Convert 120V to 12V

mtofell1 wrote:
So, if wiring off the battery I'm seeing wire size recommendations from 8 to 14 gauge in the last handful of posts. Online I found some voltage drop tables but they talk about different % (3% table, 10% table, etc.). Does anyone have an opinion as to what table is used for the motor in this macerator? There seems to be a pretty big difference depending what table is used.

Something I did not know from a lack of DC wiring knowledge is the drastic wire size increase necessary when traveling distance. In houses it rarely is an issue unless pulling a feed to an outbuilding. Sometimes, with a very large house you might put a second panel at an opposite end. But I've often suspected the 2nd panel is as much about convenience for the electricians as it is voltage drop due to long runs.

It's not an AC vs DC thing.

It's an amperage thing. The higher the amperage, the more losses. Since 12v is 1/10th of 120v, for the same power, you are pulling 10 times the amperage. This results in much shorter runs before voltage drop becomes an issue and you have to up-size the cable.

This is a big part of why many other countries run 240v household AC. It allows smaller cheaper wires to be used. 120v or 240v, if you lick the live ends, it's not a good thing, so not much safety difference. Only reason the USA doesn't switch is everything is set up for 120v.

Remember to take the full round trip when using the tables to calculate the length (If you coil up 50' of cable in the circuit, that counts towards the length)

I wouldn't use the 10% table. If you are fully charged it can work OK but if the battery is down, it tends to compound. Say you use 10AWG copper.
- 12.7v (fully charged battery) @ 20 amps = 254w with 9.45% voltage drop with 11.5v at the motor. The motor will run a little warm but likely will survive if you don't use it a lot.
- 12.0v (45% charged) with 10% voltage drop results in 24amps and 12% drop end result 10.56v at the motor and that will be hard on the motor and being weaker, may result in greater likelihood of jamming on solids.

The site below has a nice explaination:
https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity


Edit: My calculations assumed 30' roundtrip, so 15' from battery to motor and I see you just posted 20' w/ 12AWG. That should result in around 12.5% voltage drop or 10.48v at the motor if the battery is at 12.0v. Bump up to 8AWG and that gets you 5% drop or 11.4v at the motor.

I would jump up at least to 8AWG and might even consider 6AWG but that does get pretty big and costly.