another wire sizing question.

Re: another wire sizing question.

There are several reasons why wire sizes are what they are...

First, there is short circuit current--which you size the wire and fuse/breaker. And there are different ways to size it based on where it is installed, type if insulation, etc...

For example. 14 awg wire is normally rated for 15-20 amps maximum/breaker in the NEC. In the table you pointed to--it is rated for 32 amps in "chassis wiring" and for 5.9 amps when used for power transmission.

NEC uses 15 amps as a maximum current for what they consider safe in home/office wiring in conduit of a conduit in a particular setting, operating temperature and specific type of insulation. The Power Stream table makes other assumptions that the wire inside a chassis--I guess with circulating air--can carry as much as 32 amps before insulation will melt. And 5.9 amps if you are going to send the power a long distance (allowing for copper resistance drop)--but no real reason given.

So--for your case, the manufacturer assumes 8 awg cable and a short distance will carry the maximum current safely for short distances at 73 amps. 100 amps is a lot of current and the cables will probably get hot--but the charger may limit the time it will run at 100 amps (like starting a car or "boost" starting current for 15 seconds or something like that--just guessing).

But, there is the other thing we are looking at -- resistance and voltage drop. 10' cable run, x2 (there and back) is 20' of cable. If you use 8 awg at room temperature and 45 amps, you will get around 0.581 volts drop...

• P-I*V=45 amps * 14.2 volts = 639 watts from charger
• P=I*V=45 amps * 0.581 volts = 26 watts lost

If you are using solar power to charge your batteries--that is 26 watts that is going into heating your wiring instead of charging your battery bank--at $5 a watt for a smaller solar panel--that is $130 in wasted solar panel every day.

Normally, we try to recommend that you limit losses to ~3% or less--and many people try to aim near the 1.5% or less in losses.

But, the power loss in charging the battery (and heating the cables) is not really the limiting issue in this case--Notice that you will have almost a 0.6 volt drop in your cables. The battery charger will see the battery voltage as 0.6 volts higher than it really is.

You set the charger to 14.2 volts for charging and the battery sees 13.6--a nice easy float voltage instead of a fast charging one--and so you will probably get a lot less than 45 amps through to the battery bank unless the charger voltage set-point is jacked up 0.6 volts or so.

In computers--we use sense leads (a separate set of volt meter wires that connect to the load)--to allow the charge controller to measure the "real battery voltage" instead of battery+lead drop voltage. Unfortinatually, I have not seen many solar charge controllers that use sense leads.

Anyway, the longer your wires, the heavier they will need to be at high currents to reduce heat loss and ensure your battery is quickly charged. And why it is recommended that the charger to battery connections be as short and heavy as practical to limit these losses and errors. Short heavy cables are also recommended for inverters... And why sending low voltage DC (like 12 volts) any great lengths is so difficult.

Does that sort of make sense?

-Bill