cable calc

[hodgey1972]

guys and gals.
This is a little bit out of my comfort zone, im trying to provide a cable size for
100a 3ph crane
temp supply(1 yea)
in ducting
35m
could someone give me the ammo to work it out?
im sure its m/v/a/m2 x ib x L / 1000
I DONT WANT IT T BE 70MM ££££££££
THANKS

 

[Sintra]

That my friend is the formula for volt drop. Lots more factors to consider like Ib, In, Iz etc.

 

[Spoon]

Assuming all the correction factors Ca,Cd.... ect. are 1 then you will be looking around 35mm (Table 4E4A) or 50mm (Table 4D4A)
These are approx figures with the info your have given us.

 

[milliohm]

Hi, Jenkins & Coates book on installation calculations on page 22, suggests that motor circuits subject to frequent stopping and starting should have the cable csa based on the full load current times 1.4, to take into account the extra heating due to starting currents, and possibly the inadequate cool off times between starts. This is a rule of thumb, not necessarily accurate in every case. If more accuracy is required, then the following information would be necessary. (a) the actual magnitude of the starting current (b) the duration of, and rate of decay, of the starting current (c) the full load current (d) the first choice cable size to calculate its time constant. ( I think this is referring to its K^2S^2) (e) the time intervals between stopping and starting. Hope this helps a little.

 

[telectrix]

iassume for a crane it would be SWA on tray, in which case 35mm is plenty big enough, VD being acceptable.

 

[wozz]

Personally I'd be after more info before I start guessing sizes, inrush for a start and then others to consider.

 

[silverfox1]

Not on tray tel, from what op says it sounds like he is running swa in 35m of ducting.

 

[telectrix]

my bad. didn't read OP fully. still think 35mm is OK though. it's not a continuous load, is it?

 

[Darkwood]

I think the OP is as he's admitted well out of his comfort zone and should get a competent person in to work along side him so he can learn as he helps... too many unknowns here to even start the calcs ... are the motors Inverter controlled?

Regardless of the actual cost of materials the cable size you work out is what it is ..
Ive just had to run a 70mm SWA in ducting myself for a 118amp load the issue you may find is selecting the right kit to ensure it can accept 70mm cable ... I requested for 70mm terminations and the switch fuse with 125amp fuses came with a neutral block max 50mm

I get the sense this is also going to be outside your cash flow budget allowance for materials - too many things can go wrong here if your not used to this type of install and at a big cost to correct if you get it wrong.

 

[Engineer54]

Without really knowing or seeing this installation and without knowing what form of mechanical protection is going to be provided in exposed areas, i'd be looking or at least considering tri rated cables for this temporary crane supply....