working out volt drop

[darren1981]

Hi I have been looking in to wiring up a garage which has a exsiting swa cable running to it just wanted to run this by some of your for your opinion . Supply is from 20a rcbo using 3 core 2.5mm swa into isolator
In the garage there is to be 2 power sockets and 1 lighting circuit with 4 florescent luminaries this will be connected to a small rcd cu with a 6A mcb for lights and 16A mcpvfor power there are also no ECP .
I am using a design current of 17.A 16A for the radial circuit which is the rating of the mcb and 1A for the lights rated at 0.25A each
so the swa cable Vd is 18mv/a/m so 0.018×17×20 =6.12v
lighting Vd 1.5 mm t/e 29mv/a/m so 0029×1×10 =0.29v
power 4 mm t/e 11mv/a/m so 0.011x16x10 = 1.76v

giving a total from supply to end of light circuit 6.41v
which is less then the max of 6.9v or 3%
And total for power of 7.88v which is less then 11.5v or 5% given
I know this is on the limit but providing new and upgraded swa to garage is a nightmare and if it's there would like to use

can anyone find a problem with this. Any help would be great

 

[Pete999]

Why calculate for both circuits when you are only supplying 1 cable, calculate the VD for that cable that will be sufficient

 

[darren1981]

Surely I have to work out the Vd of my supply cable and then add it to the volt drop of each circuit

 

[Pete999]

Surely I have to work out the Vd of my supply cable and then add it to the volt drop of each circuit

Work out the VD for the existing cable to ascertain if THAT cable is Adequate

 

[Jonspark]

you need the total VD right from the supply, not just the cable you're installing

 

[ImpededLoop]

Surely I have to work out the Vd of my supply cable and then add it to the volt drop of each circuit

Yes, that's what I would do.

Although, as you say, you are close to 3% on the lighting circuit, in reality your incoming supply is probably someway over 230V so it won't make any difference.

 

[telectrix]

what will the sockets be used for? if no heating appliances,welders, etc., then you'd be hard pushed to pull >10A

 

[ImpededLoop]

Also, you can calculate each circuit in sections if you wish - so if you have four light fittings along a radial circuit at 10m intervals, the first 10m is calculated at full load, the second at 3/4 load, the third at 1/2 load and the fourth at 1/4 load (assuming all loads are the same).

 

[Des 56]

Where does the Swa cable originate? what is the reason for Rcd at the supply and the load ends?

 

[HandySparks]

Hi I have been looking in to wiring up a garage which has a exsiting swa cable running to it just wanted to run this by some of your for your opinion . Supply is from 20a rcbo using 3 core 2.5mm swa into isolator
In the garage there is to be 2 power sockets and 1 lighting circuit with 4 florescent luminaries this will be connected to a small rcd cu with a 6A mcb for lights and 16A mcpvfor power there are also no ECP .
I am using a design current of 17.A 16A for the radial circuit which is the rating of the mcb and 1A for the lights rated at 0.25A each
so the swa cable Vd is 18mv/a/m so 0.018×17×20 =6.12v
lighting Vd 1.5 mm t/e 29mv/a/m so 0029×1×10 =0.29v
power 4 mm t/e 11mv/a/m so 0.011x16x10 = 1.76v

giving a total from supply to end of light circuit 6.41v
which is less then the max of 6.9v or 3%
And total for power of 7.88v which is less then 11.5v or 5% given
I know this is on the limit but providing new and upgraded swa to garage is a nightmare and if it's there would like to use

can anyone find a problem with this. Any help would be great

Looks generally OK. Not looked up the VD figures. A few comments:

I'd be surprised if you really have 10m of final circuit for the lights. Certainly it won't all be carrying the full load of the circuit. However, in this case, refining the calc won't make much difference.

Have you taken power factor into account with the lights? It's not much current anyway, but just for completeness.

Why are you using an RCD board in the shed when the distribution circuit is protected by an RCBO?

Why are using 4mm² for the socket circuit on a 16A MCB?

With this sort of question, it's always worth stating the earthing system in use.

 

[darren1981]

It's suppled from a pme system
rcbo at supply end and rcd at garage end but over the top but it's not going to give any nuss tripping.
4mm fro radial (got in in the van )

 

[Jonspark]

Power factor? for 4 flories?

why bother?

 

[Jonspark]

you might want pf correction to raise to near unity as well :juggle2:

 

[spark 68]

It's suppled from a pme system
rcbo at supply end and rcd at garage end but over the top but it's not going to give any nuss tripping.
4mm fro radial (got in in the van )

What has PME go to do with RCDs at both ends ?

 

[Simonslimline]

Supply SWA connected in to switch fuse. RCD/RCBo's at the board!

Pointless having an RCD both ends unless the upstream one is S type. Not really necessary with PME?

Any thoughts on this anyone?