Battery cable size

[tred]

I'm changing the battery in my 19 ft Bullet and am going with a 280 Ah lithium. The trailer currently has a single group 24 battery with 4 gauge battery cables that are about 3 feet long but I want to relocate the new battery from the tongue to the pass through storage area. I'll need to replace the cables with ones that are 6 to 8 ft long. Can I still go with 4 guage or should I bump up to 2 guage or...? I live in a rural area and the closest dealer has 4 gauge in stock but would have to order in 2 guage.

The trailer has one slide out, power stabilizers at each corner and a power tongue jack.

[Isaac-1]

Battery cable size depends on amp load, and cable run length, I think this is the best chart to give you guidance on this



https://afe.solutions/wp-content/upl...on_chartlg.jpg

[David 70]

The chart says you can do 100 amps at 10'. You should be good. The amps the battery can give makes no difference, it depends how much the motor is drawing.

[Persistent]

David's post above is correct.

Determine load.
Determine charging current.
Determine the distance.

Size cable to match the load and charging current.

A 6 foot cable requires a circuit breaker near the battery.

Both custom and stock cable is available on Amazon. Welding cable with proper connectors works best.

[oldenavy]

Your converter charging the battery is likely to be the largest current rating in your trailer. Using that current and the chart previously posted, you can determine your minimum cable size. If in any doubt, go up one size.

[tred]

Persistant - I like your idea about a circuit breaker. What size would you recommend?

[Argosy]

The circuit breaker protects the cable, use one sized for that rating.


Oldenavy had a good point, are you changing the charging, it could be greater than the draw. Where is it relative to the battery?

[Isaac-1]

Just to clarify here:


Cable size is determined by amp requirement with acceptable voltage drop


Fuse/Breaker size is determined by absolute amp capacity of a wire.


Meaning the cable size can increases with length at a give number of amps, but the fuse size stays the same for a given size cable regardless of the length.

[Persistent]

Originally Posted by tred

Persistant - I like your idea about a circuit breaker. What size would you recommend?

Isaac is correct.

For residential 120 volt AC wiring there is a national standard. There are two tables. The ampacity table lists the circuit breaker or fuse size for any given cable specs. There is another table that lists the cable to be used based on distance and maximum load. As Isaac said, it is to make sure appliances have enough power at the far end of the cable.

12 volt DC circuits do not have a national standard. The point is you do not want hot connectors or cables in the walls of a trailer you live and sleep in. You do want appliances at the end of the cable to work properly and efficiently.

Use the table listed above. The circuit breaker should be sized according the shortest listed length of wire. So use the top box for each column.

I use the ampacity table in the 120 volt National Standard. It has more than 100 years of validation behind it.

Example:
4 awg wire is rated to carry 90 or 100 amps at 6 feet. Choose a circuit breaker of no more than 100 amps to protect 4 awg wire.

For my personal safety, I tend to use a smaller breaker or a larger wire. Different 12 volt charts show different capacities for different purposes. I plan to be safe.

For battery charging I always choose a larger wire.

Some inverters need larger wire to prevent voltage drop that causes the inverter to disconnect. In this case length of wire and the number of connectors in the circuit matter as well.

First, determine the load the wire will carry.
Next, determine wire size to safely carry the load.
Next, choose a breaker or fuse that will not allow the wire or connectors to get hot. (ampacity)
Finally, up size the wire to get the power at the end of the wire high enough to meet the needs of the load.

The ultimate test is to put your hand on the wire and the connectors when the wire is carrying the maximum load. If it gets warm, you are loosing efficiency. If it gets hot, your safety may be at risk. (Yes if your hands are wet, or you are touching a ground, you may get a shock.)

[d23haynes57]

As part of this especially for DC wire attention needs to be paid for quality of the wire and the insulation including the temperature rating of the insulation. Wires do get hot. If the cabling is going inside sleeves or other spaces the heat gets trapped. Good wire will have temp ratings. A lot of the battery cables on Amazon are copper clad Aluminum. Also pay attention to the quality of the crimp connections. There are many sizes, types, plating's available.