Downstream of FCU

[ldoodle]

Hey,

Is there any theoretical limit to the number of downstream outlets from an FCU?

In my bedroom, I have a dual accessory backbox (double + single). The single is an FCU, that feeds a flex outlet behind the wall mounted TV. There are no other devices here so a single flex outlet was fine.

I have recently got a Chromecast dongle that requires power, so I thought about getting one of the MK twin USB socket accessories and putting it after the current flex outlet, so in practice making a radial downstream of the FCU.

My understanding is that as it's downstream of an FCU, it's OK (as long as the cable between FCU and accessory/ies is adequate for the total load - in my case it's 2.5mm).

Thanks

 

[JohnW2]

Is there any theoretical limit to the number of downstream outlets from an FCU?

No limit to the number of outlets. The total amount of current available to all those outlets will obviously be limited by the fuse in the FCU, hence 13A max. Do I take it that this FCU is fed from a sockets circuit, not a lighting one?

My understanding is that as it's downstream of an FCU, it's OK (as long as the cable between FCU and accessory/ies is adequate for the total load - in my case it's 2.5mm).

Indeed - and 1.5mm² cable would nearly always be adequate (for a max 13A load).

Kind Regards, John

 

[ldoodle]

Yes, socket circuit (for upstairs/loft on a 32A ring)

 

[PrenticeBoyofDerry]

My understanding is that as it's downstream of an FCU, it's OK (as long as the cable between FCU and accessory/ies is adequate for the total load - in my case it's 2.5mm).

Your understanding is correct, but the fuse is there to protect the cable, so it also about the fuse being adequate to protect the cable if there is an overload.
So you would normally calculate the demand required, the fuse correctly for that load and then calculate the cable size that is suitable, equal or greater than that of the protective device.
The current carrying capacity of the cable would be related to cable/flex type, the CSA of the conductors and any de-rating factors that would be considered in the method/route the cable was installed.

 

[JohnW2]

So you would normally calculate the demand required, the fuse correctly for that load and then calculate the cable size that is suitable, equal or greater than that of the protective device. The current carrying capacity of the cable would be related to cable/flex type, the CSA of the conductors and any de-rating factors that would be considered in the method/route the cable was installed.

I think that one really should use a cable with an adequate CCC should a 13A fuse ever be put into the FCU, even if the current load is such that a smaller fuse is appropriate at present. In practice, of course, one is not allowed to use smaller than 1.5mm² cable, and that would almost always be adequate for 13A (unless surrounded by thermal insulation) and 2.5mm² would essentially always adequate for 13A.

Kind Regards, John

 

[ldoodle]

As it's just the TV at the moment, the fuse is 5A as that's what came in the TV's plug.

 

[JohnW2]

As it's just the TV at the moment, the fuse is 5A as that's what came in the TV's plug.

Fair enough - but, as I just wrote, I think the cable should be adequate for a 13A load, in case a 13A fuse is ever put inrto the FCU for any reason. However, as I also said, very probably 1.5mm² cable, and certainly 2.5mm² cable (such as you have), would satisfy that, so there isn't really any issue.

Kind Regards, John

 

[deadshort]

Is there any theoretical limit to the number of downstream outlets from an FCU?

No limit to the number of outlets. The total amount of current available to all those outlets will obviously be limited by the fuse in the FCU, hence 13A max. Do I take it that this FCU is fed from a sockets circuit, not a lighting one?

My understanding is that as it's downstream of an FCU, it's OK (as long as the cable between FCU and accessory/ies is adequate for the total load - in my case it's 2.5mm).

Indeed - and 1.5mm² cable would nearly always be adequate (for a max 13A load).

Kind Regards, John

"nearly always" what electrical term is that ?

Kind regards,

DS

 

[JohnW2]

Indeed - and 1.5mm² cable would nearly always be adequate (for a max 13A load).

"nearly always" what electrical term is that ?

As I clarified in a subsequent post, it's a technical term which means "unless it is surrounded by thermal insulation". As I've also said, since we've been told that the OP has got 2.5mm² cable, the suitability of 1.5mm² is all a bit moot.

Kind Regards, John