battery cable sizing

[HOTPURSUIT]

Dunno how that old kicker motor post got restarted didn't notice the date until after I put the energy into the post, so I thought I would repost this info for anyone who may need to size battery cables

To calculate wire size (circular Mils) it is based on starter amps plus the total length of wire from battery to connection point (Length of positive plus negative cable). This example is based on 225 starting amps with a total cable length of 15'

CM (circular mils) = 10.75 (constant) x 225 (amps) x 15 (total length of run) / .36 ( 3% voltage drop max)

CM = 10.75 x 225 x 15 = 36,281

CM = 36,281 / .36 = 100,780

CM = 100,780 battery cable needed is 1/0 @ 105,556 circular mils




Listed below is a chart showing the circular mils in the comonly used wire sizes:

wire size / circular mils
#18 awg 1623.74
#16 awg 2582.26
#14 awg 4106.63
#12 awg 6530.86
#10 awg 10,386.15
#8 awg 16,502.38
#6 awg 26,244.07
#4 awg 41,736.48
#3 awg 52,656.81
#2 awg 66,374.38
#1 awg 83,665.49
#1/0 awg 105,556.53
#2/0 awg 133,054.95
#3/0 awg 167,868.70
#4/0 awg 211,600

 

[Mike S]

HP - I have always calculated the length of run by measuring from the battery to the load and back to ground. In an example - 15' from battery to load is 30' when the return distance is included. I then go up one size to minimize voltage drop (resistance).

 

[DaleH]

For a free download Wire Sizer program, see:
viewtopic.php?t=350

It does the calculation for you and the run is determined by the length from the source to the load BOTH WAYS, plus the ampacity of the load needed. It also specs the wire based on desired voltage drop, i.e., 3% for critical items (safety or navigation) and 10% for 'house' items, cabin lights, etc.

 

[HOTPURSUIT]

Yes you are correct I was not clear enough. total cable length of positive and negative cable combined. Voltage drop is included in this calculation. Thank you Mike

 

[Captain Frosty]

What is 10.75 ? - Is it used for everyones problem - Or I just dont understand . I want to add a second battery in my 1801. Put it under the other stow away seat . There should be just a simple answer here for this because it's the only sensible place to put it . I imagine the bigger parkers have their set spots as well . So for an 1801 you need this much wire /size etc -done . I know bigger boats are more complicated and thats where a lot of engineering needs to be exact - but the smaller ones should be pretty standard or maybe i'm way off . I've been told I dont need a second battery - but I feel a bit more secure with it .

 

[HOTPURSUIT]

First off I too feel more secure with two batteries, and have had that set up in every boat I have owned. One good example is when you are anchored and bottom fishing, you still may be using radios, fishfinders, raw water pumps, etc, that drain your battery. I always turn off one of the batteries and save it to make sure I get started and get home. As far as the battery calculations go the 10.75 is just a constant factor used in this calculation, that number never changes. If you are planning to add a second battery make sure you add a battery switch so that you can run 1,2, both or even shut them off when the boat is not in use. If you are adding a second battery and putting it next to the original battery there is no need for a calculation just use the same size wire as the factory install and you should be ok. A battery cable calculation and heavier cables would be absolutly necessary if say you were to relocate the batteries from the transon area and put them in the console another 5 or 6 feet away and the size of the boat would not matter. What you are trying to prevent is voltage drop. Good luck, Mike