Do wire gauges have to be uniform?

[JRRNeiklot]

So, if you read my other thread, I'm in need of a new battery bank.  I have 3 100 ah batteries on order.  I'm wondering what size wire to use.  My old system used 1/0 wire.  I'm wondering if I should go a bit higher.  The inverter cables are about 6 feet long.  Those will be expensive to replace.  If I jump up to 2/0 for the wire between the batteries, and leave the 1/0 cable to the inverter, will that cause any issues, assuming I don't push more amps than the 1/0 can tolerate?  

Thanks.

 

[B and C]

The bigger the wire the less the resistance. Your idea to place the larger cables on the batteries is sound. Be sure to wire the loads and charging sources by connecting positive to one end of the string and negative to the other end of the string. The other option on battery connection is buss bars where you take three equal length positive cables and connect each battery to the buss bar and do the same for the negative connections. These two arrangements allow the real load to be spread across all batteries rather than pulling from an end (or middle) battery where the other batteries try to recharge the depleted battery.

 

[Weight]

The interconnecting cables, if using paired 6 volt, and the negative and positive cables to the bus bars are 2/0 on my builds. The cables from the bus bar can be whatever is safe for the amperes and least voltage drop to major devices. I have fuses at positive battery terminals, and on the positive bus bar where smaller wires connect.

 

[Trebor English]

How big is the inverter?  What is the max load you ever put on it?  Max watts divided by 10 is approximate inverter input amps.  That's what sets your wire size requirement. 

I have a 400 watt rated inverter so I should accommodate 40 amps.  My cheap inverter actually has the output voltage drooping, 108 volts, at 350 watts.  My jigsaw that I haven't used in years takes about 250 watts.  So depending on who you listen to I MUST have a 40 amp supply, or maybe 35, or 25 amps.  In my opinion your fuse size should be appropriate for the wire size you actually have. 

The more zeroes in your wire size the better your bragging rights.

 

[JRRNeiklot]

The question is not how many amps my inverter cables can handle, they are adequate. I just want to know if having different size wire gauges from battery to inverter than the cables interconnecting batteries will cause any problems.

 

[B and C]

No. Your cables going to lights will be much smaller yet. You mix gauges all over. Make sure you fuse the battery end to protect the wire.

 

[JRRNeiklot]

Thanks. Everything is already in place, I'm just replacing the batteries. Thought I'd upgrade some cables, but the 6 feet inverter cables would be very expensive.

 

[maki2]

there are online calculators for this type of thing. You enter the length of the wire and the amps and it spits out the cable gauge you need.

 

[JRRNeiklot]

A calculator will not answer my question.

 

[Trebor English]

The question is not how many amps my inverter cables can handle, they are adequate.  I just want to know if having different size wire gauges from battery to inverter than the cables interconnecting batteries will cause any problems.

The cables connecting the batteries will have to carry the inverter current.  That's how big they need to be.  If some wire is bigger then farther down the circuit some is smaller there will be no problem with the flow of electricity at the point of size reduction like a kink in a hose.  

All 3 batteries should be wired with the same total resistance while charging and while discharging to make them participate equally.  If all 4 battery interconnecting cables are the same size and length you can have the same resistance for each battery.

 

[Weight]

This type of calculator; https://www.genuinedealz.com/voltage-drop-calculator While there check the prices on custom battery cables as well as marine grade wire. Yes you can go from larger to smaller wire. You must fuse for the wire size where it changes.

 

[Spaceman Spiff]

 . . . If I jump up to 2/0 for the wire between the batteries, and leave the 1/0 cable to the inverter, will that cause any issues, assuming I don't push more amps than the 1/0 can tolerate?

The simple answer to your question is: no you will not cause any issues.

In general any circuit with mixed wire sizes must meet these requirements:
 - the smallest wire in a circuit must be able to carry the maximum amperage of the device(s) connected to it.
 - the voltage drop in the circuit must be less than the minimum voltage to run the device(s) connected to it.
 - the fuse must be sized to protect the smallest wire in the circuit.

 

[Weight]

what Spiff said.

 

[LoveCareThinkDo]

The question is not how many amps my inverter cables can handle, they are adequate.  I just want to know if having different size wire gauges from battery to inverter than the cables interconnecting batteries will cause any problems.

Nope, you are good. The only time you will run into trouble is if you extend a fatter cable with a skinnier cable and think the fat cable will "take some of the load off the skinnier one." Nope, all the amps gotta go through both cables. 

But you aren't "extending" a single cable. You are merely using an appropriately sized cable to go between the batteries and a separate, appropriately sized cable to go to the inverter. That is exactly how it should be.