Fused spur from ring final for exterior lighting

[ianphilip007]

Trying to calculate the voltage drop for a run on external lights supplied via 5Amp Fused/rcd spur from local ring final circuit. for a quotation and have got a bit confused re Voltage drop.

27m run in 1.5mm cable total load on lighting radial 420w. (no diversity as all lights either on or off)

(VD = Lx1.82x29/1000)

(Calculated Volt drop = 1.42 V)

estimated existing ring final circuit 60m long.( 30Amp 3036 fuse ) (VD= Lx26x18/1000/4)

Calculated Volt drop = 7 Volts)

Total Volt Drop origin to last light = 7.42V

For lighting circuits <3% 6.9V from origin to the last light

it appears the volt drop on the existing ring main will already be above the 3% or am i missing something, as it is standard practice to use a fused spur from a ring final to supply a security light.

any advice welcome

 

[telectrix]

but how often will you see 26A on that RFC? i'd let it go.

 

[Knobhead]

If it’s a row of lights there is also cumulative volt drop to take in to account.

 

[ianphilip007]

Good point, but for purpose of design 26Amps is worst case. Actual loading unknown.

 

[ianphilip007]

it appears the volt drop if spurred from the ring final is too high. Just calculated ZS and that is over the allowed Max for the 30Amp 3036 Ring main protective device, even though there would be a 5amp cartridge fuse in the rcd/fcu.

this is becoming messy, A new circuit from C.U is the answer i feel

 

[telectrix]

it's a debateable issue with Zs values on a spur. one camp says use the RFC OCPD, and the other says use the 5A FCU fuse to establish max. Zs.

IMO, if you get a fault of negligible impedance on the spur, then the 5A fuse is the one that will blow within the specified time , so as long as the Zs for the spur complies with the max. for the 5A 1362, then that's OK for me.

 

[Octopus]

it's a debateable issue with Zs values on a spur. one camp says use the RFC OCPD, and the other says use the 5A FCU fuse to establish max. Zs.

IMO, if you get a fault of negligible impedance on the spur, then the 5A fuse is the one that will blow within the specified time , so as long as the Zs for the spur complies with the max. for the 5A 1362, then that's OK for me.

This is the common sense answer!