Your switch won't like 20A DC very much at all. AC has a "zero voltage point" during its alternating cycle, at which time any arc drawn as the switch contacts separate will tend to self-extinguish. In a DC system there is no such zero point to help with extinguishing the arc, so the switch current rating must be reduced. If not, your switch might be incapable of extinguishing the arc and thus end up a molten mess, undistinguishable from the rest of the molten charred mess of you and your boat (I'm being a bit melodramatic here, but I'm sure you get the picture). And while it is continuously arcing, it is possible that it might not pull enough current to blow the 20A fuse, but still generate enough localised heat to cause lots of damage.

The low-voltage circuit breaker application guide in my bookshelf shows a derating to around 1/3 - 1/4 of the AC current rating. Go with 1/4 and you will be safe.

Good on you for asking,
Cheers, Graeme

wongaga said..
Your switch won't like 20A DC very much at all. AC has a "zero voltage point" during its alternating cycle, at which time any arc drawn as the switch contacts separate will tend to self-extinguish. In a DC system there is no such zero point to help with extinguishing the arc, so the switch current rating must be reduced. If not, your switch might be incapable of extinguishing the arc and thus end up a molten mess, undistinguishable from the rest of the molten charred mess of you and your boat (I'm being a bit melodramatic here, but I'm sure you get the picture). And while it is continuously arcing, it is possible that it might not pull enough current to blow the 20A fuse, but still generate enough localised heat to cause lots of damage.

The low-voltage circuit breaker application guide in my bookshelf shows a derating to around 1/3 - 1/4 of the AC current rating. Go with 1/4 and you will be safe.

Good on you for asking,
Cheers, Graeme

That is such good info Wongaga that you got me all excited. I'm embarrassed to admit that made perfect sense...after you explained it!
Thanks!

RubikRedDog said..
Could you get around the upscaling by using double pole/single throw? Each pole carries half the current?

Yes, but it would probably be better to wire them in series. That way each set of contacts only has to deal with a little arc driven by 6V, not 12.

wongaga said..

RubikRedDog said..
Could you get around the upscaling by using double pole/single throw? Each pole carries half the current?

Yes, but it would probably be better to wire them in series. That way each set of contacts only has to deal with a little arc driven by 6V, not 12.

Hi Wongaga, That's not correct, with switchers in series there is still 12 volts differential across each set of of contacts unlike batteries. There is still full current across each set of contacts which in fact could cause both sets of contacts to ignite. It would probably extinguish quicker as technically the total air gap in the circuit would be doubled, a bit like cascading circuit breakers. If the switches were in parallel the fault current would be shared by each switch providing the impedance is identical in each cables and switch. The best option is to go with a single suitable DC rated switch or even better a DC rated circuit breaker.

Yara, The IP rating of a switch has nothing to do with it's switching ability. It relates to its ability to keep out moisture and dirt particles.

Jode5 said..

wongaga said..

RubikRedDog said..
Could you get around the upscaling by using double pole/single throw? Each pole carries half the current?

Yes, but it would probably be better to wire them in series. That way each set of contacts only has to deal with a little arc driven by 6V, not 12.

Hi Wongaga, That's not correct, with switchers in series there is still 12 volts differential across each set of of contacts unlike batteries. There is still full current across each set of contacts which in fact could cause both sets of contacts to ignite. It would probably extinguish quicker as technically the total air gap in the circuit would be doubled, a bit like cascading circuit breakers. If the switches were in parallel the fault current would be shared by each switch providing the impedance is identical in each cables and switch. The best option is to go with a single suitable DC rated switch or even better a DC rated circuit breaker.

Yara, The IP rating of a switch has nothing to do with it's switching ability. It relates to its ability to keep out moisture and dirt particles.

Jode

No, sorry: when the contacts open there is a voltage drop across each of the arcs. Since they are in series, Kirchoff tells us these will add up to 12V. There can't be a total of more than 12V (we're assuming it's not a huge inductive load here), so each one must see 12V minus something, that something being the voltage across the other arc. So each set of contacts sees less than 12V, and hence has a better chance of extinguishing the arc than if it saw 12V. But let's not bore everybody, pm me if you want and we can delve into the theory together, and if you convince me I'll post a big mea culpa. And yes you're right, applying a switch in line with the manufacturer's rating should be the starting point.
Cheers, Graeme

Yep, all good John, OP got his answer and a good discussion.

After 40 years as an Electrical and Controls engineer I was delighted to leave it behind when I retired. But the knowledge sure comes in handy on the boat, and keeping recalcitrant appliances from the dreaded nature-strip chuck-out.