wire size calculation

[antonywardle]

Hi

Trying to get my head around wire sizes for the boat.

If a fishfinder outputs Max 250W, then with a 12volt imput

that means it is drawing 250(watts)/12(volts) = 20.8 amps

so I need a cable to the fishfinder that is at least 21amps.

Is this correct?

[hookerbruce]

power ( watts ) equals volts times current

Divide watts by volts to get current

[Spykey]

Anthony,

your cable should be able to carry at least 25A DC which is a lot though..

Will it constantly draw 20.8Amps?

Then depending on how long the cable is, the thickness will vary.

Tell me how long roughly the cable will be and i'll tell you the safe thickness size...

roughly 6mm for about 3m long...

Anything longer, you need bigger...

Edited May 20, 2014 by Spykey

[stevefish]

Your calculations are correct,

Although you will find your fish finder gives a 250w pulse of a short duration a few times a second, there should be an input figure for the unit which will be lower than 21A/250W.

[Spykey]

Your calculations are correct,

Although you will find your fish finder gives a 250w pulse of a short duration a few times a second, there should be an input figure for the unit which will be lower than 21A/250W.

This is what I thought, It will not constantly use 250w..

This one here use only 5Amps, http://www.lowrance.com/Root/Lowrance-Documents/US/HDS_GEN2_TOUCH_IM_988-10317-002_w.pdf

Edited May 20, 2014 by Spykey

[antonywardle]

Thanks for that

That's the fishfinder I was looking at (HDS7 - Huey has them at a good price)

and the only specs i found were the 250W output (Max)

I've been struggling to find any information about electrical devices in general.

That manual looks like a 12Watt draw so that makes it easier

I'd like a power cable from the back to the front where the switch panel is. I was thinking

of a 6mm 50amp tinned, twin sheathed. That distance will be around 6 metres, or a little less.

I'll have a radio, a marine cb (two) and some Led and nav lights too. Haven't looked at the radios other

than to note that they are GME. The car radio is a bit faulty so I'll pull it out and

see whats up with it.

Edited May 21, 2014 by antonywardle

[Spykey]

yeah go 6mm.. thats more than enough for it..

[antonywardle]

I went and bought some, and then spent the afternoon pulling out everything.

I bought a new switch panel and spent ages trying to cut out the hole. The was already a

hole from the other one, so I went down to supercheap and bought a renovatator tool for

$58. It tidied up the hole in about 2 minutes. I don't think I'd renovate the harbor bridge with it

but its ok for what i did and it saved me at least an hour if not more.

One thing that occurs to me is that I should have a fuse down the battery end

Is that the best place for it? I was thinking of a 40AMP fuse as the wire is 46AMP rated.