3 phase volt drop

28 Jun 2006
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United Kingdom
not strictly diy but am sure someone on here will have the know how.

am looking to calculate the volt drop on a 3 phase cable. the cable is wanted to supply a 3 phase welding (no nuetral) machine pulling 80A per phase approx. proposed run is 50 metres.

proposed cable is swa pvc 4 core 35mm (4th core as cpc). i work it out as complying but as most of my work is domestic my 3 phase calcs are a little rusty.
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The calcs are the same as for single phase ie 4%,so you are permitted 16V drop between phases (for 400V).As you are probably aware this 4% applies from the intake to the final circuit, not just on the final circuit.The tables in the regs book will give you the correct figures for mV/A/M drop,bearing in mind that there are reactance drops to be taken into consideration with cables 25mm² and over.You can avoid complicated maths by using the highest figure.
25mm seems ok to me.

What is your earthing type, what is the Ze, what type of overcurrent protection are you going to use?
hi guys, thanks for the interest.

the system is tn-s.

zdb at the busbar chamber is 0.02.

my overcurrent device is going to be a bs88's.

the welder is currently supplyed by an old 100A switch fuse fitted with 100A bs88's. it is being moved hence the new install.

few more questions.

1, on start up the welder pulls 120A just for a second or so. was wondering whether when choosing a new isolator whether to up it to a 160A switch fuse to allow for this 'surge' or whether no need?

obviously i would fuse down to acomadate the cable current capacity

2, there is no room to attach the switch fuse directly to the busbar so the only way of doing it is to have the first 2 metres of cable 'unprotected'.
cant really see a problem with this as it is protected really, just not from the very start. cant seem to find anything in the regs about this however?
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1) According to the charts in BS7671 a 100A bs 88 fuse will withstand 120A for more than 1000s.
2) The way I understand the first 2 meters will need to be protected against fault current by the protective device of the supply to the bus chamber, using the adiabatic equation, 2nd opinion would be nice though ;)
cheers spark123

i know the fuse will take it yeah but was referring to the poles of the switch fuse rated at 100A tp.

see what you mean about the fault current on the first few metres of install. the busbar chamber protection is 400A per phase so i might have to rethink this one.

anybody got any suggestions how you can get round this as there is no room whatsoever to mount the switch fuse onto the busbar and connect using busbar connectors. there's about enough room to put a bw 32 gland for the 35mm 4 core and thats it!
Normal isolators are intended to be switched off load hence only need to withstand the surge whilst closed. A 100A isolator is intended to be fused at 100A hence should have had this surge included in its design.
What is the PSCC and PEFC at the take off point on the BUS bar? Is the 400A supply on BS88 fuses?
The switch fuse in reality should bolt straight onto the busbar chamber.That way you are allowed to use tails that are rated at the outgoing cable of the switchfuse not at the rating of the cables feeding the B/B chamber (This totally reduces the chance of a fault between the chamber and the S/Fuse)
To do as you say would open up the chance (albeit still slim) of a fault on the feed cable to the S/F and this is protected at 400A!.
I have in the factory I work in always had the luxury of having space somewhere on the B/B chambers.As far as I can see you have the following options:-
1)Use a feed to the S/F which is rated at 400A which will be nye on impossible to terminate in a smaller S/F
2)Ensure that the smaller rated feed to the S/F is as short as you can possibly make it and ensure that there is absolutly no way the cable could be damaged by anything and make it as visible as possible.
3)Fit a physically larger B/B chamber!!!

The surge on the welder should not pose too much of a problem unless the welder is to be used constantly and then I would go for the larger amperage switchfuse with downrated fuses.Piece of mind would say fit it anyway ;)
Hope this helps :)
You can provide overcurrent protection downstream providing the fault current protection is provided upstream, (such as a spur from a ring final circuit).
the supply is 3x 400A. this goes through a mains isolator (also 400A fuses) and then onto the busbar chamber. fuses are bs88 yeah.

havent measured pfc although likely high as the efli is 0.01.
jim, know what your talking about. we have some busbar chambers that have the same problem where i work. luckily ive never had to add to them yet.

im sure that there is a reg saying you are aloud to do as you propose, i.e cable off the busbar chamber and have the switchfuse close by. you will have to check though as i havent got my bible to hand.

believe the reg has some conditions attached to it such as limitation in distance and that cable has to be protected. of course i could be wrong but worth looking up.
Have doubled checked today for you.
25mm² will suffice volts drop and the disconnection time of the 400A fuses will be ok for short circuit protection using the following formula:-

t=k²S²/I² where t is the time that the conductor would take to reach its maximum allowable temperature, k is a factor of 115, S is the conductor size (25), and I is the prospective fault current (415/0.02 = 20750A)

So t = (115²x25²)/20750²
=0.019 (0.02) sec

A fault of 20750 A will clear a 400A BS88 fuse in less than 0.02 sec so it will be ok.(This formula also assumes that the conductor starts at its max operating temp of 70c which it won't as it oversized slightly for voltdrop)

Sorry if its long winded but I hope it helps.Just keep the supply to the S/F as short as possible and free from any possible damage as I've said before
From memory -

No longer than 3m

MUST be larger than 4mm CSA

Must "have additional protection against mechanical damage".

Steel Flexi Conduit would seem a good solution - some 2" stuff.
guys, cheers, uve been absolute stars on this one. just goes to show you should never be afraid to ask.

if anyone knows any good books on 3 phase and three phase calcs id be much obliged if you can pass on the info. could be picking up more 3 phase stuff in he long run so would be good to gen up. must say though, ring mains and putting in cooker points is far easier.

final question, when connecting to the busbar chamber ill have to use high temp sleeving and strip the core insulation back.correct? questions are how far do i strip back the cable insulation and where can i get the high temp sleeving

regards, jim.

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