Why is volt drop so important?

[davetheglitz]

I reckon I lost a job last week supplying an outbuilding. To allow for volt drop I had to run two SWA cables - one for lighting to allow for volt drop and a separate one for power (a bit more generous). As always I try to get to the reasoning behind the regs - but to my mind one cable would have done the job. The cable would have been adequately protected by the supply fusing. Given the huge variety of supply voltages does the volt drop matter that much? Why is lighting volt drop so critical? Understand for a new building - but an outbuilding for occasional use? I'm sure they are now running on completely inadequate extension leads covered in plastic bags - but I could have given them something within budget that was safe - if not directly to regs. Can someone help me with the reasoning?

 

[Apache]

Dave - would it not have been cheaper to run SWA with a larger CSA rather than 2 separate runs?

 

[The Godfather]

(If I'm not mistaken)

In a word, to ensure the minimum voltage available exceeds the minimum operating voltage rating of the fitting concerned.

(Hence the low tolerance)

Don

 

[NozSpark]

Personally I would have used only one SWA run of adequate size...

Volt drop has become more important because flourescent and discharge lighting might not strike if the voltage goes too low, where as incandescent lighting is very forgiving!

 

[green-hornet]

Voltage drop means the difference between a cold shower and a hot shower! I will explain.

In our area we had quite a few power cuts mainly because there have been no investment in the underground supply network we have in our area, demand outstrips the supply capabilities.

After a spate of three consecutive power cuts power was re established and I chose that moment to have a shower.

My shower is a 12.5 kw electric shower supplied by a 10mm cable.

The shower never got even close to warm and was very intermittant.

Out came my tester, I knew something was wrong!

When I tested the incomming supply voltage I found that I was only getting 208 volts.

Well under the voltage drop allowed minimum hence thats why my shower was not working, and why the lights dimmed everytime it tried to access more power.

So to me thats the importance of voltage drop. Without allowing for it things do not work.

Ps A phone call to the DNO rectified my problem.

 

[Apache]

It's such a shame you had to do all that naked and covered in soap suds.............

 

[oliver1234]

Voltage drop is also a consideration because ultimately if it is too low protective devices won't operate.

 

[The Godfather]

However, electronics psu's are often switch mode these days, so these have a wide operating voltage, without loss of operation or performance, although supply current will increase with voltage drop.

(So most modern TV's (LCD for example) will be unaffected)

Don & His Boys

 

[The Godfather]

Voltage drop is also a consideration because ultimately if it is too low protective devices won't operate.

Because we may not achieve the required fault current under such a condition.

The Godfather

 

[oliver1234]

However, electronics psu's are often switch mode these days, so these have a wide operating voltage, without loss of operation or performance, although supply current will increase with voltage drop.(So most modern TV's (LCD for example) will be unaffected)

Don & His Boys

I agree that many electronic devices will work with a varied supply but the supply current actually falls in relation to lower voltage.

 

[batty]

Voltage drop means the difference between a cold shower and a hot shower! I will explain.In our area we had quite a few power cuts mainly because there have been no investment in the underground supply network we have in our area, demand outstrips the supply capabilities.

After a spate of three consecutive power cuts power was re established and I chose that moment to have a shower.

My shower is a 12.5 kw electric shower supplied by a 10mm cable.

The shower never got even close to warm and was very intermittant.

Out came my tester, I knew something was wrong!

When I tested the incomming supply voltage I found that I was only getting 208 volts.

Well under the voltage drop allowed minimum hence thats why my shower was not working, and why the lights dimmed everytime it tried to access more power.

So to me thats the importance of voltage drop. Without allowing for it things do not work.

Ps A phone call to the DNO rectified my problem.

So what did they actually do?

 

[green-hornet]

So what did they actually do?

within an hour or so they had us back up to full power.

I could imagine the boys working on the fault wishing they could go home:^O

 

[The Godfather]

I agree that many electronic devices will work with a varied supply but the supply current actually falls in relation to lower voltage.

Oliver, I trust you are referring to lamps and heating elements, because a switch mode psu will in fact as voltage decreases, it's supply current will increase (in maintaining the same wattage as required by it's load).

Don

 

[oliver1234]

Oliver, I trust you are referring to lamps and heating elements, because a switch mode psu will in fact as voltage decreases, it's supply current will increase (in maintaining the same wattage as required by it's load).Don

My apologies Don, yes we were talking at cross purposes.

I was referring to mains voltage and conventional loads in which case the supply current would fall if supply voltage dropped.

Gulp..... Harmony restored!

 

[binky]

Don't forget Dave that volt drop is a function of load ie low load equals lower volt drop for the size of cable. Hence you could have probaly got away with a smaller cable, it also explains why so many marginal installations work just fine.

 

[ianmacd]

I could imagine the boys working on the fault wishing they could go home:^O

Maybe they wanted to fix it so they could have hot showers .

 

[davetheglitz]

Sorry for the delay in relying to this thread - things have been a bit crazy here as I'm trying to tie up lots of loose ends before going on holiday!

The reason for two cables - one for lighting and one for power was cost. If both were on one circuit the allowed volt drop under load would have to be the 3% lighting stipulation rather than the 5% power. On a long run (in this case 90m) this put the CSA up drastically - and hence the cost.

I understand what you are getting at with the low voltage causing fluorescents not to strike - but round here most mains voltages seem to be around the 250v mark - so suspect this is not likely.

Regarding protective devices not working as intended - I can understand what you are getting at - but even if the voltage drops to 200v compared to 250v you will only drop 1A in 5. Under fault conditions this would surely be very marginal. Also bear in mind that as the building is remote it would be TT - so fault to earth is covered by an RCD.

As always can only follow the regs - but my gut feeling is that 5% on lighting would have made next to no difference - and that max volt drop would only have been at max load - which would be a rare event - probably occurring when lights aren't needed! As it is I reckon someone has gone in there with a cable that would carry the current, drop voltage by 7 or 8% at max current - but essentially no real hazard and no-one will know the difference.

 

[Janice]

This is one of those cases where you need to apply diversity quite ruthlessly.

It appears common practise to just tot up the value of the MCB's and calculate volt drop based on this maximum.

If there is just a 100w lamp, allow 1 amp for lighting (I know, we only need half an amp) and if there are a couple of sockets, allow 7 amps and downrate the MCB. Now how big is that cable?

Anything bigger than this that you can pull in is a bonus but will, in all probability, carry everything that is demanded of it.

Would you have got the job?

J

 

[binky]

I take it you lost the job on price then Dave?? Don't fret over it, there are lots of idiots out there at the moment undercutting prices out of desperation and ignorance. You quoted for a proper job, so stick to your guns. Volt drop over 90m is certainly an issue, but as I said earlier, calculate on actual load, not MCBs potential load. I was doing some similar calcs the other day for the re-wire I've been doing in Saltash - I was getting concerned about the extensive 2 way/intermediate switching for the hallway lights. If calclated using 6A then the cable was marginal, if calculated using actual load of 200w then cable is well within spec.

 

[davetheglitz]

Thanks Janice and Binky. Good advice! I don't know if I'd have got the job anyway - it seems some friend of a friend was going to do it but couldn't do it when they wanted. However I suspect his price was much less - and the testing, calculation and certification even more so!!!