Immersion Heater Cable

[kent]

I want to replace the cable going into our standard 3 kW immersion heater. I understand that this cable needs to be heat resistant. Can anyone tell me if HO5 V2V2 - F cable is suitable? Many thanks.

[Andy Wade]

No, those Vs mean it's a 70 deg. rated PVC/PVC cable. What you need is

85 deg. harmonised type H07BN4-F (UK CMA code 3183TQ), 1.5 mm^2 3-core.

E.g.

[NT]

capable. Not only does the flex connect to hot terminals, but it also heats up due to current flow, both need to be taken into account.

NT

[Dave Plowman (News)]

Given 1.5mm is already more than adequate at 16amps - and the run is usually short - I'd call that overkill.

[NT]

1.5mm2 is rated at 20A when used clipped direct. That means that it can rise from 20C to 70C with 20A load. 16A thus gives us a temp rise of 16/20 x 50C =3D 40C.

But in this case it connects to terminals we expect to run at around

60-70C, lets say 70. 70 plus a temp rise of 40C =3D 110C. So your 1.5mm2 cable doesnt have good long term prospects because of the combination of the 2 heat sources. 2.5mm2 cable is rated at 27A clipped direct. 16A will increase its temp less, giving it a bit more margin.And butyl rubber is better able to handle such temps than pvc, hence my choice.

NT

[Dave Plowman (News)]

There's a flaw in this reasoning. Can't quite put my finger on it.

If 1.5mm butyl wasn't suitable for immersion heaters, why is it the choice?

Oh - the bit of butyl feeding the one here seems to survived some 30 odd years without problems. ;-)

[js.b1]

Just in case a Screwfix is local to you...

Screwfix do 2.5mm butyl in 1m cut lengths for =A32.83 inc VAT.

- part 16684 - 1m cut length - HO7BN4F - 3183TQ - Butyl

- 2.5mm=B2, 3-core, heat-resistant, white flexible cable

- 240V / 20A

It is actually H07BN4F (zero-seven not ohh-seven).

H =3D Harmonised

07 =3D 450/700V rated (thicker insulation) B =3D Butyl Rubber Insulation N =3D Neoprene Sheath 4 =3D can not remember F =3D Fine flexible wire (as opposed to thinner ultrafine or tinsel)

Not sure it is a neoprene sheath, I think it is butyl too. Butyl will discolour (go yellow) in direct UV thus not for outside usage.

Quality of termination is vital. Wire termination, more surface area the better. Element crimps can be poor, some wobble, some undersized crimps. Thermostat terminals can have tiny screws, low clamping force.

[Andrew Gabriel]

Power loss is proportional to current squared (P = I**2 x R), so your multiplier should be (16/20)**2, which gives (16/20)**2 x 50C = 32C.

However, most rubber cables have max operating temperature > 70C. I don't have any current capacity tables on me, but they're probably rated at a higher current rating as a result.

[NT]

presumably because its cheaper and is usually ok for many years. But it does disintegrate at the ends sometimes.

NT

[Andy Wade]

Quite. Also NT is starting from ratings for non-flexible fixed wiring cables. Table 4F3A of BS 7671 (flexible cords, non-armoured, copper conductors) is probably a better place to start:

1.5 mm^2 - 16 A 2.5 mm^2 - 25 A

Then apply the given deratings for 90 deg. thermoplastic or thermosetting cords:

Ambient temp Factor ------------ ------ 35 - 50 deg. 1.00 55 deg. 0.96 60 deg. 0.83 65 deg. 0.67 70 deg. 0.47

Which suggests that for a 3 kW heater (13 A) you're OK up to about 60 deg. ambient. How hot do the terminals get? I don't know, but the terminal box cover is supposed to be left uninsulated (thermally). Maybe 1.5 is a bit marginal, but 99.9% of the nation's immersion heaters are wired up with that size and I can't say I've seen many problems with the insulation crumbling (except where a completely inappropriate cable type has been used).

Careful - ordinary rubber flexes (H0xRRF etc.) and old rubber fixed wiring cables like TRS are/were only rated at 60 deg. conductor temperature.

[Andy Wade]

N4 is 'CSP' - chlorosufonated polyethylene, see

example. That's similar to neoprene, I think - any chemists around?

The insulation (B) is EPR (ethylene-propylene rubber and the sheath (N4) is CSP.

[Dave Plowman (News)]

So does PVC into a socket etc if the connection isn't properly made.