Theorectical discussion on fusing down smaller cables

sparkwright · 11 Jul 2019

We have had many discussions on this kind of topic, how acceptable and different are the following scenarios:

1) a 40amp 6mm2 shower circuit with a 1.5mm2 t+e spurred off it serving a 5amp FCU serving a light again in 1.5mm2.

2) a '100amp supply' with 25mm2 tails supply a Henley block feeding a household cu, with 6mm2 tails spurred from the Henley, supplying a 2 way board serving two 16amp fixed load circuits.

To me that that sounds like two identical situations.

Yours thoughts please. Discuss.

EFLImpudence · 11 Jul 2019

2) is fine - assuming short-circuit current and distance are satisfactory.

1) 4mm² is not adequate for a 40A shower - but the spur is alright.

AndyPRK · 11 Jul 2019

6mm tails or 16mm tails?

Are you saying if 2 is acceptable, then 1 should be acceptable?

sparkwright · 11 Jul 2019

EFLImpudence said:
2) is fine - assuming short-circuit current and distance are satisfactory.

1) 4mm² is not adequate for a 40A shower - but the spur is alright.

6mm2 not 4mm2.

sparkwright · 11 Jul 2019

AndyPRK said:
6mm tails or 16mm tails?

Are you saying if 2 is acceptable, then 1 should be acceptable?

6mm2 tails not 16mm2 tails.

I'm not actually saying either are acceptable at this stage, I'm leaving this open to debate at present.

SUNRAY · 11 Jul 2019

sparkwright said:
We have had many discussions on this kind of topic, how acceptable and different are the following scenarios:

1) a 40amp 6mm2 shower circuit with a 1.5mm2 t+e spurred off it serving a 5amp FCU serving a light again in 1.5mm2.

2) a '100amp supply' with 25mm2 tails supply a Henley block feeding a household cu, with 6mm2 tails spurred from the Henley, supplying a 2 way board serving two 16amp fixed load circuits.

To me that that sounds like two identical situations.

Yours thoughts please. Discuss.

With reservations both scenarios are ok. In my experience and opinion.

sparkwright · 11 Jul 2019

Reservations.

Which are?

SUNRAY · 11 Jul 2019

sparkwright said:
Reservations.

Which are?

Being on site to survey for myself for a start...

sparkwright · 11 Jul 2019

SUNRAY said:
Being on site to survey for myself for a start...

That sounds like trying to find an alternative method.

EFLImpudence · 11 Jul 2019

sparkwright said:
6mm2 not 4mm2.

Then all is well.

SUNRAY · 11 Jul 2019

sparkwright said:
That sounds like trying to find an alternative method.

No, it's all about surveying the situation. It may very well involve a different method but it's more about safe practices.

For example I have tapped off a 1200A 3ph supply with something like 2.5mm²but there is no way on earth I would tell someone on here that it is OK.

sparkwright · 11 Jul 2019

Yes, people do react differently when there's a break from the norm.

EFLImpudence · 11 Jul 2019

What is it you think might be a problem?

JohnW2 · 11 Jul 2019

SUNRAY said:
For example I have tapped off a 1200A 3ph supply with something like 2.5mm²but there is no way on earth I would tell someone on here that it is OK.

Overload protection (if necessary) for the 'small' cable can obviously be addressed by downstream fuses, so it's achieving adequate fault protection which is the potential issue (and that's likely to be a theoretical problem with 2.5mm² cable protected by a 1200A OPD). However, pragmatically speaking, if the situation is such that the probability of a fault (short) in that smaller cable is very low (e.g. if it is in steel conduit), then I think there may well be a good argument for applying 'common sense discretion'.

Kind Regards, John

SUNRAY · 11 Jul 2019

JohnW2 said:
Overload protection (if necessary) for the 'small' cable can obviously be addressed by downstream fuses, so it's achieving adequate fault protection which is the potential issue (and that's likely to be a theoretical problem with 2.5mm² cable protected by a 1200A OPD). However, pragmatically speaking, if the situation is such that the probability of a fault (short) in that smaller cable is very low (e.g. if it is in steel conduit), then I think there may well be a good argument for applying 'common sense discretion'.

Kind Regards, John

Correct and that is precisely why I'll not be offereing further advice or opinion in this thread as someone is bound to do something silly if I say it is OK and it's bound to start a silly arguement.

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Theorectical discussion on fusing down smaller cables

Similar threads