warm doggies

[atm]

I have two metal clad double sockets and FCU that feeds the light in my shed all via SWA from a 20amp mcb on the rcd side of the cu.

can i run 2 x 2kw convector heaters 1 plugged into each of the sockets?

my guess is that the convectors draw 8.7amps each and my 60watt light next to nothing so 8.7amps x 2 = 17.40amps..

20amp mcb ok then?

look forward to hearing from anyone.. thanks

 

[RF Lighting]

Sounds OK.

What size is the SWA, and how long is the run from the CU to the shed?

 

[atm]

thanks for getting back so quickly RF.

2.5mm and 50m in length..

assume though if i have both heaters going at once doesnt give me much room to plug anything else in.. spose could get away with a tv of something but not a beefy flymo?

hows my thinking so far..

if i wanted more power then up my mcb to 32amp and up my SWA to 4mm.

yeah?

 

[Lectrician]

Hey ATM....

Your measurements here are slighty different to those at askthetrades

I thought it was a 20m run!?!

 

[atm]

Lectrician yep sorry dude was longer than i thought... will the v drop be a problem?

 

[RF Lighting]

Unfortunatly with a run of 50m, due to volts drop, with a load of 20A you need a MINIMUM of 4.0mm² SWA, but this is right on the limit for a 20A circuit, and only allows you 0.2% for any other wiring in the shed etc.

 

[davy_owen_88]

50metres is a long run for 2.5mm²...

18mv x 20A x 50m = 18v

Maximum allowable VD is 5% or 11.5V - so you'll need to go up to 4mm² atleast.

 

[atm]

ok loosing me a little on the vdrop however i think i get it.

to be on the safe side and to cater for some extras in the shed i will go 32amp mcb using 4mm SWA over the 50m..

that ok?

and thanks!!!!!!

 

[RF Lighting]

I'm afraid it doesn't work like that.

You could go with 4.0mm² cable on a 20A MCB, or if you wanted to use a 32A MCB, you would need an even bigger cable.

 

[atm]

"18mv x 20A x 50m = 18v" what is 18mv?

so in english the full 20amp will carry over the 50m better using 4mm than 2.5mm yeah because its not so squashed.. however over the 50m using 2.5 mm i will only be able to draw about 15amps ish, and increasing swa to 4 mm i will be able to draw a little closer to the full 20amp if required?

just breaking it down then i will follow i think..

 

[davy_owen_88]

ok loosing me a little on the vdrop however i think i get it.

You take your voltage drop figure which can be found in table 4D4B (page 281) and multiply that by the expected current draw and multiply that by the length of the circuit.

VD figures for SWA:

1.5mm² - 29 mV/A/m
2.5mm² - 18 mV/A/m
4.0mm² - 11 mV/A/m
6.0mm² - 7.3 mV/A/m
10 mm² - 4.4 mV/A/m
16 mm² - 2.8 mV/A/m

For 4mm² with a 20A breaker:
11mV x 20A x 50m = 11000mV or 11V - as RF said earlier, that is just within the limit.

For 6mm² with a 32A breaker:
7.3mV x 32A x 50m = 11680mV or 11.68V (just a tiny bit over the limit)

For 10mm² with a 32A breaker:
4.4mV x 32A x 50m = 7040mV or 7.04V

 

[atm]

ok loosing me a little on the vdrop however i think i get it.

You take your voltage drop figure which can be found in table 4D4B (page 281) and multiply that by the expected current draw and multiply that by the length of the circuit.

VD figures for SWA:

1.5mm² - 29 mV/A/m
2.5mm² - 18 mV/A/m
4.0mm² - 11 mV/A/m
6.0mm² - 7.3 mV/A/m
10 mm² - 4.4 mV/A/m
16 mm² - 2.8 mV/A/m

For 4mm² with a 20A breaker:
11mV x 20A x 50m = 11000mV or 11V - as RF said earlier, that is just within the limit.

For 6mm² with a 32A breaker:
7.3mV x 32A x 50m = 11680mV or 11.68V (just a tiny bit over the limit)

For 10mm² with a 32A breaker:
4.4mV x 32A x 50m = 7040mV or 7.04V

excellent many thanks..

so 5% is the minimum allowed limit and anything above that is a no no..

5% of what though?

you may not beleive this but i am taking this all in.

thanks..

 

[RF Lighting]

5% of the voltage at the supply terminals.

This will typically be around 240V on a standard domestic supply (With a nominal supply voltage of 230V)

The 5% is the total allowed for the whole installation, i.e. including any submains AND final circuits supplied from the submain.

 

[atm]

thanks RF... why is VDrop so bad then ?

 

[RF Lighting]

Basically, because if the volts drop too low, lights get very dim, and things stop working properly.