Cable Size Calculation

americanpie

This is driving me mad, I have an 70 mtr XPLE run on a lighting circuit from board to switch to lights, the draw is 5 amps.

Used online calculator's to get the size of cable needed and said 1.5mm2, then used formula in ECTI rules and said 2.5mm2.

Would I get away with 1.5sq or will I splash out on the 2.5sq?


The formula is:
mV/A/M X amps X length of run
1000


Max volt drop allowed being 9.2 Volts according to ECTI.

mV/A/M 1.5 is 31
2.5 is 19



I think I answered my own question...

Can anyone recommend a decent calculator either to download or use online?

Thankin you

.G.

You also need to take into account the cable install method,the ambient temp and the number of cables it will be running alongside to get an accurate cable size to conform with ETCI.

However assuming its install method is A or B and its run on its own and the ambient temp is not above 30 then 2.5 is needed cos the voltdrop for 1.5 is .031volts * 5amps * 70 metres =10.85volts.

davebuck

For the cost involved use 2.5 it also allows for extra points at a later stage.

Bruthal

If all the lighting load is at the end of the 70m run then its 2.5 according to the calculations but if the lighting points are spread along the run of cable then 1.5 is all thats needed.

americanpie

Put in a 2.5 just in case I need to add to it in the future, only around an extra 20 for the cable, and Im not payin for it anyway

M cebee

i usually up the cable if in doubt .

not that much in the price difference

is there anything online for calculations?

mazthespark

http://www.electacourse.com/cableselector.html

its basic enough but does an ok job

noelo

hi can anyone figure this one out for me, with a clean fully explanned breakdown, its a 32 amp 3ph welding socket, 10 meter run, grouped with 3 of the same cables on a horizontal perforated tray cheers

Bruthal

noelo wrote: »

hi can anyone figure this one out for me, with a clean fully explanned breakdown, its a 32 amp 3ph welding socket, 10 meter run, grouped with 3 of the same cables on a horizontal perforated tray cheers

Not much to explain. 4 x 6 square swa with a 4 square earth i would say would do that.

noelo

robbie7730 wrote: »

Not much to explain. 4 x 6 square swa with a 4 square earth i would say would do that.

Thanks Rob, that s what I would use too, but I need to prove this for a report ,also it would have to use the etci regs (the GF & TCF factor tables), when i calculated it, it returned a result of 6square mm but I d like a second opinion.

M cebee

http://www.tlc-direct.co.uk/Book/4.3.9.htm
there's a useful method here -i use as a reference anyhow

you can take the numbers out of ET101


that gives you your cable size for the 32amp circuit

-4% installation voltage drop-that's 4% of 380v in this case =15.2 volts

-your disconnection time is 0.4 seconds for 32amp final circuit(Uo=230v)

-and you can double the measured L-N short-circuit current at the DB,your

protective device needs to be able to break this fault-current

Dardania

M cebee wrote: »

http://www.tlc-direct.co.uk/Book/4.3.9.htm
there's a useful method here -i use as a reference anyhow

you can take the numbers out of ET101


that gives you your cable size for the 32amp circuit

-4% installation voltage drop-that's 4% of 380v in this case =15.2 volts

-your disconnection time is 0.4 seconds for 32amp final circuit(Uo=230v)

-and you can double the measured L-N short-circuit current at the DB,your

protective device needs to be able to break this fault-current

Are you safe enough assuming the volt drop only across this final sub-circuit i.e. you don't need to consider volt drop leading to the supplying DB?

Also, where's 380V coming from? UK regs?

M cebee

4% installation voltage drop

total VD -final circuit+any sub main supplying the final circuit

for a 3-phase circuit it's 4% of the line voltage


in the uk it's 3% for lighting and 5% for power(installation volt drop)

DSO's(esbn) have their own tolerances -they're not part of the calculations

noelo

Exellent work mc cabe , but would it not be 4% or under of 230, instead of 380 , I know it s 3 phase but its between phase and nuetral , no?

noelo

guys I aslo need to know how to calculate the breaker size for such a circuit, can you help in the form of a detailed step by step breakdown , cheers

M cebee

noelo wrote: »

Exellent work mc cabe , but would it not be 4% or under of 230, instead of 380 , I know it s 3 phase but its between phase and nuetral , no?

no-it's a percentage of the line voltage for a 3phase circuit

Bruthal

noelo wrote: »

Exellent work mc cabe , but would it not be 4% or under of 230, instead of 380 , I know it s 3 phase but its between phase and nuetral , no?

Yes as m cebee says its 4% of the 400v. This gives allowable drop of 16v.

But 16 volt line voltage drop gives 9.2 volt phase drop, so its the same anyway.

In other words 400v line voltage is 230v phase voltage. (400/root 3)=230v

4% volt drop is 384v line voltage, which is 221.7 or about 9.3 volts phase voltage drop allowed.

Also 16 volts line voltage drop allowed divided by root 3 (16/root 3)= 9.2

So what you can say is 4% volt drop allowed on line voltage in the 3 phase circuit will show a 4% drop on the phase voltage anyway in the same circuit.

