Current Carrying Capacity - Just wanted to check ?

[self builder 123]

Hi

I was wondering if someone could check my understanding of the current capacity of a PVC insulated Twin + Earth 6 sq.mm cable which we have installed.

The cable has been clipped direct to the timber frame and has mineral wool insulation placed to the side of it.

It is not grouped with other cables and I do not expect the ambient temperature around the cable to exceed 30 deg C.

The cable is run direct from the CU and is protected with a 32A Type B MCB.

The cable supplies an induction hob only. We powered up the hob turning up all the rings to full power and using a clamp meter ( around the live wire at the CU ) measured that the maximum current drawn was 34A.

The 32A MCB did not trip after waiting for several minutes - is this normal ?

Based on Table 4D5A Installation Method 15 that the cable will be capable of conducting 35A.

To be honest I never expect all the hob rings to be switched on to maximum at the same time. I just wanted to see just how much current would be drawn in the worst case ?

Is there anything to worry about ?

 

[scarypants]

Is the thermal insulation to the side or covering the cable?
If cable clipped direct and not groupedor or run through thermal insulation and on breaker can carry up to 47A.
Your breaker will carry a little more than it's rating without tripping.

 

[scarypants]

What output has the appliance?

 

[self builder 123]

Hi

The insulation is to the side. One of the sides ( flat face ) of the cable is directly clipped to the timber frame.

The strange this is that the hob has a total theoretical loading of 10.8kW, which assuming 230V is a current rating of 47A, but it only draws 34A ?

 

[rebuke]

It's a concept known as diversity - it may draw 47A, but probably only for very brief periods, as each ring will be cycling on and off...

As for why the breaker didn't trip - MCBs generally have two parts, a magnetic and thermal. The magnetic will trip very quickly in the event of a large overload, in the event of a small overload, if it lasts long enough the thermal part will gradually heat up and eventually cause a trip. There are tables/graphs available that will show how long this takes, but it doesn't surprise me that a 2A overload didn't trip the MCB after a few minutes - I suspect it would probably sustain that indefinitely.

 

[self builder 123]

Rebuke

Cheers - am I correct to say that the cable can handle 35A ?

 

[rebuke]

It depends - if the insulation is just to the side of it and not touching it, then I would call that clipped direct (reference method C) which gives 47A, if it's a stud wall with insulation covering it and it's not touching the inner wall surface (reference method 103) then that's only 23.5A - can you be a bit more specific about how it's installed?

 

[self builder 123]

Rebuke

The cable is clipped directly to the timber joints. The mineral wool insulation was then fitted right up against the cable. The cable does not lie on top of the plasterboard.

When you say inner wall surface do you mean the rear of the plasterboard ?

 

[rebuke]

Yes I do, but as you say joists is this above a ceiling, as if so then it's slightly different (ref method 103 is for a stud wall). Above a ceiling it would be either ref method 100 (if the insulation is <= 100mm in thickness), or ref method 101 (if the insulation is > 100mm thickness), which have capacities of 34A and 27A respectively...

 

[self builder 123]

Rebuke

The consumer unit is fitted close to the ceiling.

The cable comes out of the CU up into the ceiling where it is clipped 1/2 way down the 200mm deep ceiling joists.

It thus runs through the ceiling space between the ground floor and the upper floor.

It then drops down the 150mm deep stud partition where it connects to the isolation switch for the induction hob.

 

[scarypants]

If it is routed through thermal insulation, current carrying capacity will reduced according to the distance it runs through it anything over 500mm and it will be halved so 47A becomes 23.5A and a breaker should then be used to supply no more than 23.5A or the cable size will need increasing.
The ideal way to calculate the current rating of the protective device for your cooker is to do this sum. output/volts then calculate for diversity this will be different if a socket-outlet is on the cooker isolating switch.
you have 10800watts/230v=47A
47A is your design current.(ib)
Using Appendix 1 OSG we can offer diversity to the ib
I work it out at 21.1A if no socket-outlet and 26.1 with one.
So your cable size 6mm csa is okay if routed through insulation providing no socket-outlet on cooker isolator switch and no other factors regarding grouped cables, ambient temps and not protected by BS3036 fuse.

 

[self builder 123]

Cheers Scarypants - much appreciated !