Doubling up cables

[170968]

I recently installed a Victron shunt. I decided to use 50mm cable which I thought would be OK (I'm sure it is), but was also influenced by the fact that I only had a 50mm crimping tool.

 

I'm wondering now if I could use 2 50mm cables between the shunt and distributor to half the resistance? Is it acceptable and safe? It is only about 20cm long.

 

 

[Tracy D'arth]

It will be fine. 20cm of 50mm2 cable has a negligible resistance, you will be into several hundreds of amps load before volt drop causes a problem.

[170968]

1 minute ago, Tracy D'arth said:

It will be fine. 20cm of 50mm2 cable has a negligible resistance, you will be into several hundreds of amps load before volt drop causes a problem.

 

Thanks Tracy.

 

Would the doubling up have been something you can do though? TBH I'm more interested in knowing than actually doing it.

[Tracy D'arth]

It may be considered by some to be bad practice but if say you had a problem getting thicker cable onto terminals or into crimps I would consider it to be OK especially where both were visible and short lengths.

[Tony Brooks]

The problem might come if the current flow is more than the 50mm cable can handle. If it is and one cable termination or connection developed a fault then the remaining cable could catch fire. The chances are slim but you need to be aware of the potential. If it is just volt drop that concerns you ten fuse the two cables with a fuse to protect the cable, but then the fuses may cause more volt drop that a single 50mm cable.

[cuthound]

When I was working cables were often doubled, tripled or even quadrupled. Mind you as 630mm sq cable is the biggest you can get and the loads often exceeded its current carrying capacity there was no other option.

 

Providing the lugs are crimped properly and the bolted tightly to.the connectors there shouldn't be any problems.

[Batavia]

5 minutes ago, cuthound said:

When I was working cables were often doubled, tripled or even quadrupled. Mind you as 630mm sq cable is the biggest you can get and the loads often exceeded its current carrying capacity there was no other option.

 

Providing the lugs are crimped properly and the bolted tightly to.the connectors there shouldn't be any problems.

Totally agree.  I used to double up cables on largeish motors, as it is infinitely easier to install 2 x 70mm2 3 core SWA cables than, say, one 240mm2 in ducts. On a 1.6 MVA distribution transformer, the tails will normally  be 3 x 630mm2 AWA per phase and 2 x 630mm2 for the neutral.

It relies entirely on good workmanship.

[nicknorman]

Doubling up negative cables is fine, though in your case at 20cm it isn’t going to make any difference. Doubling up positive cables can mean that 2 fuses are required because really the fuse should be able to protect one remaining cable if the other becomes disconnected for some reason. However this is only going to be a problem if the max intended current flowing in both cable exceeds the max allowed on one cable on thermal grounds.

[170968]

Thank you all. Very interesting. I won't bother doing it.

Edited October 13, 2022 by 170968

[Batavia]

10 minutes ago, nicknorman said:

Doubling up negative cables is fine, though in your case at 20cm it isn’t going to make any difference. Doubling up positive cables can mean that 2 fuses are required because really the fuse should be able to protect one remaining cable if the other becomes disconnected for some reason. However this is only going to be a problem if the max intended current flowing in both cable exceeds the max allowed on one cable on thermal grounds.

I would like to point out that fusing of individual parallel conductors is hardly ever done in electrical power installations - on the basis that the installation should have been done properly and that nobody would be stupid enough to disconnect a conductor.  However, having seen some atrocious boat electrical installations...

[Tony Brooks]

1 hour ago, Batavia said:

I would like to point out that fusing of individual parallel conductors is hardly ever done in electrical power installations - on the basis that the installation should have been done properly and that nobody would be stupid enough to disconnect a conductor.  However, having seen some atrocious boat electrical installations...

That is exactly my worry when professional power people say it is OK. The professionals in power supply almost certainly use first class terminals and very expensive (in boater terms) crimp tools, not something from Ebay and a pair of pliers or cutters.

[robtheplod]

I've often wondered this. Some Victron kit states having 2 x 50mm2 cables each for positive and negative... often wondered if 1 x 95mm2 would be so wrong here?

[nicknorman]

3 minutes ago, robtheplod said:

I've often wondered this. Some Victron kit states having 2 x 50mm2 cables each for positive and negative... often wondered if 1 x 95mm2 would be so wrong here?

Not possible because the connectors on said Victron kit only fit 50mm2 cable! You can't get 95mm2 cable to fit. You would have to ask Victron why they did that!

[Murflynn]

I used to use welding cables in such situations.  Nice and flexible and easy to terminate with a large crimping tool.  I would be interested to learn if that was bad practice.

[Tony Brooks]

14 minutes ago, Murflynn said:

I used to use welding cables in such situations.  Nice and flexible and easy to terminate with a large crimping tool.  I would be interested to learn if that was bad practice.

As long as they were supported as per the BSS then they are fine at 12/24 V. Not sure what the insulation is rated for voltage wise, so would be hesitant to use them at higher voltages..

[IanD]

13 hours ago, nicknorman said:

Not possible because the connectors on said Victron kit only fit 50mm2 cable! You can't get 95mm2 cable to fit. You would have to ask Victron why they did that!

 

Maybe because the DC terminals are PCB mounted, and with a single thick cable there's more risk of stress from the cable causing damage, either from the cable or the bigger tag/stud needing more torque to tighten it down?

[Tracy D'arth]

2 hours ago, IanD said:

 

Maybe because the DC terminals are PCB mounted, and with a single thick cable there's more risk of stress from the cable causing damage, either from the cable or the bigger tag/stud needing more torque to tighten it down?

Very possible, I have had a few where the terminal blocks have burn off the pcb, the connecting tabs soldered to the boards are not very thick in comparison with the terminals. The thermal movement of heavy cables is a probable cause.

[cuthound]

4 hours ago, Murflynn said:

I used to use welding cables in such situations.  Nice and flexible and easy to terminate with a large crimping tool.  I would be interested to learn if that was bad practice.

 

No, not bad practice. Many moons ago the BT mobile generator group was attached to my group. They always used multiple paralleled 150mm sq. welding cables to connect the mobile generators to the telephone exchanges, because they were easy to get into often cramped switchboards.

 

Cables were cut to a length that weighed 25kg (the then maximum recommended weight for one man to lift), so the longer runs had lots of joints.

 

 

4 hours ago, Tony Brooks said:

As long as they were supported as per the BSS then they are fine at 12/24 V. Not sure what the insulation is rated for voltage wise, so would be hesitant to use them at higher voltages..

 

The insulation is rated at 500 volts, so good for three phase mains.

Edited October 14, 2022 by cuthound
To add spaces between merged posts