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I have the job of wiring a divelivaboard which is 30m long, and the customer wants 300w inverters in each cabin. He insists on a ring main feeding everything and I went ahead and bought a bunch of cable before discussing anything.

The total length of the ring is around 15-20meters, 5x inverters not including lighting and fans.

I have told him this is not practical, but to have someone else's opinion would be helpful. So what size cable should be used( considering the all inverters could be at full demand at the same time)?

MrPhooky
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Stu
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    What is a 'divelivaboard' ? What do you mean by a ring main feeding everything. Do you mean a pair of wires run from the battery to inverter one, and a pair from there to inverter two, etc?? – Bobbi Bennett Feb 13 '13 at 06:52
  • You probably should also add the kind of cable he bought. My gut feel is for 150A over the distance cable something in the order of 10mm in diameter would probably be OK, I'm not sure if that's "practical" or not. – PeterJ Feb 13 '13 at 07:52
  • dive-live-aboard, a boat with cabins for divers to live in. – posipiet Feb 13 '13 at 08:31

2 Answers2

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Lets say, no more than 0.5V drop, 130A, 30m.

Rule of thumb for cable size:

\$Q\text{[mm}^2]=\dfrac{I\text{[A]}×l\text{[m]}×0.018}{\Delta U\text{[V]}} = \dfrac{130×30×0.018}{0.5} = 140 \text{ mm}^2\$

Result: at least \$140\text{ mm}^2\$, and you need two of them. Thats about 13mm diameter without insulation, and you need two of those. You also need to attach to that cable at each cabin.

You should put the inverters near the boat´s power source and transport the high voltage over distance.

Another option would be to put an appropriate AC generator on the boat and keep the power supply completely off the Ship´s DC system.

posipiet
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  • Passengers will probably get little sleep with a constantly buzzing inverter in their cabins too, another reason to move the inverter near the boat's power source. – jippie Feb 13 '13 at 08:59
  • Can't resist: $d=2×\sqrt{\frac{A}{\pi}} = 2×\sqrt{\frac{140\text{ mm}^2}{\pi}} = 13\text{mm}$ – jippie Feb 13 '13 at 09:05
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    +1 for "Put the inverters by the battery and run mains to the cabins", although I would go one further and say put ONE BIG (and more efficient) inverter near the battery and run the mains out. That nice Mr Tesla has done the thinking for you: https://en.wikipedia.org/wiki/War_of_Currents – John U Feb 13 '13 at 10:53
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With a ring main, any wire in normal operation is only carrying half the current, so you could adjust Posipiet's answer accordingly. However in the event of (a) an open circuit in the wrong place and (b) full load on all inverters the installation must remain safe so I would endorse his choice of wire size, and reap the benefit of the ring as lower voltage loss and less wasted power in normal operation. (0.5V drop at 130A is 65W dissipated in the cables; they may get warm)

But given your situation I would suggest a star rather than a ring, a separate pair running to each inverter carrying just 25A. This means much lower loss in each cable and you may be able to use the wire he already bought. Any fault will be immediately detected (unlike a ring) and only affect one cabin (unlike a ring) and you can protect each circuit individually at the distribution board.

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    Also, the fuse for the ring must allow 150 Amps in a circuit running all over the boat. If any of those divers need to do some welding, they've got the juice! What I mean is, a 150 Amp feeder is no longer safe innocent 12V any more. With a star, at least it gets a bit more reasonable. – Bobbi Bennett Feb 13 '13 at 15:06