AWG

American Wire Gauge is a logarithmic stepped standard wire gauge used in North America.

• It is distinct from and implicitly superior to British Standard Wire Gauge (SWG).
• Other standards include IEC 60228 which defines metric wire sizes in terms of cross-sectional area (in units of $m{m}^2$).

American Wire Gauge Chart and AWG Electrical Current Load Limits table with ampacities, wire sizes, skin depth frequencies and wire breaking strength

AWG sizes

• AWG is mechanically similar to the Brown & Sharp (B&S) sheet metal gauge.
• larger numbers indicate a smaller diameter conductor.
• Stranded AWG have the same electrical properties as the equivalent solid AWG (although stranded wire will occupy a larger space than an equivalent solid wire)
  • stranded AWG is sometimes expressed as $A$ AWG $B/C$ :
    $A=$ overall AWG size
    $B=$ number of strands
    $C=$ AWG of the strands
  • E.g. a 22 AWG 7/30 refers to a 22 AWG wire composed of 7 strands of 30 AWG wire.
• There are 40 defined sizes ranging from 0000 to 36.
  • 0000 AWG = 0.46 inches in diameter
    N.B. - 0000 AWG is also written as 4/0 AWG
  • 36 AWG = 0.005 inches in diameter

convenient coincidences

• the ratio between the diameter of any two adjacent sizes is $\sqrt[39]{92}\approx 1.229$
  • therefore 3 AWG wire is $\approx D_{2AWG}÷1.229$
  • knowing that $D_{2AWG}=0.2576$ (about $\frac{1}{4}$ inch), then $D_{3AWG}=0.2294$ inch
• The ratio of diameters between any wires that are two gages apart is $(\sqrt[39]{92}{)}^2\approx (1.229{)}^2\approx 1.261$
  E.g. 20 AWG wire has a diameter which is 1.261 times larger than 22 AWG wire

AWG rules of thumb

since $(\sqrt[39]{92}{)}^6\approx 2$

doubling the cross-sectional area

• Doubling the cross-sectional area of a wire corresponds to a change of 3 in AWG
  E.g. a 6 AWG wire has nearly the same cross-sectional area as 2 x 9 AWG wires
  which means that two 9 AWG wires can carry around the same current as a single 6 AWG wire.

doubling the diameter

Doubling the diameter of a solid round wire decreases the AWG by 6
A 14 AWG wire has a diameter of 0.0641 inches $(\approx \frac{1}{16}\text{"})$ and a 20 AWG wire has a diameter about half of that: $D_{20AWG}=0.0320\text{ in}\approx \frac{1}{32}\text{"}$

power of 10 AWG

A decrease of 10 AWG increases the area, weight, and conductance by an order of magnitude.
20 AWG wire is 10x heavier and larger (and can carry 10x more current) than 30 AWG.

resistance rule of thumb

For an arbitrary gage $n$, the resistance $(R)$ of a copper wire is approximately

E.g. for 20 AWG, $R\approx {10}^{\frac{20}{10}}\frac{\mathrm{\Omega }}{1000ft}$
$R_{20AWG}\approx 100\frac{\mathrm{\Omega }}{1000ft}\approx 10\frac{m\mathrm{\Omega }}{ft}$

Aluminum has a conductivity which is $\approx 61\mathrm{\%}$ the conductivity of copper, so an aluminum wire has the same resistance as a copper wire which is two sizes smaller (and $\therefore$ has about half the cross-sectional area)

table of AWG characteristics

How to use this chart:
1. find the required wire gauge - if you know your thermal limit (defined by the lowest rating of all components in the circuit) and your design specified maximum current, you can read the maximum wire gauge off the left column.
2. For a given wire gage, use the lowest temperature rating on the circuit to determine the maximum continuous current that the system can support.
3. To find voltage drop, use Ohm's Law to solve for the voltage difference given the length of the wire (in ft) and the maximum/intended current through it.

ephemera

footnotes for the AWG table

• source: NEC Table 310.15(B)(16), data applies to copper conductors only
• Breakers are made to protect wires.
• circuit breakers are for short-circuit protection, not to protect equipment (that would be an overload heater) or you (GFCI).

ampacity

• per the US NEC, ampacity is defined as, "the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating."
  • you have to derate conductors if you have multiples in a single conduit.
  • For the purposes of rating conductors in a conduit, a ground is not counted (because in normal operation, it carries no current).
• ampacity is determined by the thermal characteristics of the insulation on the conductor.
  i.e. the temperature rating for the type of wire under consideration dictates which column in the above table applies.

wire types

• Tray Cable
  per NEC, “a factory assembly of two or more insulated conductors, with or without associated bare or covered grounding conductors under a nonmetallic sheath, for installation in cable trays, in raceways, or where supported by a messenger wire.”

• Machine Tool Wire (MTW) is resistant to heat, moisture, oil, and gasoline. This type of wire almost always has a voltage rating of 600V and a maximum temperature of 105°C.

• Thermoplastic High Heat resistant Nylon-coated wire (THHN) wire has a voltage rating of 600V. It has a temperature range of up to 90°C in dry locations and 75°C in wet locations.

resistance

• applies to copper conductors and DC current or AC current of $\approx 60Hz$.
• Other materials have different resistivity $(\rho )$. Per a common #resistance rule of thumb for AWG, the resistance of an aluminum wire is approximately the same as a copper wire two sizes smaller than the aluminum wire.

max frequency

The frequency listed in the table shows the frequency at which the calculated skin depth is equal to the radius of a solid copper wire, and is an indication that above this frequency you should start considering the skin effect when calculating the wire's resistance.

• This data is useful for high frequency AC engineering. When high frequency AC is conducted by a wire there is a tendency for the current to flow along the outside of the wire.
• This increases the effective resistance.

tables of voltage drop

The resistance of a wire increases linearly as a function of its length. The table below gives expected resistances for a wire of $x$ AWG and $l$ length based on specifications for copper conductors.