ksanders
Moderator Emeritus
Here’s how 10% voltage drop works in a 12 volt system. These are the rules of thumb that I go by.
I generally do not have to worry about wire run length because my boat has a centralized main breaker panel, and sub panels in reasonable places.
Breaker size wire size max run
20A. 12 AWG. 37’
30A. 10 AWG. 40’
40A. 8 AWG. 47’ (I AMOST NEVER RUN 8 AWG)
50A. 6 AWG. 60’
The maximum run above is the round trip 10% voltage drop wire length. If you look at almost every piece of DC gear on a boat it will be manufacturer rated for 10.5 volts to 14.5 volts.
Thesae are the exact same rules of thumb that we use in land based systems. The only difference being the max run is in practical terms never an issue in a land based system.
BTW, the OP figured out his original problem. He had a 10 amp breaker installed for a motor load of 16 amps.
Even though I never worry about the exact voltage drop calculation it’s easy math. In his case 16 amps is his current. 10% voltage drop is 1.2 volts. 1.2/16 =.075 ohms maximum resistance. 12 AWG wire is .0015888 ohms per foot. .075/.0015888= 47’ round trip wire run.
My money says his actual wire run is not longer than 47’ making 12AWG wire appropriate for his application.
I generally do not have to worry about wire run length because my boat has a centralized main breaker panel, and sub panels in reasonable places.
Breaker size wire size max run
20A. 12 AWG. 37’
30A. 10 AWG. 40’
40A. 8 AWG. 47’ (I AMOST NEVER RUN 8 AWG)
50A. 6 AWG. 60’
The maximum run above is the round trip 10% voltage drop wire length. If you look at almost every piece of DC gear on a boat it will be manufacturer rated for 10.5 volts to 14.5 volts.
Thesae are the exact same rules of thumb that we use in land based systems. The only difference being the max run is in practical terms never an issue in a land based system.
BTW, the OP figured out his original problem. He had a 10 amp breaker installed for a motor load of 16 amps.
Even though I never worry about the exact voltage drop calculation it’s easy math. In his case 16 amps is his current. 10% voltage drop is 1.2 volts. 1.2/16 =.075 ohms maximum resistance. 12 AWG wire is .0015888 ohms per foot. .075/.0015888= 47’ round trip wire run.
My money says his actual wire run is not longer than 47’ making 12AWG wire appropriate for his application.