'Bundling' of 12V cables

zoidberg

zoidberg

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I have a qwessie on the effects of 'bundling' groups of 6 x 12V 21Amp wiring within spiralwrap. I don't expect any one wire, each ~1metre long, to carry >15amps ( bilge pump ) and asked the cablewrap makers' tech support. They pointed to 'BS 7671 (18th Edition) wiring regulations. For detailed calculations refer to Appendix 4, which provides tables for cable grouping.'

Can anyone here give more helpful guidance?
 
wonkywinch said:
Better still, learn to lace, it's ropework only smaller :D

Cable Lacing | Lacing Cord | RS
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I did.. 50 years ago being taught by Derek Shaw, the workshop practices instructor at RAF Locking.

However the question posed is not about how to bundle cables. But about the effects of cable heating when tieing bundles together. Like you should always unwind an extension cable before applying power, you should be cautious bundling power cables together to prevent melt down.

Sadly as I'm busy packing and this is a coffee break, I don't have time to do the required sums..
 
zoidberg said:
I have a qwessie on the effects of 'bundling' groups of 6 x 12V 21Amp wiring within spiralwrap. I don't expect any one wire, each ~1metre long, to carry >15amps ( bilge pump )
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All 1m long ? Even the bilge pump ?

Even at 1m long, with 15A loads, 1.5mm cable is too small, should be 2.5mm.
 
FWIW, I'll do the bilge pump, rated by label at 15A, on a rugged robust tinned and physically separate 3mm cable, separately fused.

The other, far lesser stuff will have to be content with red 1.5mm cable between the switch panel and the convenient W60 Wago Junction Box - which is not quite 1 metre distant. The black stuff I'll run separately.

As for lacing, I'm undecided between Chantilly, Calais or Guipure style and certainly in what colours.

Sheesh! This is more of a PITA than arguing about anchors....
 
Can anyone recommend a good calculator to use if you want to comply with the relevant ISO standard or do you need to get the tables out?

If you plug the OPs figures into a basic calculator like the one on 12v Planet it suggests that 1.5mm2 is "just" about OK at a fraction over 3% voltage drop, although to be fair there is a disclaimer.

However, if you want to run it through an engine bay and/or bundle it with 5 other cables and stay compliant you need to apply the relevant derating, which gives you Paul's figure of 2.5mm2. The only way I can calculate this is to get the books out.
 
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'It ain't easy being green!'
 
Ammonite said:
Can anyone recommend a good calculator to use if you want to comply with the relevant ISO standard or do you need to get the tables out?
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De-rating for bundling, multiply the cable current rating by :

4 to 6 cables - 0,7
7 to 24 cables - 0,6
25 or more cables - 0,5

Ammonite said:
If you plug the OPs figures into a basic calculator like the one on 12v Planet it suggests that 1.5mm2 is "just" about OK at a fraction over 3% voltage drop, although to be fair there is a disclaimer.

However, if you want to run it through an engine bay and/or bundle it with 5 other cables and stay compliant you need to apply the relevant derating, which gives you Paul's figure of 2.5mm2. The only way I can calculate this is to get the books out.
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In almost all cases, the allowance made for voltage drop will be at least as much as the de-rating for being in a bundle. So in the case of the OPs bilge pump, in a bundle of 6 wires, de-rating the cable still gives it a current rating of 22A. Using the proposed 1.5mm cable leaves you with 14.7V, so don't use borderline cables.

But don't get too hung up on cable sizes. If your LED lighting circuits are OK with 1mm cable, use that. For everything else that can be wired with 1.5mm cable, use that. For everything between 1.5mm and 2.5mm use 2.5mm. You'll likely end up with the odd thing that wants 4mm, 6mm, 8mm, 10mm or 16mm so just buy what you need here. In other words, don't buy short lengths of 50 different cable sizes, buy a roll of 1.5mm and a roll of 2.5mm and you're covered for most things. Of course, if you only have one small job to do you might not want to buy a full roll.
 
Thanks Paul. I just wondered if there was a handy tool to work it out automatically. Using the tables if you are running a nominal 2.5mm2 30a cable through both the engine bay and bundling it with four or five other cables, assuming a one metre one way run and 15a load, the cable rating drops even further to 17a which is obviously a huge difference. A 21a 1.5mm2 cable would be derated to 12a..but as you say 1.5mm2 and 2.5mm2 should cover most things.
 
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We're getting there..

I'll ignore the 'swerve' from the Spiralwrap's tech people as evasive. I've learned another layer of 'wary' i.r.o. boaty 'lectrics...
....and have re-confirmed that, when a question pops into mind, ask about it here/there/everywhere until a working solution emerges.

The bilge-pump requirement is clear. It has a separate switch. I'll use a 2.5mm2 or 3mm2 cable ( I have some ) and keep it separated.
The other stuff can be serviced by my 10 little reels of 1.5mm - a healthy Clearance price from Aquafax - allowing for Paul's factors, and anything heftier that comes along, such as a stonking big radar or a sub-hunter searchlight, will warrant further careful provision.

Postscript: There's often a 'silver lining'. I purchased the Spiralwrap via Amazon. It turned up just fine. A week later they notified they had refunded my purchase price against a Return. Odd. I hadn't requested a Return so told them, thinking their system had 'glitched'.
So it proved, and they told me to keep the Spiralwrap AND the returned purchase price.

So, I've purchased TWO Lottery Tickets this week.....

;)
 
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