working out volt drop

Discuss working out volt drop in the Electricians' Talk area at ElectriciansForums.net

D

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
 
Why calculate for both circuits when you are only supplying 1 cable, calculate the VD for that cable that will be sufficient
 
what will the sockets be used for? if no heating appliances,welders, etc., then you'd be hard pushed to pull >10A
 
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).
 
darren1981 said:
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
Click to expand...


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.
 
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 )
 
spark 68 said:
What has PME go to do with RCDs at both ends ?
Click to expand...
he is a trainee mate with 8 posts.

to op ask your mentor what they would recommend but you shouldn't be designing it yet because there are errors already and you might make a hash of it.

before you do this answer a couple questions first.

ccc of 2.5 swa,twin etc
why do we use rcd's?
why is having two rcd's in series a bad idea?

in a TT system what is the difference between the upfront rcd and a 61008?
 
Simonslimline said:
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?
Click to expand...

That is what I was getting at, the SWA does not need RCD protection on a TN system, two 30mA devices won't discriminate anyway, I was wondering why the OP brought PME into a (possible?) reason for the supply RCBO ?
 
spark 68 said:
That is what I was getting at, the SWA does not need RCD protection on a TN system, two 30mA devices won't discriminate anyway, I was wondering why the OP brought PME into a (possible?) reason for the supply RCBO ?
Click to expand...
he is a trainee, i wouldnt be suprised if he is a first year

it sounds like a homework question, if it is its in the wrong section


he mentions mv/a/m on basic domestic circuits so its homework, why would you need to work this out when it gives you all that info in bs7671 and onsite guide if we wanted it
 
I wasn't having a go at him Shanky :) , just curious as the thinking behind it that was all :) , I must admit that I had dark thoughts of someone thinking a source/supply RCD may somehow negate possible bonding issues with PME, as I have heard this before from someone who should have known better :)
 
darren1981 said:
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 )
Click to expand...

Over the top is one one thing but wasting money and having no discrimination of 2 rcd in series thus does not comply to regs is another, yes its not going to give nuisance tripping par' ce but its poor design from a professional all the same.
 
No, it won't. Or am I missing some of what you missed?
 
I have thought for a moment and I cannot see why an ordinary RCD test button would trip an upstream RCBO. Pray tell...
 
just out of interest, can anyone quote a reg. that forbids 2 30mA RCDs in series?
 
Lucien Nunes said:
I have thought for a moment and I cannot see why an ordinary RCD test button would trip an upstream RCBO. Pray tell...
Click to expand...

Right, I have had a thought :), and it seems maybe you are correct and I was wrong for this scenario being that the RCBO is upstream and the RCD is downstream, however if the RCBO and RCD were the other way around then it trips both devices for some reason, maybe something to do with the functional earth on the RCBO ?

I know this as a fact because I once was testing a custom built SP DB (by the firm I as working for), and all of the test rigs were in use so I used a normal socket outlet and "widow maker" lead to power this DB up to check the metering etc., anyhow I accidently caught one of the test buttons on one the many RCBOs fitted to this board and promptly tripped out the supply RCBO for this supply too, the howls of protest from a co-worker whom was using a computer powered from this supply revealed this lol :)
 
Archy Styrigg said:
Reg 1: Use Common sense.

or it'll be something out of Chapter 3 about discrimination.
Click to expand...

yes. i know, but was seeking a specific reg.

531.2.9 only asks for discrimination between RCDs when it is necessary to prevent danger.
 
ImpededLoop said:
I was always taught that the test button mechanically tests the device. I'm not sure about RCBO's.
Click to expand...

well yes to an extent, it checks the electro-mechanical aspects out, the test button usually switches a resistor from the incoming L terminal to the outgoing N terminal so it checks the trip coil operation too.
 
shanky887614 said:
he is a trainee, i wouldnt be suprised if he is a first year

it sounds like a homework question, if it is its in the wrong section


he mentions mv/a/m on basic domestic circuits so its homework, why would you need to work this out when it gives you all that info in bs7671 and onsite guide if we wanted it
Click to expand...

Sounds like he wants to do the job because he's applying for elesca registration in another thread he started.
 
spark 68 said:
well yes to an extent, it checks the electro-mechanical aspects out, the test button usually switches a resistor from the incoming L terminal to the outgoing N terminal so it checks the trip coil operation too.
Click to expand...

That makes sense, but wouldn't cause an upstream device to operate as it only diverts current to neutral so no imbalance would be detected on the other device.
 
ImpededLoop said:
That makes sense, but wouldn't cause an upstream device to operate as it only diverts current to neutral so no imbalance would be detected on the other device.
Click to expand...

Hence my earlier reply to Lucien ;) , I am not sure why the RCBO tripped out the upstream one that I did at work though, but it did.

I might tomorrow fit my spare RCD that I keep, to a plug and lead , and see if it trips out the house RCD lol :)
 
My downstairs sockets at home are fed from a rcbo and I spurred a outside rcd socket off this circuit, now every time I use the outside socket I always push the test button and it has never tripped the upstream rcbo.
 
Jonspark said:
My downstairs sockets at home are fed from a rcbo and I spurred a outside rcd socket off this circuit, now every time I use the outside socket I always push the test button and it has never tripped the upstream rcbo.
Click to expand...

I wonder if it only happens with an RCBO downstream ?, as I said maybe it was due to only certain types or possibly with the functional Earth when this is present.

I originally opened my mouth based on my earlier experience without thinking , then I thought about it and came around to Luciens way of thinking. :)
 
some one asked what earthing system was in place that's the only reason there shouldn't be any bonding issues as there is no extraneous conductive parts. two rcd in line yep I agree the only reason that the rcbo is there is because it was is all ready in place. what would be best the rcbo at source or rcd on garage board???
 

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