wire sizing

Solar Expert Posts: 57 ✭✭
Hi I have been searching the forum for the answer to my question but I am still confused. I just received my crown 6 volt 245 amp batteries. I have a total of 4. Will wire series parallel. What size wire do I need to use for the connections? Right now they are going on the shelf for a float charge until I can install them this spring. Thanks for your help.
Craig

• Solar Expert Posts: 48
Re: wire sizing

Are you going to run a 12v or 24v system

If you are running 12v. wire the 2 sets in series then parallel them together. If you are running 24v, they will all need to be in series.
As for wire size, that also depends on how many amps you are using and over how long of a run. How many amps/volts need to travel over the wire and how far?
Re: wire sizing

Assuming 12 volt (series parallel), you would need only about 1% rate of charge for "floating" the battery bank.
• 490 AH * 1% = 4.9 amps minimum float charger rating
If you use 14 awg wiring and want a maximum of 0.1 volt drop (float drop voltage is less critical vs trying to charge a battery bank quickly), using a generic voltage drop calculator:
• 12 awg @ 4.9 amps and 5 feet (one way) run of wire -> 0.1 volt drop
I would not worry if you even had 0.3 volts or more of drop with float charging...

Check the battery manual and see what they recommend for your battery bank (usually around 13.2 to 13.6 volts or so).

If the float charger appears to recharge at too high of voltage (voltage cannot be set), get a heavy duty lamp timer and set it for 1-8 hours a day or so (watch battery resting voltage, amount of bubbling, and water use).

Even a 1 amp charger can eventually boil a battery dry over time if it floats the batteries at too high of voltage.

If you were going to do this with solar panels and a small charger controller:
• 4.9 amps * 17.5 Vmp * 1/0.77 derating = 111 Watt panel (roughly)
You should probably also look at getting a good quality hydrometer (hydrometers with internal thermometer/float--may not be the reliable) and double check that all of the cells are fully and equally charged.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 57 ✭✭
Re: wire sizing

12 volt system. Per battery mfg, float is at 13.5 volts. Charger specs say float output is 13.2 volts. I'll add another question. When I end up putting this all together, I will be using an cotek 1000 watt pure sine inverter. What size wiring then? 2/0 from the battery bank t the inverter? I have "fooled around with the voltage drop calculator and I believe I have a good idea of what I need from the panels to the charge controller. Not quite sure all the placement of fuses and disconnects but I'm working at it. I hope to be able to get things set up some time this summer. Thanks again.
Re: wire sizing

Using some generic numbers:
• 1,000 watts * 1/0.85 inverter eff * 1/10.5 volts cutoff * 1.25 NEC safety factor = 140 amp minimum wiring/fusing
Using a generic voltage drop calculator:
• 5' one way trip cable, 1 volt max drop, 140 amps load, 2/O cable -> 0.13 volt drop
So, your cable is way over kill (based on my above assumptions). But very nice.

The minimum cable would probably be:
• 5' one way trip cable, 1 volt max drop, 140 amps load -> 6 awg @ 0.7 volt drop
Using the ABYC chart, you should probably use 2 awg minimum based on current requirements (the more conservative NEC would even be larger wire).

Remember that fuses and switches will add voltage drop too--12 volt battery bank does not leave a lot of headroom for voltage drop (11.5 volt minimum battery voltage--charged under load, 10.5 voltage cutoff for most inverters). Add a surge current (typically a good inverter will surge 2x rated voltage)--You don't want the inverter to shut down when trying to start a heavy load/pump/motor.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 48
Re: wire sizing

Wire can never be to big.. only to small!
Re: wire sizing
shift wrote: »
Wire can never be too big.. only too small!

Technically, that is not always true... For example, say you have a breaker rated for 10,000 AIC (amps interrupt current) and a really large power source (a 10,000 Amp pole transformer or a large DC Battery bank).

If you have a dead short and too heavy/short of cabling, it is possible for the power source to exceed the AIC rating of the protective device. For example, commercial power users in (at least) Northern California that have a very short run from the pole/pad mounted transformer to the main disconnect cannot use standard main panel breakers. They have to use special high AIC fuses instead. If the main panel is more than XX feet from the transformer, then they are able to use standard rated breakers because of the additional resistance/impedance of the longer drop.

-Bill "generalizations are always wrong" B. :roll::p:D
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: wire sizing

i'm going to remind you that the series battery interconnects add to the run going to the inverter. i concur the 5ft run is good for #00, but if you have a 1ft intertie on the batteries this means the run to the inverter is now limited to a 4.5ft run as a 4.5ft run consists of 2 wires both 4.5ft long adding up to 9ft total to be added with the 1ft battery intertie for a total of 10ft of wire and that is a 5ft run. if the wire lengths are longer, or as was mentioned with fuses/breakers or even a switch, this adds extra resistance making a thicker wire necessary.