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Catalina/Capri 25/250 Sailor's Forums

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How many amps?

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delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Initially Posted - 03/02/2008 : 18:59:06

I'm replacing the wiring from my alternator & solar panel to my battery. The PO or PPO didn't install any fuses, breakers, etc. and the wires have obviously been arced together at least once, there is a burn mark on both the battery box & battery where one of them got hot enough to melt plastic. Theoretically this was fixed before I bought the boat, but never actually was. I think he just cut out the worst burnt parts and put new ring fittings on.

I've already replaced the external wiring with 12 & 14 gauge marine wire for the alternator & solar collector respectively, and that's what I'll be running on the interior as well. My question is how many amps do I need for a fuse to protect both circuits. I'm guessing I can get by with 1-2 amps on the solar panel at most, and maybe 10 on the alternator?

I've got a '97 Johnson Sailmaster engine with (what I presume is the) factory alternator. But I have no idea how many amps it'll put out at full throttle. How do I find this out?

My solar panel is maybe 18" square, maybe 40 watt? Not sure on it either, there's nothing on the nomenclature tag on the underside of of it that indicates how many amps it should be protected from.

David
C-250 Mainsheet Editor

Sirius Lepak
1997 C-250 WK TR #271 --Seattle area Port Captain --

Edited by - on

JohnP
Master Marine Consultant

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1520 Posts

Response Posted - 03/02/2008 : 20:35:02

Wow, you are lucky that the boat didn't catch on fire! I don't have a solar panel, but in Don Casey's Sailboat Electrics Simplified there is a chapter explaining the wiring for them.

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glivs
Admiral

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USA
836 Posts

Response Posted - 03/02/2008 : 20:38:32

David,
Max. alternator output on our '89 9.9HP Evinrude is only 4A. I think output on modern 4-strokes is about 6A, although Honda's may be as high as 8A at full throttle.

Curious your PV-panel does not list its Amperage. If you knew the manufacturer, perhaps you could search if they make a comparable sized model today. For reference, Kyocera's 20W panel (about 14x21") puts out about 1.24A.

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Voyager
Master Marine Consultant

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USA
5369 Posts

Response Posted - 03/02/2008 : 21:30:59

David -
Gerry's estimates are reasonable.

If you have a Digital Multimeter (DMM), many come with a 10A or 20A setting, so you can measure the actual current with this device. If you don't have one with a 10A or 20A setting, don't use this approach.

To do this, set up your circuit with your fuse block, remove the fuse and put the DMM in series with the fuse block (red on one side, black on the other) and you can measure the actual value. Then, PULL THE CORD to start the engine, don't use the starter.

One word of caution, however, make sure you DO NOT disconnect the DMM while the engine is running, or creating an "open circuit" will likely fry the diodes in the alternator.

Check the alternator under full load and full throttle, and the DMM will tell you the max current. Then assume 100% more for your fuse.

For example, if you measure 7A, add 7 more and you should use a ~15A fuse. If you measure 10A, use a 20A fuse. And so on.

Once you get your max current, shut off the engine and replace the fuse.

One problem: If you get OL on the meter, the alternatior is putting out more than 10A. This is very unlikely.

The trick is to use a fuse that will not blow unless something totally unforeseen happens. Some people will actually wire a 20A light bulb in series with the fuse block to prevent a blown fuse from creating an "open circuit".

The solar panel is not as critical. Provided it has a "blocking diode, nothing bad will happen if the fuse blows. Just use a larger one. Start with a 5A, and if that blows, use a 10A, and so on.

Edited by - Voyager on 03/02/2008 21:32:38

Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 03/03/2008 : 00:33:43

My 96 Sailmaster is 60 watts, that is 5 amps. If the motor specs list the output in watts, it is watts=amps x volts so amps = watts divided by volts (12). The rated listing will be the maximum output under optimal conditions. But the motor circuit is not usually fused because that is also the current path for starting and that is a much higher current draw. Fusing the solar panel wouldn't hurt and an in line fuse within a few inches of the battery for the primary feed to the breaker or fuse panel is also a good idea. Add up your total anticipated amperage and round up to a larger fuse, but keep it below the recommended load for the primary gauge you have chosen

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Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 03/03/2008 : 00:39:30

Also, fuses protect the battery and wires from down stream failures. Fusing the solar panel protects the battery from a shorting failure in the solar panel or wires thereby preventing electrical fires.

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OLarryR
Master Marine Consultant

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USA
3462 Posts

Response Posted - 03/03/2008 : 05:40:15

I have a Kyocera 20 watt solar panel and it has dimensions of 20" X 14". Each manufacturer has varying dimensions even for same wattage panel based on how the cells are configured in the panel. However, when I was checking the wattage versus dimensions, the Kyocera 20 watt has about the smallest footprint for that wattage. So...if you have an 18" X 18" it is doubtful it more than say 30 watt. If it were a 20 watt, then a 2 amp fuse should do the trick. For your sized panel and not knowing the exact amps, you may want to wait for a full sun day and angle the panel perfectly to max out on the amps ...then with a multi-meter and in series for amp measurement (otherwise if done across/in parallel you will blow the multi-meter ! Done that already !) you will know if you need a 2 or 3 amp fuse...or just go with a 3 amp.

As far as the motors go, before the latest generation Honda Outboards, most outboards were putting out 6 amps or less. The latest Honda Model puts out 12 amps.

You may want to ask West Marine or someone here with some electrical smarts a bit more about fuses. They have what I believe they call fast fuses or something like that. Those should be used for electronics versus motors, etc that can use regular - slow fuses. The fuse for the solar panel probably should be one of those fast fuses since electronics can be fried in milli-seconds.

The circuit breaker in the switch panel protects the wiring and should be sized for max amps of the wire gage. Since you are using a heavier gage for some of your wiring, if you have only a 1 circuit breaker switch panel, then the one circuit breaker is sized to protect the lighter gage wiring. Switch panels that have separate circuit breaker switches for each circuit can have circuit breakers sized according to each circuit's wiring gage....but even then there is sort of a mismatch in that many of these switch panels come standard with certain sized circuit breakers already installed and probably most of us do not get the mfr to exchange them for more appropriate breakers for each of our circuits.

In any case, if each wire load has a designated fuse in-line, then that is best in that it protects the equipment and by default, also protects the individual wire...at least the one going to the load. If a specific electronic gear goes out and the main breaker is not tripped, then it is pretty easy to find out which fuse has blown and meanwhile all other electronics should be still working since only the fuse to the one load has blown.

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