Vic, et al,
Sizing wire for voltage drop so that the electronics function properly makes sense. Sizing wire for max current may be overly conservative and expensive. The Ohmic (Joule) heating of a wire has a time factor associated with it and it also depends not only on the material used for the wire (copper, aluminum, steel, etc) but on the wire construction as well (i.e. solid wire or stranded wire as well as the number of strands). Further, the resistance of the wire changes with temperature.
Often in automotive applications a wire may have a short term (transient) high current flow which drops quickly to a lower amount of current. This may only produce a minimal or acceptable amount of heating. This is part of the reason many battery jumper cables are made with only 6 gauge wire and they actually work without getting dangerously hot (generally one does not crank on a starter for more than 20 or 30 seconds - nor should one do so for longer than that before letting the starter and all the wires cool).
I recognize that every installation is different depending on the battery type(s) used as well as on the specific devices (ACR or Relay or connector, or etc) that are used and that it is probably easier to assume a worst case steady state current flow than to go through the calculations. I'm simply pointing out that doing so may result in unnecessarily oversizing the wire but at the same time there is no safety downside to doing that, only the cost and the inconvenience of dealing with larger gauge wire.
Standard automotive wire is rated for use between -40ºC to 80ºC (-40ºF to 176ºF) if PVC insulation is used or -40ºC to 125ºC (-40ºF to 257ºF) if crosslinked GXL, SXL or TXL insulation is used - Note that there are both hot and cold temperature limits. Depending on the location and use it may be perfectly acceptable for a wire to get hot to the touch and maybe as hot as 150ºF (66ºC) to 212ºF (100ºC) in regular use. I am not recommending anyone run wire that routinely gets that hot but as long as it it within the rated temperature range it should be safe.
The heating effect produced by an electric current, I through a conductor of resistance, R for a time, t is given by
H = I^2Rt
this is called Joule heating (or Ohmic heating).
Here is a chart showing ampacity for various wire sizes (temperature in the column titles are the max temperatures for that particular type of wire insulation):
EDIT 1: With apologies the chart at this link is for AC circuits I need to recover the link I found for the DC wire chart. In general, It is my understanding that wire gauge ampacity limits are determined by the maximum steady state current the wire can carry which produces a 30ºC temperature increase over ambient. As mentioned above there is a time factor associated with the heating so a short term higher current may not overheat the wire. I found a great site a year or two ago with the time dependent heating calculator for AC and DC wire and I posted that link to a comment to the WTW forum somewhere but I have not been able to find that link ror relocate the site so far.
EDIT 2: Here is a link to a calculator for constant current and temperature. There is another calculator like this that I have not yet relocated that includes a time (duration) of the current and resultant temperature rise.
The version at this next link is specifically for 12 v wire.
Stranded DC wire resistance chart:
Stranded Ancor brand wire specifications table for 18 to 4/0 (000) Awg wires (gives resistance, no of strands, etc):
Edit 3: Calculating Joule heating in a wire:
Here is a short time (transient current) Joule heating calculator (for solid wire I think) - this is close what I have been searching for (need one for stranded wire):
For example: Assuming 13v and using 6 awg wire at an initial temperature of 70ºF and allowing a maximum wire temperature of 125ºF and assuming an 80A short term current run through it, it will take 157 seconds (about 2-1/2 minutes for the wire to heat up to 125ºF. In 4 awg wire it would take 6.6 minutes.
Increasing the allowable maximum temperature to 150ºF to 6 awg wire would take 3.7 minutes and the 4 awg wire would take 9.4 minutes.
Dropping the current to 50 amps and allowing the wire to heat up to 150ºF 6 awg wire would heat up to that max temp in 9.5 minutes. At 30 amps it would take 26.4 minutes.
AGM Battery Bulk charging:
AGM charge current should be between 0.10 and 0.15 of the 20 hour rate AH rating. So a 100 AH battery should be charged at between 10 and 15 Amps. Using the short term current calculator (for solid wire) 6 awg wire can carry 20A for about an hour before heating up to 150ºF (4 awg 2.5 hours) and at 15A it takes about 1.5 hours for 6 awg and 4.5 hours for 4 awg to heat to 150ºF.
Assuming your 100 Ah battery is at 50% and bulk charging you need a bit less than 50 amps before the current flow drops off and the charging transitions to stage 2 (ref charging AGM or Lithium Ion batteries at the links below). At 15A per hour you would need about 3 hours of bulk charging time. 6 awg wire may get hot if your charger is flowing 15A instead of 10A. 4 awg wire should be OK.
Note: if you charge a battery with too high a charge voltage excessive current will flow into the battery, after reaching full charge, causing decomposition of water in the electrolyte and premature aging. At high rates of overcharge a battery will progressively heat up.
It is my understanding that recharging LiFePo4 batteries is also best done with modest current (on the order of 1 amp per cell).
While there may be momentary inrush current transients in our charging systems, as best as I understand high current draw (more than 15 amps) in our campers should not be a common occurrence nor last more than a minute or so (assuming 100 Ah battery size - a 200 Ah battery size will roughly double the charging current).
Indeed, if we see high current flow (say more than 20 to 40 amps depending on battery size) and lasting more than 10 seconds or so (when not cranking the engine) I think we should be suspicious of a short or some other electrical problem that would cause high current flow. If anyone knows any of the above to be untrue please comment and explain.
Bottom line: If you are running a 100Ah house battery and the one way distance from the charging source (assuming truck) is less than 25 feet and your charger allows a maximum current of 12A then a 6 awg wire is probably OK (do the calculations yourself or talk to an expert). If you are using a house battery larger than 100Ah, have a longer distance or a higher charging current then you should use 4 awg or larger wire (again do the calculations or rely on an expert for appropriate wire sizing).
I hope his information is helpful
Edited by ckent323, 28 March 2023 - 02:05 AM.