Electrical extension length limit

[john2k]

Is there a limit to how far a electrical extension cable can go? I want to add a few low power amplifers inside a suspended ceiling and only run one cable from the power source and then just daisy chain from there.

 

[John D v2.0]

The limit is i think about 100m last time we calculated, and that was using a socket at the limit of the circuit and a 13A fuse.
If you are running low power items, and are concerned, just change the plug fuse to 3A.
The volt drop would be small for the typical loads from small power supplies

 

[SUNRAY]

The limit is i think about 100m last time we calculated, and that was using a socket at the limit of the circuit and a 13A fuse.
If you are running low power items, and are concerned, just change the plug fuse to 3A.
The volt drop would be small for the typical loads from small power supplies

100m of 1.5mm² cable is about 2.5Ω and volt drop @ 13A about 32v or 198v assuming a perfect 230v at the source [191v for 1.25mm²]

IIRC the limit for 1.25mm² is about 30m for PAT.

 

[SUNRAY]

Is there a limit to how far a electrical extension cable can go? I want to add a few low power amplifers inside a suspended ceiling and only run one cable from the power source and then just daisy chain from there.

I take it this is domestic, in which case It is likely to be trouble free. I wouldn't have it meandering all over the house plugging in one lead after another though.

Chances are it would not be acceptable in a commercial property.

 

[John D v2.0]

100m of 1.5mm² cable is about 2.5Ω and volt drop @ 13A about 32v or 198v assuming a perfect 230v at the source [191v for 1.25mm²]

true although I was assuming the very small loads mentioned by the OP. The 13A was the fuse I assumed in for the fault protection hence the suggestion to change to 3A.

 

[crystal ball]

A slight off topic, when I was working in Zimbabwe, one of the sites I was looking after asked me why all the power tolos were running slow, the main supply was via a flex strung along poles, I walked the route and after 2 kilometres found the source

 

[SUNRAY]

true although I was assuming the very small loads mentioned by the OP. The 13A was the fuse I assumed in for the fault protection hence the suggestion to change to 3A.

In a domestic situation there is little likelyhood of getting anywhere near the limits, assuming there are not large amounts of spare length and yes 3A is most probably more than adequate.

100m of 1.25 is probably about right to maintain the factor of 5 assuming the source has a very low loop impedance. However the volt drop limit kicks in around 1.7Ω, 65m of 1.5mm² or 55m of 1.25mm² all figures are approximate and based on 0.5mm² being 7.5Ω/100m
this:
http://info.ee.surrey.ac.uk/Workshop/elecsafe/FAQ.html#Exleads
shows 15m max and no more than 3 daisy chained [12m for 1.25mm²]

IIRC there is a 0.8 factor for flexible cable which nearly matches these figures.

 

[John D v2.0]

sorry just to be clear, I was meaning the volt drop with "a few low power amplifers" on the end of 100m cable would be next to nothing. I agree that it would be excessive if there were high power loads at the end.
although having said that it would depend on what was anticipated, many loads are voltage tolerant nowadays anyway.

 

[SUNRAY]

A slight off topic, when I was working in Zimbabwe, one of the sites I was looking after asked me why all the power tolos were running slow, the main supply was via a flex strung along poles, I walked the route and after 2 kilometres found the source

I had a similar problem fitting lighting in a new 300m road tunnel. We had a 100KVA genset about 50m from the tunnel, 16mm² 3ph SWA feed into each tunnel then 10KVA 110v transformers every 30m or so with 3kw of flood lamps. The volt drop caused by the lights was such that our 40W flou fittings would struggle to strike and the Dewalt 14" 1.7KWn chop saw would reach speed but stalled easily while cutting SS Unistrutt by the time we got halfway along the tunnel. The conduit threadcutter was even worse. We had to avoid turning lights on in order to use them.

 

[Risteard]

100m of 1.5mm² cable is about 2.5Ω and volt drop @ 13A about 32v or 198v assuming a perfect 230v at the source [191v for 1.25mm²]

IIRC the limit for 1.25mm² is about 30m for PAT.

12m for PAT CoP.

 

[SUNRAY]

12m for PAT CoP.

Yes I saw that in post #7 which is a little worrying when I see things like these in regular commercial use, complete with PAT sticker:
https://www.diy.com/departments/masterplug-4-socket-cable-reel-45m/33668_BQ.prd
https://www.wickes.co.uk/Masterplug-4-Socket-Thermal-Cut-out-Open-Cable-Reel---Blue-50m-13A/p/160630

I didn't know if that site was 'current reg's' or just their figures.
I'm not sure but I have in my mind that 30m of 1.25mm² was the limit when I did my PAT training in late 70's/80's with BT and got a lot of abuse when I followed the instructions to cut leads down to length. Mind in those days there would have been a lot of 70/0.0076" in use which is a fair bit bigger than 1.5mm².

EDIT: I haven't worked out the metric size of 70/0.0076 it was thicker and heavier than 1.5mm² but I don't know how much of that was due to the insulation

 

[crystal ball]

Its been a while since I did any PAT, but you could get round the max length rule by fitting an RCD, not sure if it still applies

 

[Risteard]

Its been a while since I did any PAT, but you could get round the max length rule by fitting an RCD, not sure if it still applies

It does.

 

[ericmark]

Recommended length for 2.5 mm cable to supply a caravan is 25 meters (±2 meters) it has always puzzled me why (±2 m) as one would think nothing wrong with (+2 -20 meters) as if close to supply point shorter cord reduces tripping hazard.

But with a caravan or boat supply it will be TT and it will also be RCD protected, in properties where there is no RCD protection as @Risteard says there was a limit of around 12 meters unless fitted with a RCD plug, it was not so much the volt drop but ability to rupture the 13A fuse with a short to earth.

Loop impedance of 13 amp fuse is given as 2.42Ω if the socket outlet is 1.44Ω then 0.98Ω maximum for the cable, so 1.5 mm² is around thickest into 13 A plug so 35 meters would be maximum length to ensure fuse will blow within time limit on a short circuit and volt drop would be 12.74 volt after using a 0.9546875 correction factor on the 29 mV/A/m rating for 1.5 mm cable.

If a 3A fuse was fitted with same cable then 592 meters with volt drop of 44.88 volts. As to if the volt drop would cause a problem with your equipment I don't know, so much today is supplied with switch mode power supplies rated at 100 - 250 volt so volt drop can some times be huge, however with the old power supplies as the voltage drops you get an AC ripple, I know with a caravan site I was stopping at the voltage would at times drop to 190 volt and as a result my transceiver with transmit mains hum.

 

[SUNRAY]

Its been a while since I did any PAT, but you could get round the max length rule by fitting an RCD, not sure if it still applies

I've never understood this one...