Calculating current (A) for electrical cables

i know nothing · 20 Mar 2006 at 14:15

I need to run some cable before I complete some building work.

If I know the current that the cable will carry I can look it up in my handy little table and it will tell me what size cable to lay. Problem is I don't know the current. The only reference I have is that my 220Kw three phase supply (at 380V) comes in on a 240 sq. mm four core cable.

I do know that the load is a 3 phase motor (Star Delta job, not sure which but I know the motor has a soft starter) which is rated at 30 Kw at 380V (not in UK by the way). The power factor is 0.8 (genset in use here).

I'd really like to see the formulation rather than just the answer as I have a few more cables I need to run (all three phase loads) for loads ranging from 5-42 Kw.

TIA

i know nothing · 20 Mar 2006 at 14:29

It's the same calculation as for single phase?

i know nothing · 20 Mar 2006 at 14:34

To overclocker, I'm still digesting your post ta!

i know nothing · 20 Mar 2006 at 14:57

danrok said:
You are working with a 220kW supply and you don't know how to work out the amps?

I'd suggest getting a qualified electrical engineer to do the work. There are many things you need to be aware of when working with 3-phase power.

I need to lay a cable as I'm doing some building work. Obviously you didn't read the post as I simply want to run the cable, I never said anything about connecting it. Now bugger off and annoy someone else, there's a good boy

i know nothing · 20 Mar 2006 at 15:18

danrok said:
There's no need for the childish and derogatory comments. This is not your forum and it is not your place to tell other members to bugger off.

You're trolling, now bugger off!

i know nothing · 20 Mar 2006 at 15:20

The_blue said:
Out of intrest, what you you use a 30kw motor for? that must be huge!! :O

It drives a piece of equipment called a "simplex refiner", it would take me too long to explain exactly what it does, suffice to say I make soap.

i know nothing · 20 Mar 2006 at 16:16

Adam_151 said:
Remember that there is more to it than selecting a cable capable of carrying the load, volt drop (so you arn't loosing too much power in cable looses) and disconnection time (in the event of a phase to earth fault the current that flows must be sufficent to operate the over current device within a set amount of time) and protection against short circuit (if there is a short circuit, the cable must be able to withstand the let through energy of the protective device)

I'm looking at it from a prospective of british regs (BS7671), you should check your local regs.

That said, my starting point would most likely be 16mm² SWA on a TP C63 then you do the relevant calculations and decide whether its right, or whether it needs 'tweaking'

There are no local regs here!

I am the local regs! If the sparkies here had their way they would use 2.5mm and twist it together

When I first put the equipment in there was a local "electrical contractor" that came to spec up my requirements, I was offerred a choice of 4 core SWA XLPE 10mm or........4 core SWA XLPE 10mm, why? Because that was all they had in stock! I ordered the cable myself from Singapore and shipped it up, that was 8 years ago (and crossing fingers

) and I've never had a problem.

I recall a couple of years ago I found an online cable selection calculator, you entered all the variables (ambient temp, cable length, voltage, current etc) and it would work it all out for you. I know I have to de-rate the cable on ambient temp alone. I appreciate what you're saying about voltage drop, I've managed to find a spreadsheet on the net which takes care of that.

Interestingly it's pointing at 16mm

i know nothing · 20 Mar 2006 at 16:17

MrMatteh said:
tyler durden style?

It got too expensive, we had to shift to using palm oil

Tomsk said:
We get electricians to do ALL our cables, from 240V single phase to 20+KW three phase. If we don't, we're not insured.

There is no insurance here (and I'm really not joking!)

There's no H&S either. A qualified electrician is defined as "an individual who can make a lightbulb work without getting fried in the process".

i know nothing · 20 Mar 2006 at 16:57

That's the control panel I have to wire to (except I don't actually have it yet, it's being shipped at the moment).

That's the actual refiner itself.

As you can see from the pictures the cable will ultimately be connected to the control panel, not directly to the machine.

The cable run (shown in red on the picture above) is about 40-45 metres from the main distribution panel. I intend to put the control panel in question against the wall of the factory. The ambient temp can reach as high as 45 Deg C. I did not intend to use SWA as I can't see it being necessary. The cable will lay on the steel girts and girders, clipped in places (they make excellent cable trays!).

If I use cable that meets BS7211 (non armoured multi-core cable) I could just about get away with 10mm, except I need to de-rate it because of the temp (by a factor of 0.77) and voltage drop (4.92V over 45 metres). 16mm looks good.

Anybody care to calculate what size I need?

i know nothing · 20 Mar 2006 at 18:07

big_white_dog84 said:
SNIP...

So you will need a 57A cable for each phase.

I'm assuming the 'soft start' thing means your motor is not going to have a large inrush current. Even if you do have inrush the cable will have a cyclic rating that will allow a temporary over-current without burning out.

Don't quote me on this stuff though.

Found a formula here:-

It works out:-

(30 x 1000) / (1.732 x 380 x 0.8) = 56.98 Amps

So you're spot on with the 57A.