Running Parallel Cables for 400A Sub Main

[SparkyKris88]

Hi All, I’m after some advice. I’ll cut to the chase but I can give more detail if needed. A 400A sub main supply needs to be ran 100m, a pretty direct route on ladder tray. Instead of one large cable (I think it’s 240mm), I was looking into running parallel cables instead. I’ve not done this before hence why I’m after the advice, but as far as I know as long as the conductors are identical in type and lefty’s etc then they’ll share the 400A between them, so I just need two multicore SWA’s rated for 200A each. So that’d be 2x 70mm 4-Cores and join them both ends. But my two questions are as follows

1) Would the volt drop calculation work the same way? Will I just put 200A as the design current for each of the two formulas as in theory the 400A would be share between to two? I did this earlier and I think I had to up them to 95’s.

2) Besides the CCC & VD, is there anything else that needs to be taken into consideration when running parallels?

Please bare in mind that this will most likely be put to an estimator as it’s not really my job to design the cable calc as I’m more on the tools, but this is more for my benefit for curiosity and so I know for the futures. All the cable calc tool I’ve found online don’t give the option to add parallels into the equation.

Your advice would be appreciated, thanks

 

[James]

Hi All, I’m after some advice. I’ll cut to the chase but I can give more detail if needed. A 400A sub main supply needs to be ran 100m, a pretty direct route on ladder tray. Instead of one large cable (I think it’s 240mm), I was looking into running parallel cables instead. I’ve not done this before hence why I’m after the advice, but as far as I know as long as the conductors are identical in type and lefty’s etc then they’ll share the 400A between them, so I just need two multicore SWA’s rated for 200A each. So that’d be 2x 70mm 4-Cores and join them both ends. But my two questions are as follows

1) Would the volt drop calculation work the same way? Will I just put 200A as the design current for each of the two formulas as in theory the 400A would be share between to two? I did this earlier and I think I had to up them to 95’s.

2) Besides the CCC & VD, is there anything else that needs to be taken into consideration when running parallels?

Please bare in mind that this will most likely be put to an estimator as it’s not really my job to design the cable calc as I’m more on the tools, but this is more for my benefit for curiosity and so I know for the futures. All the cable calc tool I’ve found online don’t give the option to add parallels into the equation.

Your advice would be appreciated, thanks

1 yes, 200A per cable
2 not much, they should run the same length and ideally the same route however they may if needed run different routes, I.e. down each side of a building in a ring.

 

[plugsandsparks]

When using parallel cables, the calcs are straight forward enough , thought is needed on the switchgear at either end, so you can gland and have enough room to get the two cables lugged onto the switchgear. Some are easier than others, Schneider do some nice 400A switches with extension boxes and theres enough room for 2 X 120 lugs if you have to go that big

 

[pc1966]

Protection can be more complicated in some cases, see 434.4 and Appendix 10 in the regs.

The usual end of circuit Zs applies for sub-main disconnection on the supply fuses/MCCB.

There can be issues with a fault mid-way on a parallel path exceeding the cable's I2t limit from a single OCPD (hence the options for linked breakers to measure and disconnect on any one of the parallel set exceeding its reasonable share) but for long runs you are likely to need over-sized conductors from a VD point of view so unlikely to be an issue. But a quick check that one of the pair is safe for supply OCPD max I2t would be a good idea.

 

[pc1966]

Also to add you might need to consider adding a separate copper CPC in parallel to keep R1+R2 down to meet disconnection times as the SWA armour might not be good enough, again check the I2t limit as well as end of circuit Zs.

 

[pc1966]

An alternative is to have a fancy MCCB + RCD for the supply protection set to a high-ish earth fault trip current (1-5A sort of thing) and delay of 0.5-1s to meet sub-main disconnection with tolerable selectivity down-stream.

They are not cheap, but you might find they are cheaper than one or two 100m runs of heavy copper CPC (depending on your supply Ze, etc)!

Of course you can have odd issues of partial faults down stream are not cleared by RCD/MCB and so take out the whole sub-main, so a bit more analysis would be wise before taking that route.