Fuse rating required for fluorescent fittings?

[crj10]

I recently installed non-corrosion fluorescent fittings in my garage to replace the mess left to me by the house's previous owner.

What size fuse must be used to protect a fluorescent fitting? The instructions didn't say, but the fitting's internal wiring looks like it could only take 5A or so. Presumably wiring it to a 30A ring main would be a bad idea, but how about a 13A fused spur, for example?

As it happens I wanted to use thinner cable anyway, so I put the lights on a 3A fuse. But I'm still curious.

 

[Spark123]

The norm in domestic is a 5A or 6A lighting circuit. If there are a lot of lights on one circuit then you may find them on a 10A.
If you don't have many fittings on them then you may get away with a 3A fuse.

 

[crj10]

Mmm. Which tends to suggest that a fluorescent fitting can cope with up to 10A? I know that's the limit for most light switches, so it make sense. But I'm surprised not to see it written down anywhere.

As I said, the wiring inside the fitting I have looked pretty flimsy, but I suppose you don't need very thick wire for 10A carried half a metre clipped to a metal tray.

 

[JameZZ]

The light fitting isn't going to use 10A all to it's self though... unless it had a 2300W bulb

 

[crj10]

The light fitting isn't going to use 10A all to it's self though... unless it had a 2300W bulb

Mmm. But what if someone wanted to, I dunno, run a hundred 36W fluorescent tubes off a single 20A circuit?

 

[Spark123]

I doubt that would be a good idea! Your MCB would almost certainly trip with startup current
I think the biggest fuse I have come across with fluorescents is 16A, not to say that is a good idea in domestic instances.
As a rule of thumb you need to multiply your loads by 1.8 when using discharge lighting, also consult the MCB guidance from the manufacturer as to how many lights they can take.

 

[EFLImpudence]

What size fuse must be used to protect a fluorescent fitting?

The fuse is there to protect the cable - not what you put on the end of it.

 

[Taylortwocities]

What size fuse must be used to protect a fluorescent fitting?

The fuse is there to protect the cable - not what you put on the end of it.

Hurrah, the correct answer, at last!

PS Don't forget the manufacturer may require a specific fuse value.

 

[plugwash]

To at least some extent it's there to protect both the cables and the connected devices. The connected devices may have some protection of it's own but that protection if present at all will have limitations .

Really to allow a safe design manufacturers should specify what level of protection is adequate for their devices. Unfortunately they all too often don't requiring the installer to make an educated guess as to what is reasonable.

IIRC the regs specify a maximum rating of 16A for lighting circuits with normal ES and BC lampholders and I would generally treat this as an absolute maximum for lighting circuits in general (with the exception of circuits feeding a single very large light). Also IIRC the regs specify a lower maximum (I think it's 6A but i'm not positive) for circuits with SES and SBC lampholers.

 

[EFLImpudence]

Which tends to suggest that a fluorescent fitting can cope with up to 10A?.

It doesn't have to.

But what if someone wanted to, I dunno, run a hundred 36W fluorescent tubes off a single 20A circuit?

Then you would need larger cable than the usual 1mm² used for domestic lighting circuits. Would you then expect each lamp to 'cope' with 20A?

PS Don't forget the manufacturer may require a specific fuse value.

True, but manufacturer's should fit their appliances with an integral fuse, if necessary, to save us having to dig holes in people's walls for another (ugly) FCU.

 

[JohnW2]

The fuse is there to protect the cable - not what you put on the end of it.

I keep reading that here, but personally would not think that it's strictly true.

Am I the only person who has plenty of FCUs and 13A plugs with 3A or ('non-standard') 1A fuses in them, despite the fact that the cable could happily carry far more current than that - because I am attemping to afford some degree of protection to 'what is on the end of the cable'?

For a start, the load is going to have some internal wiring (which can present a fire risk), very probably with smaller current carrying capacity than the supply cable. Not only fire risk, but even the extent of internal damage due to a fault can be minimised by having as 'tight' prtection as is possible.

As far as I am concerned, the common sense approach whenever there is a fixed load (FCU, 13A plug or MCB of a dedicated circuit) is to use the lowest rated fuse (or MCB) that will allow normal functioning of the load (including any start-up surges etc.), even if that means a protective device rated far below what the cable could stand.

Furthermore, if you really believe that protective devices exist only to protect the cables, why do we normally use a 6A MCB to protect a lighting circuit run in 1.5 mm² cable, which could happily cope with more than double that current?

