Power Consumption in 4 Parallel Circuits

Don S/V ILLusion · Jul 22, 2013

Nothing of any importance. Every device presents a unique load.

Perhaps I don't understand what you are asking.

Bill Roosa · Jul 22, 2013

First things first. Stop thinking in watts. DC sizing, capacity......everything is volts, ohms, and amps. Watts is right out.
Most appliances will have an amp rating (assumed at 12.6ish volts) use that value to calculate the circuit protection or better yet use the manufacturers recommended fuse/Circuit breaker amp rating. The wires have the added burden of carring the current over a distance. So long wires need to be larger for a given load than short wires. aka voltage drop due to wire resistance. West marine and others have wire ampacity vs wire length. Also be mindfull of the differance between safety and non-safety appliances as they have different charts and wiring that goes through hot engine spaces has a derating factor that has to be applied.
I would highly recommend that you ignore the ohm readings that you have posted as the VHF would seem to be drawing 12.6/.029=48 amps!!!! no way is that correct. I suspect you measured the resistance at the power and ground terminals. Unfortunately the current has impedance as well as resistance. The impedance is on the order of 12 ohms as most vhf draw less then an amp in standby and only about 2-5 in transmit mode. Similarly the light would draw 12.6/1.12 or 12ish amps. Again they only draw an amp or two. Don't know where you are getting those numbers but they are not correct.

Ralph Johnstone · Jul 22, 2013

As Bill has mentioned, you only want to use amps in calculating circuit loading.

However, first you need to find out the amps drawn by each appliance. Electronic instruments make less than no sense if you measure their resistance. Best to get the manufacturer's rated wattage and calculate the amperage from there. Also, incandescent light bulbs have almost no resistance when cold but build up resistance when hot. Again, look at their wattage to get the amperage. And that VHF that Bill just doesn't believe pulls 48 amps, maybe it's got a built in coffee maker you haven't discovered yet

. Again, best to check the manufacturer's wattage figures.

Once you have everybody's amperage draw, THEN go ahead and see if anyone is overloaded

Blue Sea Systems has some excellent info on wire sizing.

Maine Sail · Jul 23, 2013

DinoKanuck said:
Here is each device followed by Watts, Volts, Amps and Ohms
........Device............ Watts.. Volts...Amps....Ohms
Chartplotter MFDisplay 10.4.. 13..... 0.80....1.250
3G Broad Band Radar 19...13.8.... 1.37......0.726
VHF/AIS Lowrance L8 81.6...13.6... 6.00..... 0.167
Deck Light................ 15.... 12.6... 1.50..... 2.143

Is there an obvious error? Can it be 108 amps?

Dino K

I don't see 108A there I see:

Plotter = .8A
Radar = 1.37A
VHF = 6A
Deck Light = 1.5A

Total = 9.67A

I see that you've figured some at 13.8V and some at 12.6V. Without the engine on all of these circuits will be below 12.6V...

Forget the OHM's and deal in amps, amp hours and wattage. Size your critical circuits for 3% voltage drop.

I like this calculator: http://genuinedealz.com/voltage-drop.html

And this one: http://boatstuff.awardspace.com/awgcalc.html

Watts to Amps

To convert Watts to Amps at fixed voltage: Amps = Watts/Volts

For example 12 watts/12.6 volts = .95A

Volts to Amps at fixed wattage

To convert Volts to Amps at a known wattage: Amps = Watts/Volts

For example 120 watts/110 volts = 1.09A
For example 120 watts/12.6 volts = 9.5A

Power Consumption in 4 Parallel Circuits

DinoKanuck

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Don S/V ILLusion

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Bill Roosa

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DinoKanuck

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Ralph Johnstone

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Attachments

Maine Sail

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DinoKanuck

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