'c.

It also provides rating factors for 110'c, 150'c flexible cables, 35-80'c x 1 & 35-120'c x 1 respectively, i.e. no effect on current carrying capacities for those cables at those temperature ranges.

If we are to assume the ambient temperature of an immersion heater cupboard to be 40'c (i'e just measured mine at 33'c), a rating factor of 0.82 would apply.

Therefore, a standard flexible cable of 1.5mm & 2.5mm, would have capacity of 13.12a & 20.5a. Heat resistant flexible cables would not have their capacities, affected by this particular ambient temperature.

I see that some heat resistance flexes are sold as heat resistance to 85-90'c range.

My observations are; standard flexible cables would be at or above their limit, in terms of their current carrying capacities in the above scenario, whereas heat resistance would not. To use 2.5mm heat resistance flex for 3kw immersion heater, is not necessary whereas 1.5mm is more than sufficient.

 

[GEOFF S]

Looking at table 4F3A, which states current-carrying capacity (no mention of minimum or maximum). I note the current carrying capacity of flexible cables (single phase & non armoured) for 1.5mm & 2.5mm to be 16amp & 25amp respectively. The table does not submit an ambient temperature for those particular figures. However, does start to give rating factors for ambient temperatures from 35'c.

It also provides rating factors for 110'c, 150'c flexible cables, 35-80'c x 1 & 35-120'c x 1 respectively, i.e. no effect on current carrying capacities for those cables at those temperature ranges.

If we are to assume the ambient temperature of an immersion heater cupboard to be 40'c (i'e just measured mine at 33'c), a rating factor of 0.82 would apply.

Therefore, a standard flexible cable of 1.5mm & 2.5mm, would have capacity of 13.12a & 20.5a. Heat resistant flexible cables would not have their capacities, affected by this particular ambient temperature.

I see that some heat resistance flexes are sold as heat resistance to 85-90'c range.

My observations are; standard flexible cables would be at or above their limit, in terms of their current carrying capacities in the above scenario, whereas heat resistance would not. To use 2.5mm heat resistance flex for 3kw immersion heater, is not necessary whereas 1.5mm is more than sufficient.

Hi

I find this a strange remark, 1.5mm is a minimum and with some cables inadequate. Unnecessary means excessive, so what is excessive about installing 2.5mm cable, the cost, crikey?

Ill give you some pro's, you give the con's

Lower operating Temp
Less Volt Drop
Lower I2R
Can be used as a supplemetary bonding conductor
A longer life span
More Robust
Pretty much all flexible cables even derated will be okay.

Cheers