Showers! 6mm T&E cable to be used for shower

[Mike2007]

It seems a HUGE question in this forum, the cable to be used for an electric shower.
I am replacing my electric shower with another one, but have just found out after using the shower for 8 years the cable is under sized at 6mm csa. Still can not locate the wattage of the shower but obviously it would contravene the regs surely??
Whats the current carrying capacity for 6mm cable versus 10mm cable (in ideal conditions of course).

 

[GaryMo]

If you don't know the KW rating of your shower then how do you know the cable is undersized?

What's protecting the cable? Rewirable fuse, cartridge fuse, MCB?
Installation method of cable (really important) - Clipped direct, run in trunking, run within thermal insulation?
Length of run? Again, it's very important to access volt drop.

Answer the above questions and we may be able give you an idea of the size of shower to purchase.

 

[Mike2007]

The cable run is roughly 16m long, through floor cavity and initially 2.5 m up a plaster wall from CU (generally, its a cold 100 year old house).
I think its a rewirable fuse but not investigated. Will go with any shower size that is safe on this cable!

 

[Mike2007]

Whats the right page in the OSG or 16th edition for these tables of current rating/derating?

 

[GaryMo]

Current carrying capacity

BS7671 - Table 4D2A, pg222
OSG - Table 6E1, pg123

Volt drop

BS7671 - Table 4D2B, pg223
OSG - Table 6E2, pg 125

Corrections factors

OSG - pg115-117

Going off your information and possible 30A BS3036 rewirable fuse then I would say a 7KW shower would be safe.

7000/240=29.17A

6mm (mehod 1, clipped direct) can carry 46A

Assuming 30A BS3036 fuse

30/0.725=41.38

46A is more than or equal to 41.38 so we're fine there.

Assume 30 degrees so no correction factor, no thermal insulation and no grouping of cables - all best condition.

Voltdrop, assuming your 16m is correct - 3.5v which is less than the maximum of 9.2v

 

[Mike2007]

I think the present shower is slightly larger than that and its never caused problems? I know that with diversity (only is used for 10 mins in morning, once at night) this may say why its lasted and not degraded to a dangerous level!

You have probably heard this statement hundreds of time before?

Thanks for the info.

 

[GaryMo]

I know that with diversity

Isn't to be taken into account on a circuit containing an instantaneous water heater (shower).