Mixing cable sizes

[Darkness]

I'm wiring a large building using conduit and singles and at various conduit/termination boxes I will be running a foot or so of 3-core flex from it to a 100w light out side.

For example I've one circuit with two 100w lights on it since I'm using 1.5mm² singles, should the flex also be 1.5mm²?

I've used 1.5mm² in the past but several jobs I've seen in the past number of weeks, the flex was either 0.75mm² or 1.0mm² with 1.5mm² singles.

What should I be using?

 

[supersparks]

are you qualified to be wiring this large building?

 

[Darkness]

Nope, its mine and I will be doing so anyhow.

I thought if the circuit was wired using 1.5mm² then in my case the flex should be 1.5mm².

 

[andy]

Nope, its mine and I will be doing so anyhow.

I thought if the circuit was wired using 1.5mm² then in my case the flex should be 1.5mm².

cable should be suitable protected by overload, in this case its probably the MCB in CU. so depending on MCB rating (id imagine 6A) you should be using cable what can take over 6A. altho i would just use 1.5 3core

 

[Darkness]

Cheers for that Andy.

It's just that I was puzzled to why lower rated flex was used in the other installations wired by a Pro.

Thanks for the reply.

 

[Spark123]

As long as "Iz > In >Ib" (taking into account correction factors) is fulfilled and volt drop is OK, using a smaller CSA cable is OK.

Ib = design current
In = nominal current or current setting of protective device
Iz = current carrying capacity of cable

 

[Lectrician]

As the light fitting is a fixed load, you can feed it using 0.75mm flex from the 1.5mm cable no probs.

Think of a domestic pendent drop.



Tips:

With conduit wiring, you may share the earth between circuits. If you wire a 2.5mm socket radial, and a 1.5mm lighting cct in tube, just run one 2.5 earth for both circuits, looping in and out as required.

Rather than joining flex to cable in the circ boxes, use gewiss 4x4x2 boxes, drilled as appropriate. More space for term blocks etc.

When pulling cables in, always pull ALL cables in at once - don't try pulling one circuit in, and then the next. Also, don't try pulling extra through if your short, you need to move all cables at once, other wise they will friction burn each other.

Make sure you put a drain hole in any accessories that you have conduit entering into the top of.

Dont use inspection tee's or elbow's - use circ boxes and pulled bends.

 

[Darkness]

Thanks for the tips Lectrician.

Using Gewiss/Saip boxes when more room is required but some sparky's said they were a bad idea as the tops pop off easily and are not as weatherproof as your standard conduit box with rubber gasket etc. I find them great for their job though and the lids aren't that easy to get off sometimes.

When joining flex in the circ boxes, I drill a hole in the plastic lid and fit a Nylon Gland to it and run the flex though it.

I find it the simpliest way to take a length of flex from circ boxes.

 

[Lectrician]

Trust me, a 4x4x2 box is the way to go.....they have 4 screws, one each corner for a start

You are thinking of the ****e circular ones with pvc stepped grommets - they are crap.