I am completely rewiring my Pearson 323. Using Casey's formula for wire size if, I use 1 gauge battery cable I will have a 3.3% voltage drop in the primary starter circuit and a 3.9% drop in the secondary/emergency circuit. Using 0 gauge the numbers are 2.6% and 3.1%. It seems that for this type of circuit the voltage drop is not as critical and the 1 gauge would be fine. Is this correct or should I go to the 0 gauge? There is a significant difference in expense but also in the ease of working with the larger wire. Thanks. Geoff Kloster
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Just to be sure....
make sure you are doubling the distance from battery to starter when doing your V drop calculation.( i.e a 5 foot cable run=10 feet total in the full circuit for your calculation.)
make sure you are doubling the distance from battery to starter when doing your V drop calculation.( i.e a 5 foot cable run=10 feet total in the full circuit for your calculation.)
Starting current
I am curious to know what your starting current is that you based your wire percentage drop on. And where did you get the starting current from? A 3.3 percent drop in voltage for a 1 gauge from 12V is 396mV. Resistance for 1 gauge is 0.000127 ohms per foot. Using your number of 396mV and the resistance of the wire, a quick Ohms Law sanity check is: 10ft = 311 Amps 15ft = 207 Amps 20ft = 155 Amps For a typical installation, 15 - 20 ft is typical because you have to add the TOTAL length of wire = positive and negative. You should use the shortest path(starting battery should be the closest of the two batteries) and the thickest wire you can manage. Make sure you use Marine wire - it's only difference is the strands of wire are tinned.
I am curious to know what your starting current is that you based your wire percentage drop on. And where did you get the starting current from? A 3.3 percent drop in voltage for a 1 gauge from 12V is 396mV. Resistance for 1 gauge is 0.000127 ohms per foot. Using your number of 396mV and the resistance of the wire, a quick Ohms Law sanity check is: 10ft = 311 Amps 15ft = 207 Amps 20ft = 155 Amps For a typical installation, 15 - 20 ft is typical because you have to add the TOTAL length of wire = positive and negative. You should use the shortest path(starting battery should be the closest of the two batteries) and the thickest wire you can manage. Make sure you use Marine wire - it's only difference is the strands of wire are tinned.
Thanks, but...
Ross, I used 12v to determine voltage drop. I'm not sure what you mean by using a dead battery to determine if the engine will start. No matter; I don't have a battery. The boat is in my back yard getting a complete refit. When it's ready for launch I'll buy the batteries. camaraderie, I did double the lengths, though I forgot the first few times I went through the calculations. Quite a shock to realize my relatively inexpensive cable, when the length doubled, now was very expensive! Guy, the primary route, battery to switch to starter to battery is 22'. The secondary/emergency route house battery to switch to starter to house battery is 26.5'. The engine is an MD11c that should draw 140 amps. The starting battery is close to the engine but there was no place for the main switch panel close to the engine; thus the long run. but... my question; How much voltage drop is acceptable for a starter motor without compromising it's function significantly? Will it turn over the engine adequately at 11.9v? 11.8v? 11.5v? Using 12v, the 4% drop in the secondary circuit will give me 11.52v. 12v is 75% discharge according to East Penn's data. (A level of discharge I hope to avoid.) A 4% drop (secondary circuit, 1g cable) then gives me 11.52v. Will that be OK? Sorry to sound so picky but if I get the boat in the water and find I need to do it over there will be a lot more work than doing it right now. Thanks much for your help. Geoff
Ross, I used 12v to determine voltage drop. I'm not sure what you mean by using a dead battery to determine if the engine will start. No matter; I don't have a battery. The boat is in my back yard getting a complete refit. When it's ready for launch I'll buy the batteries. camaraderie, I did double the lengths, though I forgot the first few times I went through the calculations. Quite a shock to realize my relatively inexpensive cable, when the length doubled, now was very expensive! Guy, the primary route, battery to switch to starter to battery is 22'. The secondary/emergency route house battery to switch to starter to house battery is 26.5'. The engine is an MD11c that should draw 140 amps. The starting battery is close to the engine but there was no place for the main switch panel close to the engine; thus the long run. but... my question; How much voltage drop is acceptable for a starter motor without compromising it's function significantly? Will it turn over the engine adequately at 11.9v? 11.8v? 11.5v? Using 12v, the 4% drop in the secondary circuit will give me 11.52v. 12v is 75% discharge according to East Penn's data. (A level of discharge I hope to avoid.) A 4% drop (secondary circuit, 1g cable) then gives me 11.52v. Will that be OK? Sorry to sound so picky but if I get the boat in the water and find I need to do it over there will be a lot more work than doing it right now. Thanks much for your help. Geoff
Geoff, You have answered your question concerning
a dead battery. If you only have 11.5 volts "no-load" and try to start the engine will you get enough juice to turn the engine or just get clicks? I think your 140 amp starter draw is high. That is a 2 hp starter. On my 2001 volvo (1 cylinder 10 hp) it is only a 1/2 hp starter.
a dead battery. If you only have 11.5 volts "no-load" and try to start the engine will you get enough juice to turn the engine or just get clicks? I think your 140 amp starter draw is high. That is a 2 hp starter. On my 2001 volvo (1 cylinder 10 hp) it is only a 1/2 hp starter.
Some more answers Geoff
Starters are performance rated at a low voltage of 10VDC. So, as long as you have 9.5 to 10 VDC at the starter, it will work fine. Your 1 Gauge wire will work just fine. When you initially engage a starter like mine that is rated 1Hp(62Amps @ 12VDC) The current draw can be several hundred amps to overcome the locked rotor current and the initial force to spin the engine. So, just because it is rated at 1 Hp does not mean that it will only draw 62 Amps. To error on caution is good. It will give more starting power. Also, you could split up the wire gauges. Make the positive wire lighter than the negative wire. Just match your total percentage for the mixed wire combination. This may be a good way to getting all those positive wires into the selector switch. You may want to ensure that all the wires and lugs can fit into the space dedicated for the switch. Larger wires can be problematic with bending radius. The other alternative is to run parallel wires and crimp them into one lug. Two 4 Gauge wires in parallel is the same as One 1 Gauge. Good luck.
Starters are performance rated at a low voltage of 10VDC. So, as long as you have 9.5 to 10 VDC at the starter, it will work fine. Your 1 Gauge wire will work just fine. When you initially engage a starter like mine that is rated 1Hp(62Amps @ 12VDC) The current draw can be several hundred amps to overcome the locked rotor current and the initial force to spin the engine. So, just because it is rated at 1 Hp does not mean that it will only draw 62 Amps. To error on caution is good. It will give more starting power. Also, you could split up the wire gauges. Make the positive wire lighter than the negative wire. Just match your total percentage for the mixed wire combination. This may be a good way to getting all those positive wires into the selector switch. You may want to ensure that all the wires and lugs can fit into the space dedicated for the switch. Larger wires can be problematic with bending radius. The other alternative is to run parallel wires and crimp them into one lug. Two 4 Gauge wires in parallel is the same as One 1 Gauge. Good luck.
How long is the roundtrip from battery
to engine? Stick to the recommended gauge for the length required.
to engine? Stick to the recommended gauge for the length required.
I added a battery 4' from the starter
and it works great I have a #4 run to the starter and added a batery 4' awaY. Now i can have my house bank under my seat . I have 0 and #1 wire from my ALT 110 amp and the inverter charger.
and it works great I have a #4 run to the starter and added a batery 4' awaY. Now i can have my house bank under my seat . I have 0 and #1 wire from my ALT 110 amp and the inverter charger.
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