Kind Regards, John.

 

[EFLImpudence]

For a start, the load is going to have some internal wiring (which can present a fire risk), very probably with smaller current carrying capacity than the supply cable.

These wires are not going to be overloaded as they only supply their own equipment.

As far as I am concerned, the common sense approach whenever there is a fixed load (FCU, 13A plug or MCB of a dedicated circuit) is to use the lowest rated fuse (or MCB) that will allow normal functioning of the load (including any start-up surges etc.), even if that means a protective device rated far below what the cable could stand.

Alright if you don't mind (ugly and) unnecessary FCUs all over the place. Dedicated (single) load circuits aren't going to be overloaded either .

if you really believe that protective devices exist only to protect the cables, why do we normally use a 6A MCB to protect a lighting circuit run in 1.5 mm² cable, which could happily cope with more than

I don't really understand the 'fashion' to quote 1.5mm² for lighting circuits when 1mm² is more than adequate (normally).
To attempt to answer your question, although I stand to be corrected, I would say because light fittings and switches are only rated at 6 or 10A

 

[JohnW2]

For a start, the load is going to have some internal wiring (which can present a fire risk), very probably with smaller current carrying capacity than the supply cable.

These wires are not going to be overloaded as they only supply their own equipment.

I'm thinking of fault conditions within the equipment. In the absence of a lower-rated fuse within the equipment, that wiring could be damaged and/or catch on fire at a current allowed by a protective device rated only to protect the supply cable.

As far as I am concerned, the common sense approach whenever there is a fixed load (FCU, 13A plug or MCB of a dedicated circuit) is to use the lowest rated fuse (or MCB) that will allow normal functioning of the load (including any start-up surges etc.), even if that means a protective device rated far below what the cable could stand.

Alright if you don't mind (ugly and) unnecessary FCUs all over the place. Dedicated (single) load circuits aren't going to be overloaded either .

It just seems common sense to me. It's not just about FCUs (which don't have to be visible, even if ugly!) but also 13A plugs. Dedicated circuits could also present the same problem if they were wired in unnecessarily large cable and protected by a device appropriate for the maximum current carrying capacity of that cable (which I would call a design error - but could be justified if the protective device were only there 'to protect the cable').

if you really believe that protective devices exist only to protect the cables, why do we normally use a 6A MCB to protect a lighting circuit run in 1.5 mm² cable, which could happily cope with more than

I don't really understand the 'fashion' to quote 1.5mm² for lighting circuits when 1mm² is more than adequate (normally).

I agree, but 1.5 mm² is frequently used, and that's not the issue under discussion.

To attempt to answer your question, although I stand to be corrected, I would say because light fittings and switches are only rated at 6 or 10A

So how are you going to get out of that one, then? You appear to be saying that the rating of the protective device is being determined by 'what is on the end of the cable', not the cable itself which you suggested is the only thing that it is protecting?

Kind Regards, John

 

[bernardgreen]

For a start, the load is going to have some internal wiring (which can present a fire risk), very probably with smaller current carrying capacity than the supply cable.

These wires are not going to be overloaded as they only supply their own equipment.

I am with JohnW2 here, if the equipment goes faulty and starts taking more than normal current then its internal wiring could be overloaded and present a fire risk.

 

[ericmark]

1) The regulations say:- 559.6.1.6 Lighting circuits incorporating B15. B22. E14. E27 or E40 lampholders shall be protected by an overcurrent protective device of maximum rating 16A. As a result it is very rare to use over 16A per circuit.
2) The ceiling rose is rated 5/6A so in the main lighting circuits are limited to 6A as somewhere there is likely a ceiling rose.
3) Florescent fittings normally have the BS 1363 fuse holder and the BS 1363 fuse has two preferred sizes 3A and 13A so in the main fittings have a 3A BS 1363 fuse fitted.
4) Lighting track normally have larger physical size fuses so tend to fit 5A fuses. Most track rated at 16A so even with very large buildings three phase 16A supply to a lighting track as about the biggest supply used for lighting.
5) As already said the cable is protected by fuse and since 1.5mm common then again 16A likely to be max size.
6) Florescent lamps take more power than the tube says so for 16A supply 2500W is about the max. For a 6A supply no more than 900W unless all fittings are HF type.

Personally I would be normally consider 6A Max and with something very special may go up to 16A.