How many amps?

Discussion in 'Boat Tips' started by Ictalurus Punctatus, Oct 8, 2007.

  1. Ictalurus Punctatus

    Ictalurus Punctatus New Member

    Messages:
    353
    State:
    Greensboro, NC
    I just put a new switch/fuse gang in my boat. I want to properly match my fuse capacities (for obvious safety reasons) but I can't come across the info for how many amps my nav/anchor lights draw. The lights are the SeaSense brand from Walmart (both nav and anchor) that uses an Attwood #90, 7.5 volt bulb. What size fuse does it take? Any help would be appreciated.

    Jon
     
  2. Mark J

    Mark J New Member

    Messages:
    9,407
    State:
    Four Oaks, NC
    You'll need to know the resistance of the bulb(s) or the amps to apply ohms law. Once its determined then you can size the wire. The bulb probally has an R value on its base.

    DC is alot different then AC current in wire sizing.
    For example if you put your starting battery in the front of an 18 foot boat it would be expensive in just the cost of the wire. 5' feet of additional length can have a large impact on wire size on a 50 amp DC circuit. 15 additional feet can have a huge costly impact on wire size for a 50 amp DC circuit.
    Where as on an equivilent AC circuit there would be no loss of voltage over 15 feet or 30 feet.
     

  3. wolfman

    wolfman Well-Known Member

    Messages:
    9,227
    State:
    Triadelphia, WV
    Name:
    Walter Flack
  4. BKS72

    BKS72 New Member

    Messages:
    3,361
    State:
    East of KC
    They're going to draw almost nothing. A 500 gph bilge pump draws around 4 amps, my LCD depthfinder has a 2 amp inline fuse. There's no spike when you turn them on like there might be for pump or other device with a motor so you don't need to size it to compensate for that. I wouldn't put anything over 5 amp on it and would probably use smaller if you have it handy. Good Luck!

    Branden
     
  5. Mark J

    Mark J New Member

    Messages:
    9,407
    State:
    Four Oaks, NC
    Watch those pumps! That amperage rating listed on the pump is water in and water out with no hose.
    An outlet hose too small, bends in the pipe and height of rise will increase amperage.
    The same applies to the rated pump capacity (500GPH).
    After you get it set up you can test its capacity with a bucket and a stopwatch.
    I dont know of any pump that pumps its rated capacity once its installed.
    Some brands are alot worse then others.
    Its why I always suggest two bilge pumps of atleast 1500 GPH.
    You've got a back up pump and a combined capacity that will help you get to shore if you are moderately holed.
    The lower capacity pumps arent good for anything other then pumping standing bilge water out or collected rain water. If you are holed its a praying situation with a small pump. Pray that you dont have to go far to beach it.
    That is one area dealers often skimp on the rig out. Most folks look to see if its got a pump and thats good enough if it does.
     
  6. olefin

    olefin New Member

    Messages:
    3,908
    State:
    Texas
    Mark, are not those centrifugal pumps?
    On centrifugal pumps or compressors motor load will decrease if flow is restricted.
    Right the opposite on positive displacement pumps or compressors..
     
  7. GaryF

    GaryF New Member

    Messages:
    3,649
    State:
    O.P., KS
    The Atwood 90 bulb draws 7.5 watts, which is .625 amps in a 12 volt environment. Two of them would be about 1.25 amps. But I would probably go with a 5 amp fuse rather than a 2 amp.

    3 and 4 amp fuses would be fine, but seem pretty scarce, and I've noticed that 5 amp is pretty standard even for low draw applications such as automotive interior lights. The goal in this application is to have something that will blow rather than start a fire, and it doesn't take much to blow a 5 amp fuse.

    For sensitive electronics such as a depthfinder it would be prudent to go with the manufacturer recommendation, as the goal is also to protect sensitive electronics, and the manufacturer will generally specify the lowest workable size. Navigation lights are a lot more forgiving.
     
  8. crazy

    crazy New Member

    Messages:
    2,090
    State:
    Kansas CIty, MO
    Ahh who needs a fuse just stick a wire in there. Really though my fuse board has all 5 amp fuse's.
     
  9. olefin

    olefin New Member

    Messages:
    3,908
    State:
    Texas
    All three pumps on mine are 3 amp. Sometimes I accidently drop a minnow on the pump side and it blows the fuse while grinding the minnow.
     
  10. Duckpoor

    Duckpoor New Member

    Messages:
    184
    State:
    Illinois
    Properly fused DC circuits are always a debate .. and were finally explained to me so I could digest it.
    Fused circuits are either designed to protect the device down stream or protect the feed side of the wiring.

    Little of what we "plug" into a 12 volt system in a boat, is in need of tender loving care .
    What we are protecting from is the gross failure that might translate into fire or damage the delivery circuit.
    Anything fused at the "device" Generally requires additional protection... almost every thing else is and should ultimately be Source protected from a "To Ground" type failue.

    Maybe over simplified.. but just trying to share what we are trying to accomplish.
     
  11. smhmc6

    smhmc6 Member

    Messages:
    744
    State:
    Kansas
    I think I asked the same question a while back... or one like it. Someone reminded me that Power=Voltage*current. Sometimes the things just tell you what wattage they are so this equation may be more convenient. So current (amps)=Power(watts)/Voltage(12v). Hope this helps.