Electrical help, armoured cable sizes?

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Electrical help, armoured cable sizes?

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Bazil

Joined: 26 Sep 2005
Posts: 181
Location: Near Shrewsbury, Shropshire

Posted: Tue Oct 11, 05 1:03 pm Post subject: Electrical help, armoured cable sizes?
I need armoured cable for my turbine and want to know the size for the cable. The turbine is 36V at 500 watts, what size cable would I need? I have some 1.5mm2 but I think thats too small. Any help appreciated, otherwise its ring the sparky

hoarebag

Joined: 03 Oct 2005
Posts: 5

Posted: Tue Oct 11, 05 9:03 pm Post subject:

Power =Volts X Amps. Which means that Amps = Power/volts (I guess you know that already). Your installation is able to give about 14 amps max. There are lots of calculations for sizing the cable that include the length of run, number of cables bunched together, type of protection and ambient temperature. As a rule of thumb I would look at the rating of the cable and half it. I think 1.5mm SWA is rated at 16Amp but I would not be happy using at that rating- try 2.5mm.

If a cable burn out won't matter to you!!!??? then give it a try but keep an eye on the joints and terminations thats where a problem will show itself first.

Bazil

Joined: 26 Sep 2005
Posts: 181
Location: Near Shrewsbury, Shropshire

Posted: Tue Oct 11, 05 9:38 pm Post subject:
Thanks for the info. IT's only over a distance of 8.5 metres and I have connections blocks to go on the cable rated at 30 amps. I think I will give it a try and if it all goes well then I will stick it in the ground. I can't remember what the sparky I asked said but I think it was that 1.5mm would cope over that distance.

dougal

Joined: 15 Jan 2005
Posts: 7184
Location: South Kent

Posted: Tue Oct 11, 05 11:05 pm Post subject:

Ummm.

1st off. Its DC. So beware any tables relating to AC mains.

2nd. The heating in a resistance (like a cable) is proportional to the *square* of the current. It goes up fast.

3rd. I know that for mains use, cables must be de-rated (given a lower limit) if they are enclosed so that they cannot shed heat so well. You have to rate the cable based on its hottest point, which might just be where it goes through a joist, or is buried in roof insulation, goes through a warm area or is bunched with other cables... My guess is that armoured cable might have a *lower* current rating than the same conductor size in simple pvc. And that it should be further derated if it is going to be buried in the soil.

4th. The lower the voltage, the more significant the voltage drop, and the power loss. That power loss can be reduced by using lower resistance, bigger cross-section, cable - quite apart from whether the smaller cable could carry that current without danger.

I found this size calculation webpage
https://www.unlimited-power.co.uk/cable_sizing.html
the site being that of the "Energy Development Co-operative" who are all about renewables. There's an invitation to contact them for more advice...
I reckon you might need two or three times your 1.5mm^2 cross section of conductor. Which, if you have stock of that stuff, you might run in duplicate or triplicate...

nathanbriggs

Joined: 23 Mar 2005
Posts: 35
Location: Chester

Posted: Thu Oct 27, 05 2:10 pm Post subject:
1.5sqmm will give you a volt drop approx 1.37V watts loss approx 19W ignoring terminations 2.5sqmm will drop about 0.82V approx 12W As with the other posters my take is that 1.5sqmm is close to the knuckle and for 8.5m length the cost differntial shouldn't be too great

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