M cebee

ya same thing- but you may only have 3-phase so you'd be measuring line voltage

i was thinking that it's prob no longer 380 -it's 230/400

Bruthal

Thats right yes for 230 phase v it would be 400v line.

noelo

ah great stuff!, so now how can i size breakers ??? cheers for the help lads.:)

Stoner

Dardania wrote: »

Are you safe enough assuming the volt drop only across this final sub-circuit i.e. you don't need to consider volt drop leading to the supplying DB?

correct this is often missed. Rule of thumb would be to allow 1.5% to the sub board, so you are working with 2.5% from the sub board on. That is just a quick rule but a quick assumption rule is required here unless you can properly determine the voltage drop at the main board- sub boards.

I know some guys who work off UK IEI charts for current sizes (for larger sized runs).
I have found that calculators differ a lot because they seem to pick and chose the variables they take into consideration when working out calculations.
Many ignore grouping, installation method, PFC, temperature. If you use the ETCI regs and the method thought to electricians you can sometimes get a slightly different answer compared to results from Amtec and other programs.
I also have never seen a calculator take continuous loading into consideration, I know that we had to go a cable size up for both the breaker amp rating (not cut out) and cable size for refrigeration as the load was 24/7 and the cable never had a break.

I use a quick free app on my Android phone, Diarsoft Voltage Drop Calculator, it works fine I guess most apps are set up for american UOMs. This is only a quick tool, and I often ignore the results as it works on percentage voltage drop and does not have a look up for cable current saturation for some reason but it has a parallel run option.

noelo

robbie7730 wrote: »

Yes as m cebee says its 4% of the 400v. This gives allowable drop of 16v.

But 16 volt line voltage drop gives 9.2 volt phase drop, so its the same anyway.

In other words 400v line voltage is 230v phase voltage. (400/root 3)=230v

4% volt drop is 384v line voltage, which is 221.7 or about 9.3 volts phase voltage drop allowed.

Also 16 volts line voltage drop allowed divided by root 3 (16/root 3)= 9.2

So what you can say is 4% volt drop allowed on line voltage in the 3 phase circuit will show a 4% drop on the phase voltage anyway in the same circuit.

Robbie, would you know how to size a breaker? and could you break it down for me? cheers

Bruthal

noelo wrote: »

Robbie, would you know how to size a breaker? and could you break it down for me? cheers

Well the way its normally done is you find out what the load will be, then use the next size breaker above this load, and then use a cable suitable to continously carry the current the breaker is rated at, taking into account the length of the cable run etc. Some circuits like lighting and sockets are set at a certain size as standard and use the appropriate size wiring for the breaker sizes.

noelo

robbie7730 wrote: »

Well the way its normally done is you find out what the load will be, then use the next size breaker above this load, and then use a cable suitable to continously carry the current the breaker is rated at, taking into account the length of the cable run etc. Some circuits like lighting and sockets are set at a certain size as standard and use the appropriate size wiring for the breaker sizes.[/QThats what i would usually do, but is there not a mathamatical way of doing it? cheers

noelo

robbie7730 wrote: »

Well the way its normally done is you find out what the load will be, then use the next size breaker above this load, and then use a cable suitable to continously carry the current the breaker is rated at, taking into account the length of the cable run etc. Some circuits like lighting and sockets are set at a certain size as standard and use the appropriate size wiring for the breaker sizes.

Thats what i would usually do, but is there not a mathamatical way of doing it? cheers

PODSIE

noelo wrote: »

Thats what i would usually do, but is there not a mathamatical way of doing it? cheers

i was asked to run in a cable for floodlights around our training pitch. the club are putting in ten 400watt lights around the pitch. the total length of run from distribution board to last light is about 400 metres. any idea of what size cable to run in. is 3x2.5 swa ok or should i run in 3x4swa. thanks for any replies.

noelo

PODSIE wrote: »

i was asked to run in a cable for floodlights around our training pitch. the club are putting in ten 400watt lights around the pitch. the total length of run from distribution board to last light is about 400 metres. any idea of what size cable to run in. is 3x2.5 swa ok or should i run in 3x4swa. thanks for any replies.

Hi this is how you ll get the size bang on and save money the trouble by not over or under sizing ;
Using the Notations IB (Design Load), IN (Protective Device Rating), IZ (Continuous Current Carrying Capacity), mV (Volt-Drop per metre) and the formula’s IZ = IN / (TCF x GF) and mV x A x M (were (A) is ampere and (M) is metre) just plug in all your digits..................
You will need the regs table and dont for get to use heavy connectors in junction boxes for looping your supply cable through to your next light just "spuring-off" each one to your light.

M cebee

PODSIE wrote: »

noelo wrote: »

Thats what i would usually do, but is there not a mathamatical way of doing it? cheers

i was asked to run in a cable for floodlights around our training pitch. the club are putting in ten 400watt lights around the pitch. the total length of run from distribution board to last light is about 400 metres. any idea of what size cable to run in. is 3x2.5 swa ok or should i run in 3x4swa. thanks for any replies.

Youve got 4kw of discharge lighting and a multiplier(used to be 1.8) for design current

youll have to limit voltage drops at furthest light/s

either radials or a ring-dont think 2.5 or 4 will be suitable here

Dardania

I reckon 4mm sq. in a ring will do it if the fittings are power factor capacitor corrected...

OP, what is the lighting configuration like? Have you gotten a lighting simulation done?