Doubling DC wiring - but not all the way

[sailorbenji]

A hypothetical question (for now) that's caused quite some differing opinions here :

Let's say, according to manufacturer's guidance, I need DC wiring of 100mm2 each for the relevant run and current requirements.

If I ran 2 lengths of 70mm2 cable, side-by-side, but only 80% of the distance, could the run be considered to have been done in 112mm2 cable ((2*70)*0.8)...and thus be considered to be in excess of the 100mm2 requirement?

If not, what is the impact of doubling up a cable run cable like this, but not completely from one point to the other?

 

[B27]

Work out the resistances.

But personally, I'd try to avoid joins whenever possible, more so in heavy cable.

 

[AngusMcDoon]

You could choose the length of doubled cable length so that in theory the resistance was the same as a single larger cable thereby giving the same voltage drop. However, that's only in theory. In practice there would be resistances in the connections and also the single section of cable may get hot. Both will increase the actual resistance and hence increase the voltage drop.

 

[neil_s]

Hypothetically - If one of your 70mm2 wires was damaged, broken or simply suffered corrosion in its terminals, your appliance might well power up and not exhibit any signs of malfunction until you demanded full power from it. Then, the single 70mm2 would carry all the load and would overheat and perhaps start a fire.

 

[sailorbenji]

Thanks for the feedback, of course aware of all said, but below is the issue.
Of course it’s completely impractical to run 140 mm2 cabling, so even the manual states 2 x 70mm2. I can do that, my run is maybe 8m, but I can only get 1 x 70mm2 cable the last 1m or so due to access.
I think I’ll have to live with that.

 

[andsarkit]

Check with your cable spec but I think the problem is more likely to be volt drop rather than current capacity.
If you work out the volt drop of 5m of 70mm and compare that with your combination of 8m doubled and 1m single and I think you will find they are exactly the same. Joining the cable could be a problem but proper lug terminals on a bolt in a sealed box should work as the cables probably use lugs for the end connections already.
.

 

[KompetentKrew]

If I ran 2 lengths of 70mm2 cable, side-by-side, but only 80% of the distance

I'm confused. What runs the remaining 20% of the way?

 

[justanothersailboat]

The spec sheet is pretty clear: 1x 70mm2 is enough to carry the current, but over 5m, they think it'll have too much volt drop, so they recommend the doubling.

I looked up 70mm2 wire and found some rated at 485 amps - this is much more than the 200A the device is designed to demand. So it seems pretty clear that the single cable won't actually be overloaded, and if one cable in the double run drops out, the remaining one won't be overloaded either. There will just be enough volt drop that the performance will fall a lot.

You'll be fusing for the rating of the device (plus a bit), not the cable, so there's not much risk of overloading the cable.

I would only worry about the connections. Nearly all connections have less capacity than the wire when new, and are at greater risk of degrading. The bigger the wire, the harder to make a connection that is only a small bottleneck rather than a large one.

My guess is that your plan is ok if your connections are really good quality (and well protected) and not ok if they are not.

 

[William_H]

You'll be fusing for the rating of the device (plus a bit), not the cable, so there's not much risk of overloading the cable

You should be fusing for the smaller portion of cable but of course fuse must be suitable for cable and device. Yes possibly consider fialure of one of the parallel cables or joins. Resistance of the total cable (or better thought of as conductance the mathmatical inverse of resistance) is a function of the total resistance of the 3 parts of cable (2 parallel plus single portion) Short lower size should be no problem as resistance is a function of length however small size could conceivably get hot at high current (burn). Plus of course the same concerns re negative return cable(s). So no concerns except as said the reliability of joins. ol'will

 

[sailorbenji]

I'm confused. What runs the remaining 20% of the way?

Only 1 cable.

 

[thinwater]

At least in the US, the electrical code says parallel feeds are not allowed smaller than 1/0 AWG, which is about 100 mm2.

Another problem is that if high resistance develops in one wire, the fuse cannot protect the remaining leg.

 

[VicS]

At least in the US, the electrical code says parallel feeds are not allowed smaller than 1/0 AWG, which is about 100 mm2.

Another problem is that if high resistance develops in one wire, the fuse cannot protect the remaining leg.

1/0 AWG is only 53.5mm²

The recommended 70mm² cable is rated at 485 amps. The recommended 200amp fuse will adequately protect this.
.

 

[VicS]

Thanks for the feedback, of course aware of all said, but below is the issue.
Of course it’s completely impractical to run 140 mm2 cabling, so even the manual states 2 x 70mm2. I can do that, my run is maybe 8m, but I can only get 1 x 70mm2 cable the last 1m or so due to access.
I think I’ll have to live with that.

IF you take the resistance of 70mm² cable as 0.25 ohms/1000m you can calculate the total resistance of your proposed wiring to be 0.0023 ohms ( I assume your negative wiring will be the same as the positive)

You don't state the current draw for the equipment you are installing but at 150 amps the volts drop will be 0.35 V which is 2.9% for a 12 volt system. This is within normally acceptable limits

However, have you considered a local battery charged by a B2B charger? This will avoid all that heavy wiring and what you save will more than pay for the B2B charger.
.

 

[madabouttheboat]

If you can only run a single cable the last couple of meters, what about running a single but larger cable the whole way, say a 90mm2?

 

[VicS]

If you can only run a single cable the last couple of meters, what about running a single but larger cable the whole way, say a 90mm2?

Given that the the electrical resistivity of copper is 1.724 x 10 ^-8 Ωm, what will be the maximum current this could carry over a total distance of 16m without causing excessive volts drop.
.

 

[madabouttheboat]

Given that the the electrical resistivity of copper is 1.724 x 10 ^-8 Ωm, what will be the maximum current this could carry over a total distance of 16m without causing excessive volts drop.
.

You tell me. 90mm2 was a suggestion, but I would exect the OP to do the calculations. The point I was making was if only one cable can be fitted, why not use one slightly bigger than 70mm2. If 1 x 70 is good for 5m, and 2 x 70 is good for 10m, then at 8m surely 1 x ?? would work fine once the voltage drop calcs have been done.

 

[Aja]

Be really interesting to find out what is being powered at the business end and how long it would run continuously. Windlass?

 

[thinwater]

1/0 AWG is only 53.5mm²

The recommended 70mm² cable is rated at 485 amps. The recommended 200amp fuse will adequately protect this.
.

Correct. I looked at the OD.

Yes, if you are doing it for voltage drop, not max amps, no problem.

 

[VicS]

You tell me. 90mm2 was a suggestion, but I would exect the OP to do the calculations. The point I was making was if only one cable can be fitted, why not use one slightly bigger than 70mm2. If 1 x 70 is good for 5m, and 2 x 70 is good for 10m, then at 8m surely 1 x ?? would work fine once the voltage drop calcs have been done.

For 90mm² it would be just shy of 120A but AFAIK 90mm² is not a standard size. For 95mm², which is a standard size, it would be 140 A.

If you extrapolate from 70mm² being recommended for up to 5m you get 112mm² for up to 8m. The next nearest standard size would be 120mm².

 

[PaulRainbow]

Be really interesting to find out what is being powered at the business end and how long it would run continuously. Windlass?

The OP is fitting a 48v 5000w inverter with a built in 70a charger.

Continuously rated at 5000w, which at 48v will draw about 100a or so, but also rated for probably double that for peak current, hence the 200a fuse. The fat cables aren't there for current ratings, they are there for voltage drop.