SHORT Answer - it depends ...
What I mean is ...
does your existing wiring back there have enough extra spare juice available to run your handheld spotlight?
MEDIUM Answer - I bet that light draws 8 amps max, so figure 15 MAX amps for worst-case draw due to peak amps at start-up, plus to leave some 'overhead'. Therefore if your existing run can handle an additional 15 amps - go for it.
LONG Answer - OK, here goes ... but remember - you asked for it!
1) You need to know and
identify ALL circuits on the helm panel and then simply
add up the amperage of all those items. If, like the spotlight, you only know the wattage of the bulb, then divide the wattage by 12.6 volts (typical full charge battery rating) and the result gives you the amp draw or load.
Example, I bet that light uses a ~100 watt bulb, so dividing by 12.6 volts gives you an 8 amp load.
2) Once you know the total amp load on your circuits, determine HOW MANY are “on” for a true worst-case scenario. Meaning, you won’t likely throw on ALL switches at the same time, but figure in your worst case and make sure you add for “intermittent use items” like bilge or spotlight use, livewell, and/or trim tab cycling (20 amps).
FYI - an OB starter draws ~235 amps at peak when starting; while this load is NOT on your house circuits, nor should it be, it is to the battery. This is 'why?" some items can shut-off when you restart your motor (which also means there is TOO much resistance in the existing wiring wire [i.e., terminals or wire is corroded or corroding] or the wire is sized too small.
3) Check your worst-case loads (always good to have sufficient overhead) to the wire ampacity capacity given the TOTAL LENGTH of the existing circuit run - see #4 as it is CRITICAL to this calculation.
4) NOTE: You'll have to know the TOTAL round-trip distance from the load to the battery or power source to adequately determine the amp draw and capacity for the wire rating.
Shoot for no more than a 3% voltage drop for critical items (nav lights, radar, radios, etc.,) and no more than a 10% voltage drop for non-critical items (livewell pump, house lights, am/fm radio, etc.).
5) Use this wire size calculator
click here[/b], to determine the proper gauge of wire for any load or combination of loads you need.
6) See this post
here[/b] for sample
wiring diagrams that TomS and I made up for our boats using Visio or similar drawing program. TomS kindly provided me with his sample to modify. He is a wiz with software and that's why he's our resident IT/IS and graphics guro on CP. Thanks Tom!
7) Fuses and breakers are installed to protect your device per se ... they are there to PREVENT your boat's wiring from burning your boat to the waterline IF it should ever be over-loaded :shock: ! That is why you NEVER insert a replacement fuse/breaker rated higher than demanded for the item and the total ampacityrating of the wire.
FYI - Boat wiring is ‘simple’ once the basics are understood. I have started a
“Boat Wiring Tutorial 101” article on
How To DIY Rewire your boat when/if/as needed … but it is not complete and is in DRAFT form. Someday I would like to post it on Classic Parker. That said, if anyone wants to use – PM me, provided you understand and acknowledge 2 things: (1) You use it at YOUR lone risk and do not hold me or CP liable for any personal or equipment damages and (2) you provide feedback in case or errors or omissions.
CP members may PM me for any wiring question/comment, or continue to add to this post here.
