volt drop

james king · Mar 22, 2011

hi all i have been to collage today and we did a bit on volt drop. can some one plase help explain this to me? does this mean on a long run if the cable is not big enuf it will loose volts? eg. 221V?

yellowvanman · Mar 22, 2011

You'll lose volts on any type of cable, you just need to manage it correctly.

Volt drop is dependant on:
size of conductor CSA
length
current flowing

james king · Mar 22, 2011

so 4 example 90m run of 3core swa 10mm and current of say 36A

yellowvanman · Mar 22, 2011

Volt drop per amp per meter for a 10mm 3 core (1 phase) = 4.7 mV/A/m (Page 291 of BS7671)

Total Volt drop = 4.7/1000 (because its in mV) x 36A x 90m = 15.228V

james king · Mar 22, 2011

so there for would this be ok?

Guest123 · Mar 22, 2011

jiga said:
so there for would this be ok?

No, as it is greater than the 5% of 230V maximum permitted VD of 11.5 V.

(3% if there are lighting circuits involved....6.9V)

james king · Mar 22, 2011

so would vd play a part in mcb or rcd tripping

yellowvanman · Mar 22, 2011

The amount of volt drop depends what is being supplied. If you know what is specifically being supplied and the spec for the machine has a better tolerance for voltage drop then it could be OK. The 5% rule is for when you don't know whats going to be connected - eg. on a ring main.

Volt drop has nothing to do with tripping breakers.

james king · Mar 22, 2011

what would would happen to the cable melt?

yellowvanman · Mar 22, 2011

Cables will get damaged if there is too much current flowing - gets hot - insulation gets damaged, thats why cables have a 'current carrying capacity' that shouldn't be exceeded.

james king · Mar 22, 2011

so surly the mcb will trip to protect it from over current

yellowvanman · Mar 22, 2011

yes - but thats nothing to do directly with volt drop.

fire · Mar 22, 2011

it was in my local newspaper recently that someone connected a 10KW shower on a 6mm cable, lady was having a shower it switched off half way, she came out to her room to get changed, to her surprise the fire man put out the fire on the roof. the 6mm cable got hot, melted and started buring the insulation.