Electrical systems are always figured the same, even in your house. Simply put, the first statement about this in the National Electrical Code is that the supply MUST meet the demand. Then they de-rate ampacities to give a 20% safety factor in each branch circuit. Your car is no different except that car wiring is not hidden in walls, the environment vibrates and it is exposed to heat and harsh chemicals.
Start by adding all your accessories. (Everyone's car is different.) Consider battery losses when cold and 'sitting for days or weeks', trailer hauling, inverters, stereo systems, headlight upgrades, etc. Now look to the future and always consider this question, 'what do modern cars use?' If your system falls short you will be buying a bigger alternator very soon.
Never depend on the battery to deliver power that the alt should have produced. I tried running a 75-amp alt with an electric fan. During the Woodward Dream Cruise our cars are in a big parking lot for miles, all day long. It ends up where the electric fan cools the engine but the alt is too small to charge the battery AND run the fan at idle. So, the battery drains and I'm looking to get off Woodward so I can run the car at 40-mph to shed heat and charge the battery. Now, I have a 130-amp alt and all is very well.
Also consider the progression:
1955 T-birds had a 30-amp 6-volt gen that output 180 watts.
1956 T-birds had a 30-amp 12-volt gen that output 360 watts. (doubled the year before)
Alternators appeared, producing much more power at idle and low engine speeds.
1980's saw 75-amp alternators
1990's Fords came with 100 amps and now 130 amps is very common. Some alternators output 200 amps (200 X 12-v = 2,400 watts, a far cry from the original 180 watts).
I've never heard anyone say, '****, I wish I had bought a smaller alternator.'
Also consider the horsepower your new alt will need... 746 watts = 1 hp. 2,400 divided by 746 = 3.2 HP. <--this needs enough belt surface area or it will slip and scream. That's why new cars come with that wide serpentine belt with so many ribs. You can do the same with a double-groove pulley or a slightly larger diameter pulley. All the old pulleys (Ford and GM are interchangeable) fit new alternators if you use the spacer behind the pulley (it looks like a ring) then tighten the nut and lock washer. - Dave
Start by adding all your accessories. (Everyone's car is different.) Consider battery losses when cold and 'sitting for days or weeks', trailer hauling, inverters, stereo systems, headlight upgrades, etc. Now look to the future and always consider this question, 'what do modern cars use?' If your system falls short you will be buying a bigger alternator very soon.
Never depend on the battery to deliver power that the alt should have produced. I tried running a 75-amp alt with an electric fan. During the Woodward Dream Cruise our cars are in a big parking lot for miles, all day long. It ends up where the electric fan cools the engine but the alt is too small to charge the battery AND run the fan at idle. So, the battery drains and I'm looking to get off Woodward so I can run the car at 40-mph to shed heat and charge the battery. Now, I have a 130-amp alt and all is very well.
Also consider the progression:
1955 T-birds had a 30-amp 6-volt gen that output 180 watts.
1956 T-birds had a 30-amp 12-volt gen that output 360 watts. (doubled the year before)
Alternators appeared, producing much more power at idle and low engine speeds.
1980's saw 75-amp alternators
1990's Fords came with 100 amps and now 130 amps is very common. Some alternators output 200 amps (200 X 12-v = 2,400 watts, a far cry from the original 180 watts).
I've never heard anyone say, '****, I wish I had bought a smaller alternator.'
Also consider the horsepower your new alt will need... 746 watts = 1 hp. 2,400 divided by 746 = 3.2 HP. <--this needs enough belt surface area or it will slip and scream. That's why new cars come with that wide serpentine belt with so many ribs. You can do the same with a double-groove pulley or a slightly larger diameter pulley. All the old pulleys (Ford and GM are interchangeable) fit new alternators if you use the spacer behind the pulley (it looks like a ring) then tighten the nut and lock washer. - Dave
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