I have recently installed solar and dual bettery systems on two buses, refreshed my own installation and am starting to think like an electrical engineer. After some extensive research, I have produced some cable selection tables for my toolbox. These tables can be used for auto electrics, marine electrics and small solar power systems. Before presenting the tables, I discuss the data sources.
Ampacity (amps capacity) is the maximum continuous current an electrical cable can carry without the insulation melting. Some major factors that determine ampacity are:
- Cable size (small conductors have high resistance and become hot at low currents).
- Insulation temperature rating.
- Ambient temperature (heat dissipation from hot cables is slow at high ambient temperatures).
- AC or DC current.
Also beware that old cables with corroded conductors and weathered or perished insulation will not perform like new.
There can be large differences between ampacity data from different sources. The following plot compares five standards. These curves show that AC ratings tend to be lower than DC ratings (e.g. NEC versus ABYC). For DC applications, the JASO and ABYC ratings are similar and the ISO ratings are about 15% lower. I selected the ABYC data, which seems reliable and covers a broad range of cable sizes and conditions.
Voltage losses usually decide power cable size rather than ampacity. Resistance is largely determined by copper cross-sectional area. The following plot compares DC resistance data from five sources and the differences are not substantial. I selected the ABYC data, which covers a broad range of cable sizes.
Cable ratings table
I merged all data for common power cable sizes into one reference table below. For cables not in the ABYC E-11 table (e.g. ‘auto cables’), I estimated ratings from quadratic curves fitted to the ABYC data.
|Cable ratings chart sorted by wire size (white = IEC/ISO cables, yellow = AWG cables, grey = ‘Auto cables’). Ampacity for 30°C and 60°C (‘engine room) ambient temperatures. Most data are from ABYC E-11 found at Blue Sea Systems. Auto cable cross-sectional areas are from the manufacturer Tycab. Blue values are from quadratic interpolation of the ABYC data. Red values are extrapolated. I have also added a column for insulated crimp terminal selection.|
The most common insulation for copper power cables seems to be PVC, which is rated for 75°C conductor temperature (V-75). Usage up to 90°C (V-90) is limited. I selected ABYC data for 75°C insulation.
Ampacity should be derated for ‘engine room’ conditions (e.g. inside the engine bay of a vehicle). The ABYC data decrease ampacity by 25% at 60°C ambient temperature. Tycab recommend a larger derating, 40% at 60°C, perhaps because they provide ampacities for V-90 insulation.
When routing cable inside an engine room or engine bay, be careful not to run PVC insulated cables near exhausts, cyclinder heads, radiators and other hot parts > 75°C. PVC insulation will melt at high temperatures!
To use the above chart, one needs to identify the cable size and insulation:
- Sometimes, the insulation of a power cable is marked with the conductor area (mm2 or AWG) and temperature rating of the insulation (°C or maybe °F).
- When buying cable off the reel, a label on the reel should identify the conductor area (mm2 or AWG) and insulation type (e.g. V-75).
- For scrap pieces of cable, I have to judge whether the insulation is PVC or not and then estimate the copper area using a wire stripper. PVC insulation is ‘opaque’ (never clear), ‘firm’ (not soft), fairly tough (when cutting and stripping) and softens when heated lightly (but doesn’t burn, melt away or shrink by a large amount).
Beware that the size of ‘auto cables‘ refers to the total cable diameter, including the insulation. Plastic is cheaper than copper and the cross-sectional conductor area of auto cables may not be well standardised. For Australia, Electra auto cables and Tycab auto cables have the same cross-sectional area as in the above table. Avoid buying cable from auto parts stores. Better cable and better prices are found at electrical trade stores and sometimes on Ebay.
Also note that ‘gas cable’ is double-insulated cable for hazardous applications. It’s better than single insulated cable, but is slightly more expensive.
Voltage losses and cable selection charts
Resistance in a power circuit results in voltage losses and less power reaching the load (DC power = voltage × current). For example, the compressor in my Evakool fridge may have starting problems and runs slower when voltage is low.
Voltage loss is a function of current and resistance (Ohm’s law: volts = amps × ohms). Cable resistance is a function of cable length and cross-sectional area (ohms = ohms/m × m; see the cable ratings table above).
Below are some cable selection tables for both 12 V and 24 V systems. I calculated these tables in five steps:
- Calculate maximum cable resistance (ohms/km) from voltage drop, cable length and current.
- Calculate minimum copper conductor cross-sectional from cable resistance (using power function fitted to ABYC data, see above).
- Calculate ampacity for 75°C insulation and 60°C ambient temerature (using quadratic function fitted to ABYC data, see above).
- Upgrade results smaller than 0.5 mm2 to 0.5 mm2.
- Upgrade results with ampacity less than the current in Step 1 above.
These cable selection tables specify the minimum conductor area. An appropriate cable can then be selected by reference to the cable ratings table above.
|Cable selection chart for 12 V systems and 2% voltage loss (sensitive loads). Read minimum cross-sectional copper area (mm2) at the intersection of current (top row) and cable length (one of the two left-rows). For single core cables, it is assumed the chassis return has zero resistance.|
|Cable selection chart for 12 V systems and 4% voltage loss (normal loads). Equivalent to 24 V and 2% voltage loss (sensitive loads). Read minimum cross-sectional copper area (mm2) at the intersection of current (top row) and cable length (one of the two left-rows). For single core cables, it is assumed the chassis return has zero resistance.|
|Cable selection chart for 24 V systems and 4% voltage loss (normal loads). Read minimum cross-sectional copper area (mm2) at the intersection of current (top row) and cable length (one of the two left-rows). For single core cables, it is assumed the chassis return has zero resistance.|
Cable selection tips
When selecting electrical cables, biggest is not best. Bigger cables are more difficult to work with (e.g. routing, making connections), heavier and more costly. Use electrical engineering know-how and always select the thinnest cable that is fit for purpose. For example, you will not find many thick cables in OEM installations because they are not necessary for most applications.
When routing cables, shortest is not best. Leave a generous amount of extra length to allow for future work. Investing in a little extra cable is cheaper than having to replace a whole section of cable that is found to be too short or better than having to join two pieces of cable.
Exceed the ampacity and risk an electrical fire! Use appropriate fuses, especially for smaller cables with low current ratings.