Conductor size is based one or more of the following considerations:
Current carrying capacity [inducing a rise in wire temperature, in high-voltage lines], Short circuit current, or Voltage drop [long low voltage lines].

Current carrying capacity varies depending on the type of insulation, current and use.

American wire gage every 6 gauge decrease gives a doubling of the wire diameter, and every 3 gauge decrease doubles the wire cross sectional area.

The wire size is dependent on amperage not voltage. The NEC code is for household AC power wiring, the others are generally for DC, but they should be about the same.

Conductor Size (AWG)	Maximum Amperage									Diameter (in)		Resis- tance ohm/ 1000' *
	NEC prem- ises †	NEC General ‡		ARRL2		TFE insulated3		Chassis wiring1	Power Trans- mission 1	stranded	Solid
		Bun- dled 140°F insl.	Single 194°F insl.	Bun- dled	Single	Bun- dled	Single
0000 4/0		195	405			380		302			0.46	0.05
00 2/0	200	145	300			87	169	283	190		0.36	0.08
0		110	220			75	147	245	150	0.38	0.32	0.1
1	150	95	190					211	119	0.33	0.29	0.13
2		85	165			50	108	181	94	0.3	0.26	0.16
4	100	70	140			40	81	135	60	0.24	0.2	0.25
6	50	55	105			30	60	101	37	0.19	0.16	0.4
8	40	40	80			23	44	73	24	0.15	0.13	0.64
10	30	30	55			16	33	55	15	0.12	0.1	1.02
12	20	25	40	23	41	12	25	41	9.3	0.09	0.08	1.62
14	15	20	35	17	32	8	19	32	5.9	0.07	0.06	2.58
16		-	24	13	22	6	13	22	3.7	0.06	0.05	4.08
18		-	18	10	16	5	9.2	16	2.3	0.05	0.04	6.52
20				7.5	11	3.7	6.5	11	1.5	0.04	0.03	10.36
22				5	7	2.5	4.5	7	0.92		0.03	16.46
24						2	3.3	3.5	0.58		0.02	25.67
30						0.7	1.3	0.86	0.14		0.01	103.2

AWG - American Wire Gauge

† National Electrical Code [NEC Article 240.3]
Also National Fire Protection Association (NFPA 70)
- Cable sizing for premises wiring.
Wire must be two sizes larger (lower AWG) for Aluminum Wire.
‡ NEC Article 310 Conductors for General Wiring. (2008 Edition)
There two tables in this article one for bundled conductors and one for free air conductors. Each has three voltage ranges for insulation with different melting temperatures.
We used the low (60°C insulation bundled) and high (90°C insulation free air) values.
- Table 310.17 - Allowable Ampacities of Single-Insulated Conductors Rated 0 Through 2000 Volts in Free Air, Based on Ambient Air Temperature of 30°C (86°F)
- Table 310.16 - Bundled cables
Insulation temperature ratings:

• 60°C (140°F): Types TW, UF
• 75°C (167°F): Types RHW, THHW, THW, THWN, XHHW, ZW
• 90°C (194°F): Types TBS, SA, SIS, FEP, FEPB, MI, RHH, RHW-2, THHN, THHW, THW-2, THWN-2, USE-2, XHH, XHHW, XHHW-2, ZW-2

1 - Handbook of Electronic Tables and Formulas for American Wire Gauge, H.W. Sams and Co., 6th edition (March 1986)
The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very very conservative.
Chassis Wiring is meant for wiring in air, and not in a bundle.

2 - ARRL - Amateur Radio Relay League Handbook.
Based on max. wire temperature 100° C with ambient temperature 57° F

* Stranded wire

3 - MIL-STD-975 replaced by the NASA Parts Selection List. (NPSL)

The suggested MAXIMUM fuse rating is no MORE than 50% of the maximum current load a wire can handle.

Auto Charging Cable:

AMPS	Length ft
	≤7	7-10	10-13	13-16
85-105	6	4	2	2
105-125	4	4	2	2
125-150	2	2	2	0

American specs covering automotive wire are SAE J 1127 and J-1128, specifying resistance to heat and mechanical damage.
There are two basic types of insulation covered in these specs: Polyvinyl Chloride, (PVC ), and Cross-linked Polyethylene, (XPE).
GPT, TWP, and HDT wire insulations are all Polyvinyl Chloride (PVC) based insulations used in cooler parts of the car with a 185°F/85°C rating.
SXL, GXL, and TXL (thin, medium, and thick insulation) wire are Cross-linked Polyethylene, (XLP) and used under the hood. They have a 257°F/125°C temperature rating.
See Automotive WireTables

Anderson Power Connectors for battery cable at: solar-electric.com

Maximum current capacity depends on the melting temperature of the insulation.
TFE - Tetrafluorethylene.
Teflon TFE- Teflon a DuPont trademark.
PTFE - Polytetrafluoroethylene
ETFE - Ethylene Tetrafluoroethylene

	Insulation Type
Conductor Size	Polyethylene Neoprene Polyurethane Polyvinylchloride (Semi-Rigid)	Poly- propylene Polyethylene (High Density)	Polyvinyl- chloride PVC (Irradiated) Nylon	Polyethylene (Crosslinked) Thermoplastic Elastomer	Kapton Teflon Silicone
Max Temp	80° C	90°	105°	125°	200°
	176° F	194°	221°	257°	392°
AWG	Maximum Current - Amps
2	170	180	200	225	240
4	125	135	145	170	180
6	95	100	105	125	135
8	65	70	75	90	100
10	47	55	58	70	75
12	36	40	45	50	55
14	27	30	33	40	45
16	19	22	24	26	32
18	15	17	18	20	24
20	10	12	13	14	17
22	8	9	10	11	13
24	6	7	7	8	10
26	4	5	5	6	7
28	3	4	4	5	6
30	2	3	3	3	4

Source: Alpha Wire High ambient temperature increases the chance of insulation meltdown, so max. amperage is reduced. De-rating for high temperature:
Copper Wire Correction

Temperature	40°C	45°C	50°C	55°C	60°C	70°C
	104°F	113°F	122°F	131°F	140°F	158°F
DerateAmpacity	0.88	0.82	0.75	0.67	0.58	0.33

Low Voltage Cable Types
MP - Multipurpose cable
CM - Communications Cable
PLTC - Power-limited tray cable
Article 725, Class 2
   725-38(b)1      CL2X    Class 2 cable, limited use
   725-38(b)1      CL2     Class 2 cable
   725-38(b)2      CL2R    Class 2 riser cable
   725-38(b)3      CL2P    Class 2 plenum cable
   
Class 2, Class 3 - Remote-control, signaling, and power-limited cables
Class 1 - Power and lighting cables
FPL - Power-limited fire protective signaling cables 
Maximum Power Supply size for Class 2 Circuits is 100 VA