Need help on sizing cable runs

Discussion in 'Electrical Systems' started by bntii, Sep 1, 2006.

  1. bntii
    Joined: Jun 2006
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    bntii Senior Member

    Just about to start a rewire job.

    Have a good understanding of awg sizing for cable runs but would like to know have the primary cables are sized.

    I have a cable run of about 12' (one way), from the batteries to the positive bus at the panel.

    Is there a convention for how many of the loads at the panel are used to calculate the primary cable runs?

    Should one use the total capacity of all the breakers, or a total load of all individual circuits? Perhaps an intelligent guess of how many things one might run at the same time?

    Vessel is cruising sailboat of 40' with normal loads- including refrigeration, radar, autopilot, pumps, etc

    Thanks
     
  2. Tim B
    Joined: Jan 2003
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    Tim B Senior Member

    Probably a good idea to take the total load of all the breakers, then you have a reserve factor built in and it will save the wires warming up.

    Engine power is usually dealt with on a separate panel, and, on a 40 footer I would suggest local power relays near the engine.

    Tim B.
     
  3. bntii
    Joined: Jun 2006
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    bntii Senior Member

    I take it I should total all loads in this case. I will need to find another voltage drop table as the one I have just shows load to 100 amps.

    Additional question- Seems odd to sweat the voltage drop in the system and be forced to place an inline fuse on a cigar sized wire. My mighty 2cm of copper drops to 2mm across the fuse :(
    What do the electrical engineers think about the resistance cost of placing this necessary circuit protection in place? Is there a better option? A high load breaker?

    T
     
  4. Tim B
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    Tim B Senior Member

    I wouldn't have a fuse or breaker between the panel and battery. Do you really want to lose the VHF when a NAV-light blows? Also, consider running things like the Anchor Windlass and Winches etc. off a completely separate circuit. Possibly from a separate battery.

    Lighting is about 10 - 15 Amps. VHF is about 5 Amps, GPS 2 Amps, Pumps, wipers etc. 15 Amps. Blowers 10 Amps

    That's 47 Amps from the battery just off the top of my head. That is why it is so important to keep the cable runs short.

    Tim B.
     
  5. bntii
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    bntii Senior Member

    delete
     

  6. bntii
    Joined: Jun 2006
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    Location: MD

    bntii Senior Member

    Ok-

    Boy am I dumb. I opened the ABYC book sitting beside my desk.

    To calculate panel loads, section 9.9.1 states that:

    1)List the current rating (amps) of the loads that must be available for use on a continuous duty basis for normal operations
    2)List the current rating(amps) of the remaining loads that are intermittent, and total these loads. Take 10% of the intermittent loads and add to 100% of the continuous duty loads to establish total electrical load.
     
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