30 amp supply to garage

[ianphilip007]

hey there

customer currently has a supply fused at 30A via 4mm SWA to their garage. they want a new garage cu fitted. they currently have two radial socket circuits 1 feeds a single outside socket (fused at 16) and the other feeds 2 internal double sockets (fused at 20). the 2 lighting circuits consist of one circuit to external pir flood (150w) and one circuit to 2 36w batten fittings.

now i cant see any problem with replacing the cu but not sure as to weather the existing supply cable fused at 30amps is big enough ?

any advice would be appreciated.

 

[ianphilip007]

i guess its the fact that there's 4 circuits, if there were a 20 amp socket radial and a 6amp lighting circuit i wouldnt see a problem but i guess its the same load just split into more circuits

 

[ianphilip007]

just concerned about there being a 16amp and 20 amp radial. the internal sockets will be used for low power use, and the most the external socket will have to power is a lawn mower.

 

[Deleted member 26818]

Well Ian, if you use the method to determine maximum demand and diversity in the OSG, you would add 100% of the current demand of the largest circuit, to 40% of the current demand of the other circuits, which would amount to about 28.291A.
As such a 30A fuse would appear to be acceptable.

 

[ianphilip007]

thanks spinlondon was forgetting about diversity.

internal lights = 80w = 0.34 Amps
external light = 150w = 0.65 Amps
internal sockets = 20 Amps (no diversity 100%)
external socket = 16Amps ( 40% diversity ) = 6.4 Amps

total = 27.39 so i agree 30 Amp supply should be acceptable.

 

[Engineer54]

If you manage to see 10A flowing at any one time, i reckon you'll be lucky to see it!!!

Why is it trained electricians these days, can only see breaker ratings as the load that's going to be drawn?? ...They are protective devices, full stop!!!!

 

[brav]

the two batten fitting should be written as 100w each when using diversity, as this would be the maximum watt light bulbs they could possibly find.. altho there are technically no 100w bulbs for sale anymore.

 

[Deleted member 26818]

I only ever use 100W for incandescent lamps, with flourescents I always use the actual value.
Of course with the increasing use of energy saving lamps, 100W is probably a bit over the top.

 

[platty1005]

Your question is don't know if existing supply cable is big enough ?
Rule of design is Demand current is equal or less than protective device using diversity etc
Protective device (30A) is equal or less than current carrying capacity of the cable so what is the current capacity of your cable this depends on a number of factors See page 123 OSG Appendix 6

Is the Garage detached and is the cable RCD protected ? if so use Table 7.1 OSG page 49 30amp fuse/ 4.0/1.5 cable maximum length of run RCD protected 36 metres

Lets assume a 4mm SWA cable buried feeds the garage then using Table 4E4A column 6 in BS7671 Ref Method D
the maximum current carrying capacity is 43 amps much higher than 30 Protective fuse so is OK
If the circuit draws more current than 30 Amp the fuse will operate before damage to the cable

Answer to original question YES supply cable should be OK commonsense applied regarding loads

 

[malcolmsanford]

Your question is don't know if existing supply cable is big enough ?
Rule of design is Demand current is equal or less than protective device using diversity etc
Protective device (30A) is equal or less than current carrying capacity of the cable so what is the current capacity of your cable this depends on a number of factors See page 123 OSG Appendix 6

Is the Garage detached and is the cable RCD protected ? if so use Table 7.1 OSG page 49 30amp fuse/ 4.0/1.5 cable maximum length of run RCD protected 36 metres

Lets assume a 4mm SWA cable buried feeds the garage then using Table 4E4A column 6 in BS7671 Ref Method D
the maximum current carrying capacity is 43 amps much higher than 30 Protective fuse so is OK
If the circuit draws more current than 30 Amp the fuse will operate before damage to the cable

Answer to original question YES supply cable should be OK commonsense applied regarding loads

Sorry Platty but I can't work out what your post is about with regards in applying diversity. You have just done a cable calc for overload as per reg 433.1.1

 

[platty1005]

Hi Malcolmsanford read ianphillip007 original post he wanted to know if a 4mm supply cable on a 30 fuse is OK. The following posts all talked about the load current and ways of determining the load current using diversity. Basically he has garage with a maximum current draw of 30 A limited by the supply fuse. If the radial circuits attached each draw their full load current then the 30A fuse will blow but as he says the radial circuits are likely to draw small currents. He has a 20A, 16A and 6A radial circuits on the garage CU adding up to 42A maximum load IF ALL CIRCUITS are fully loaded. Even at this unlikely scenario the supply cable is still OK. I can only assume he was asking if he changed the CU did he need to change the supply cable as well answer NO