Should We Use In or Iz if Changing the Core Size of Cable

If you take your max. current divided by your derated value you already have your safety factor, so as you say: 2806/254= 11.05, especially if you use 12 runs.

However you must check the current rating environment, is it the same spec for each cable? Usually two cables with the same cross section as a single cable together will carry more current between them compared to the larger cable. They have a bigger surface area which is important to capacity & cooling.

Some cables may have special instructions for multiple parallel runs in trunking, check with the supplier / manufacturer.

On the information supplied 12 runs sounds fine, however look at the protection of each cable / run.

Hi JohnBob,

I guess you are referring to the SKIN effect when you say that cables with more surface area carry more current for a given cross section.

What is the reason you think cables are only made in circular shape?

Well actually they are not just made with a circular cross-section, we have worked with some 3 phase [aluminium conductor, underground armoured] cables where the conductors are made up of 3 x 1/3 circle quadrants [packed into a 'circle']

I am no cable specialist, but at an exhibition a cable manufacturer was telling me of lightweight cables with a hollow centre [a tube effectively] the centre being taken out as it has a lower power carrying capacity / density. Also the hollow centre helps with cooling so they have a high capacity for the weight.

One answer; from a mechanical viewpoint going round corners with a square cable would be difficult! & cause stress if twisted & cut through the insulation. [well true ]

Seriously - a square cable would have a flux density at the corners I guess, producing heat and so increasing losses, but please educate me! thank you.

omg, in case you weren't paying attention, the 2806A was listed as the Nominal Current. This means that 2806A is the current that it runs at in normal condition and is NOT the Maximum current. I personally would not try to derate down to such a smaller cable. you are going to come into line loss and possibly heating issues because of going from such a large conductor down to a smaller conductor. however, if you must do so for physical cable flexibility or availability issues, I would use the 3430A rating number to determine the number of conductors/cables you need to use. also, with multible conductors you are going to come across more problems with emf radiation or interference (ie. harmonics). just some things to consider. I would call the equipment(generator) manufacture and talk to their engineers before changing the cable size because there is a chance it could mess with the governor.

V

Hi V & JohnBob,

Thanks for joining this discussion.

JohnBob - the hollow cable sounded interesting. I hope some more cable experts can add their reasons as well in this discussion.

V- The generator supplier also supplies the main switchboard to which the generator is connected and so the incoming circuit breaker in the switchboard is sized by him. So when they give us the nominal current, I presume that the circuit breaker will trip if it exceeds this nominal current. So dont you think that 2806 is enough for cable sizing. Thank you.