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Cable sizes... parallel wires


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Hopefully a quick question...

 

To get a sufficient cross section and therefore current handling, is it acceptable to run parallel cables?

 

I understand that there may be differences in the cables and therefore slightly unequal resistivity but it offers a more flexible solution for a tight space.

 

Thanks

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Its possible to do to "get you out of a fix" but I personally strongly dislike it because if ONE of the two wires breaks or its connection fails etc, the device will still work but maybe poorly (and maybe you'd never know), thus leading to possibly the other wire overheating, and/or live current flapping around within the bodywork from the other broken wire.

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We used to parallel cables frequently on HV installations, often 2 X 400 mm sq.
Providing secure connections are used (as all connections should be!), I can't see a problem. As others have said, the biggest risk is if one cable loses its connection,it leaves the other to bear the full load.

Parallelled cables can offer better cooling to ambient, though this isn't usually an issue on boats, where volt drop is the usual consideration.

Edited by Guest
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Remember the fuse is there to protect the cable so you may need to consider an inline fuse in both cables so that if one disconnects for any reason the fuse in the remaining supply will blow thus preventing overheating.

Seems like a good idea.

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I have used up to 6 cables in parallel when I used to work in the critical power industry. It is necessary to ensure a sound connection, either by connecting all e D's to the protective device, or using a busbar connection to the protective device and then taming the parparallel cables from that.

 

If only using 2 cables in parallel it should be feasible to crimp the ends on each cable and put both crimps onto the termination points.

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I'm looking to put twin 2.5mm three core 230v a down the length of the boat (future proofing for 32A supply) rather than getting the next cable size up (which seems to be 3 x 6mm csa).

 

If someone could suggest a reasonably priced source for the larger cable then I may think about re-routing it but the price and radii required are my reasons for thinking parallel.

 

Thanks so far

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Ok look at this slightly differently

In domestic wiring its common place to wire in a ring, breaker > socket1 > socket2 > socket3 >>>> last socket > breaker. This is perfectly acceptable practice.

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Why not run two 16a banks of sockets? No single item is going to use more than 13a.

 

I presume you are derating the 2.5mm for running in insulation?

 

Also the ring main idea as mentioned will help here. Effectively parallel cables joined at a socket.

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Sorry, I didn't make that clear... This is the feed (through RCB within the distance reqd) to the fuse and distribution board from the front appliance inlet. There rings or spurs are after that and will be 2.5mm the cable doesn't run through insulation or conduit/trunking

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Ok look at this slightly differently

In domestic wiring its common place to wire in a ring, breaker > socket1 > socket2 > socket3 >>>> last socket > breaker. This is perfectly acceptable practice.

All true. The downside of a ring main is that the continuity of the ring can be lost, whilst some sockets remain live, leading to what is effectively a radial circuit of 2.5 mm sq cable with a 32A breaker feeding it. I have known this to happen more than once when we used to perform tests in public buildings. I have often wondered if some kind of crimp should be deployed where even if the cables leave a socket due to a loose connection, the integrity of the ring main remains intact.

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Sam, currently moored nose in and planning an inlet both ends.

 

The cabin end option is troubling me as to how to get it to look right. Especially as I'd like that to be 32A too.

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One question why 32amp I can only think of one Mooring/marina where there is a bigger supply than 16a and from memory they are 63a.

Unless you are running an electric cooking system or electric heating 16a should easily be enough.

Edited by Loddon
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Remember to work out the circuits overcurrent protective device settings (OCPD) make sure you take into account relevant factors, e.g a 2.5 ring final circuit (2 x 2.5mm2) in a house may have to be downrated to 25 or 20 amps due to various de-ratings like bunching, thermal constraints etc. These are all in the tables along with the formulas in BS7671, although not sure if these need to be complied with on a boat (although it would be best practice)

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Why all this talk of rings !!

 

Davey is not wiring a ring, he is wiring a shoreline feed from the font of the boat to the rear.

 

 

Davey

 

Another point, if you are wiring a plug front and rear of the boat, you will need to ensure that whichever one is not being used is not live.

 

A change over switch manual or automatic will be the way to go.

 

I did not query the 32 amp supply before but as Loddon says Why?, I assumed your mooring possibly had it but as I understand it it is rare.

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Manual change-over switch to separate front and rear inlets as ithe is fairly easy and cheap (and replicates the inverter/inlet switch that stops the inverter making the inlet live).

 

The 32A idea was as I suggested at the start, a belt and braces future proofing idea.

 

Thanks for all the info... I may look into 4mm csa or simply not bother and stick to 16A

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