cable size to a sub-fuse

[briwire]

What are the regs regarding the size of a cable feeding a fuse. Does it have to be large enough to be protected by the suuply fuses regardless of distance?

For example. Imagine a DB fed from a 100A fused supply. I need to fit a 20A fused switch at the side of the DB. There are no spare ways to connect to and I have to go on the live cables coming into the DB. There isn't enough room to get another 25mm cable into the terminal. Am I allowed to use say a 6mm to feed the new fused switch, which is approximately a meter away.

I've always tried to use the same size cable but I have a project coming up where this is a problem. I've looked through the 16th edition with no success.

 

[baz1968]

Are you Part P registered, all works on a DB are notifiable.

 

[Pensdown]

Yes you can do that but it would be classed as a lash up.

There are a couple of options. If you can isolate the supply you could fit henley blocks and install a new DB. If you can't isolate the supply you could remove one circuit from the existing DB and use the way to feed a new DB. The circuit you have removed and your new one could then be fed properly from the new DB.

 

[Adam_151]

Are you Part P registered, all works on a DB are notifiable.

Close but no cigar... while the work the OP intends to do would certainly be notifiable! (new circuit), there is nothing in the SI which makes work inside a DB automatically notifiable, for example the following would be non notfiable works:

Adding a spur from a ring final circuit at the orgin of the circuit
Replacing a damaged cable for a circuit with the same size and type
Refixing a loose DB enclosure to the wall


In answer to the original question, there are instances where protection can be placed downstream providing criteria are met, but IMHO what you intend to do sounds rough and I'd question the suitability of the terminals to make a sound connection on 25mm² and 6mm² conductors...much better in my mind to remove the lowest rated two circuits from the DB and put them on a DB at the side and feed that from a breaker as a submain, and voila enough ways (still doesn't look the neatest though, for that you'd have to replace the DB with one with more ways!)

 

[briwire]

It's probably best to describe the exact situation. It's in a factory, and up in the northlight area of the roof there is an old DB. Its source is in the switchroom of the factory about 200 yards away, fused at 100A, its supply looks to be 25mm.

On the side of the DB there is an isolator and a contactor that are in series, so both actually supply the DB. The contactor is 2 way switched from near entrance points to the area. So de-energising the contactor will isolate the whole DB. The lighting for the area is then fed from the various fuseways in the board.

However the DB also supplies outside lights. The factory workers turn the lights off on a sunday afternoon. Then when the morning shift turn up at 6am on the monday, the outside is in darkness.

I want to take out the supply for the outside lights. Connect it to a new switched fuse mounted next to the DB. Pick up a permanent live from the supply side of the contactor and use this to supply the outside lights which are on photo cells.

The problem is that the contactor terminals are only just big enough for the present 25mm. I might be able to get a 6mm in with them.

 

[Adam_151]

SP or TP DB?

And check i've got this right, the submain runs through a contactor in a separate DIN enclosure and then into the boards isolator?

 

[Pensdown]

200 yards for a 100amp supply to a dis board in 25mm? Somethings very wrong there.

 

[briwire]

You want to see the rest of the factory!

The 3 phase and neutral supply comes into a seperate isolator fixed to the side of the DB. From there it goes through a contactor situated in an enclosure again screwed to the side of the DB. It then goes into the DB and directly to the busbars.

So all the DB goes dead when the contactor is de energised.

 

[plugwash]

Afaict it is acceptable to have overload protection downstream of the cable to be protected but short circuit protection must be upstream.

In other words you need to calculate the adibatic to check if the cable size you intend to use will be acceptablly protected or not.

 

[Adam_151]

for 100A on 25mm² assuming no factors and a sensible install method, then it'll be volt drop that'll be biting, not current carrying capacity

And anyway it would be iffy in this circumstance anyway, suppose you could use the undiversified sum of ratings of the prot devices in db as like the total that could flow due to the nature of the load, but it would be strange and someone might add more breakers without realising... if there was an overcurrent device as the incommer to the board then it'd be more likely to fly! anyway I digress!