How to Read a Wire Gauge Chart

A wire gauge is the diameter of the wire. The diameter determines how many amperes (amps) a wire can conduct without overheating. If a wire overheats, the insulation could melt off and cause a fire. This is why it is important to know which gauge size is necessary for the wire used to connect outlets, appliances and switchboxes to a breaker box.

A standard round gauge tool is the most common tool for measuring wire size. To measure a wire gauge, strip the wire and try to fit it into the numbered grooves that appear closest to it in size. The correct size should just fit the wire, but still allow it to pass through to the hole to be removed. The written gauge size will be the number corresponding to the groove it fits into, followed by “AWG.”

AWG wire sizes are measured by their gauge, which can be converted to imperial measurements. Wire sizes get larger as the gauge number gets lower, from 40-gauge all the way down to 0-gauge or 0.325 inches. Sizes larger than 0-gauge are listed as 00, 000 and so on. Wire with a low gauge will be harder to work with, and larger wire cutters may be necessary.

AWG is measured differently for solid wire than it is stranded wire. The standard measurement tool will find the size of each, but the written gauge size will be more specific. This is because the measurement accounts for the very small gaps between each woven strand. The gauge of each strand will be the same, which is listed alongside the total number of strands.

For example, a 6-gauge wire could be made up of seven wire strands, each with a gauge size of 14. The AWG size of the wire would be “6 AWG 7/14.”

It is important to remember that this is different from the conductor counts that are listed on the front of wire packaging. A spool of wire labeled “12/2” refers to the gauge size of each cable within the wire, followed by the number of conductors. Each conductor can be separated to make connections with individual outlets or appliance connectors, and is part of a circuit on the breaker box. Every wire will also have a grounding cable, which means a “12/2” wire would have three total cables inside it.

A wire gauge chart for converting AWG wire sizes is shown above. The chart converts AWG sizes to diameter in inches, millimeters and the cross-sectional area of the wire. The resistance and rated ampacity for common wire types are also listed. Each type of wire is listed by the metal used, its maximum temperature rating and the types of insulation that wrap the wire. Each row lists a wire gauge size and its properties.

Non-metallic (NM) and Underground Feeder (UF) cables are the most common types of wire for homes. NM cable supports outlets, appliances and connecting them to the breaker box. UF cable can be used as outdoor electrical wires to power outdoor lights and other electronics.

Other types of wire on the chart are labeled by their insulation. The meanings of these abbreviations are listed below according to each family of insulation types.

THHN cable is predominantly used in construction, machine tools, control circuits and large appliances.

• T: Thermoplastic
• H: Heat Resistance
• HH: High Heat Resistance
• N: Nylon-coated
• W: Water Resistance

XHHW cable is found in many residential, commercial, and industrial buildings. The “X” stands for cross-linked polyethylene, and the rest of the letters carry the same meaning as in the THHN family.

Underground Service Entrance (USE) and Service Entrance (SE) cables are responsible for providing electrical service into a home or building. Both are heat and fire resistant, but USE cable is also moisture resistant.

The maximum amperage a wire can conduct before it overheats is called ampacity. A wire cannot be attached to a circuit that exceeds its ampacity as determined by AWG size and insulation, or rated ampacity. Insulation can withstand differing amounts of heat, which can increase the relative ampacity of the wire. A lower gauge wire will have a higher-rated ampacity than a high gauge, because the larger diameter of the wire will be able to carry more electrical current.

Electrical wires will not always operate close to their maximum-rated ampacity. All wires have some resistance to electricity, which will effectively decrease the output of the wire. The tendency for electrical current to travel mostly along the surface of conductors will also increase resistance.

A wire must have an ampacity higher than or equal to the maximum amps of the connected circuit. If the circuit exceeds the ampacity of a wire, the wire will not be able to handle the electrical current and may overheat. When assembling circuits in a home or building, it is necessary to know what types of electronics the circuits are servicing. Some common uses for wires of different ampacities include:

• 3/0-gauge, 200 amps: Service entrance
• 1/0-gauge, 150 amps: Service entrance and feeder wire
• 6-gauge, 55 amps: Feeder wire and large appliances
• 10-gauge, 30 amps: Appliances, dryers, air conditioners
• 12-gauge, 20 amps: Appliance, laundry, and GFCI circuits
• 14-gauge, 15 amps: Lighting, ceiling fans, and outlets

All of these types of circuits can be connected to the same breaker box without overheating the main breaker. Most home breaker boxes can handle 100-200 amps. However, no circuit should ever run above 80% of its maximum rated ampacity. Overtaxing the circuit can lead to surges, outages and fires.

It is possible for every circuit on a 200-amp breaker box to run simultaneously if they do not demand more than 160 amps combined. A bedroom or living room typically has a 15- or 20-amp circuit that services all lighting and outlets. Conversely, bathrooms have a 20-amp circuit for outlets and a 15-amp circuit for lights. A kitchen will need six or seven 20-amp circuits to cover the large appliances, microwave, garbage disposal, dishwasher and outlets. However, homeowners typically do not draw all of that electricity at one time. A house with 10 to 20 circuits conducting 15 to 55 amps would rarely cause a main breaker to cut off power.

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