Wire Size Chart: Ampacity & Voltage Drop Guide

How to Read Wire Size Charts

Wire size charts contain essential information for selecting the proper conductor for any electrical installation. Understanding how to read these charts is fundamental to safe and efficient electrical design. Most charts include wire gauge, cross-sectional area, resistance, and ampacity—each serving a specific purpose in the selection process.

The wire gauge column shows the AWG (American Wire Gauge) or kcmil designation. Smaller AWG numbers indicate larger wire—14 AWG is smaller than 10 AWG. For very large conductors, the designation switches to kcmil (thousands of circular mils), where larger numbers mean larger wire.

Cross-sectional area, typically shown in square millimeters (mm²), provides a universal measurement that's useful for international applications and for understanding the actual conductor size. This measurement directly relates to the wire's current-carrying ability and resistance characteristics.

Resistance values, usually given in ohms per 1000 feet (Ω/kft) or ohms per kilometer, determine how much voltage drop occurs when current flows through the wire. Lower resistance values mean less voltage drop for the same current and length. These values are essential for voltage drop calculations.

Ampacity columns show the maximum current a conductor can safely carry without exceeding its temperature rating. Ampacity depends on conductor material, insulation type, and installation conditions. Charts typically show different ampacity values for different temperature ratings (60°C, 75°C, 90°C).

AWG Wire Gauge System Explained

The American Wire Gauge (AWG) system is the standard for measuring wire size in North America. This system can seem counterintuitive at first because smaller numbers indicate larger wires. Understanding the logic behind AWG helps make sense of wire sizing.

The AWG Scale

The AWG scale was developed based on the number of drawing operations required to produce a given wire size. Starting from a standard rod, each drawing through progressively smaller dies reduced the diameter. More drawing operations meant smaller wire and higher gauge numbers. This historical process explains why 22 AWG is smaller than 10 AWG—it required more drawing steps to produce.

The AWG system follows a geometric progression. Each three-gauge decrease (larger number) approximately halves the cross-sectional area. For example, 10 AWG has roughly double the area of 13 AWG. Similarly, each six-gauge decrease approximately halves the resistance per unit length.

AWG for Large Conductors

The AWG scale extends from 40 AWG (very fine wire) through 0000 AWG (4/0). Beyond 4/0 AWG, wire sizes are specified in kcmil (thousands of circular mils). A circular mil is the area of a circle with a 1 mil (0.001 inch) diameter. Common large conductor sizes include 250 kcmil, 350 kcmil, 500 kcmil, and larger.

The notation 4/0 AWG (read as "four-ought") continues from 1 AWG. The progression goes: 1 AWG, 1/0 AWG, 2/0 AWG, 3/0 AWG, 4/0 AWG. These larger gauges are essential for main service conductors, large feeders, and high-current applications.

Common Wire Sizes and Uses

Different wire sizes serve different purposes in electrical installations. 14 AWG is the minimum size for most 15A circuits. 12 AWG is standard for 20A general-purpose circuits. 10 AWG handles 30A circuits like dryers and water heaters. 8 AWG and larger sizes are used for ranges, air conditioners, and subfeeder circuits. Understanding these common applications helps quickly identify appropriate wire sizes for various installations.

Ampacity vs. Voltage Drop

When sizing conductors, electricians must consider two separate requirements: ampacity and voltage drop. Both must be satisfied, and the larger wire size wins. This dual requirement often surprises those new to electrical design.

Ampacity: The Safety Requirement

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. Exceeding ampacity causes the conductor insulation to overheat, potentially leading to insulation damage, fire hazards, and shortened wire life. NEC Article 310 specifies ampacity values based on conductor material, insulation temperature rating, and installation method.

Ampacity is a code requirement enforced by electrical inspectors. A circuit with insufficient ampacity is a safety violation that must be corrected. The NEC provides tables (310.16 through 310.21) listing ampacity values for various conditions.

Voltage Drop: The Performance Requirement

Voltage drop affects equipment performance rather than safety. When voltage at the load is too low, motors produce less torque, lights dim, and electronics may malfunction. The NEC recommends (but doesn't require) limiting voltage drop to 3% for branch circuits and 5% total.

Voltage drop often requires larger wire than ampacity alone would indicate, especially for long circuit runs. A wire might have adequate ampacity for the load but still produce excessive voltage drop at the end of a long run. In these cases, larger wire is needed despite having sufficient current-carrying capacity.

When Each Determines Wire Size

Short, high-current circuits are typically limited by ampacity. The current is high enough that the wire needs to be large for safe operation, and voltage drop over the short distance is minimal. Long, low-current circuits are often limited by voltage drop. The current is low enough that a small wire could handle it safely, but the long distance causes unacceptable voltage drop, requiring a larger wire.

Copper Wire Specifications

Copper is the most common conductor material for building wiring due to its excellent conductivity, corrosion resistance, and workability. The following table provides complete specifications for copper conductors from NEC Chapter 9, Table 8 and Article 310.

AWG	Area (mm²)	Diameter (in)	Resistance (Ω/kft)	60°C Amp	75°C Amp	90°C Amp
14	2.08	0.064	3.14	15	20	25
12	3.31	0.081	1.98	20	25	30
10	5.26	0.102	1.24	30	35	40
8	8.37	0.128	0.778	40	50	55
6	13.3	0.162	0.491	55	65	75
4	21.2	0.204	0.308	70	85	95
3	26.7	0.229	0.245	85	100	115
2	33.6	0.258	0.194	95	115	130
1	42.4	0.289	0.154	110	130	145
1/0	53.5	0.325	0.122	125	150	170
2/0	67.4	0.365	0.0967	145	175	195
3/0	85.0	0.410	0.0766	165	200	225
4/0	107	0.460	0.0608	195	230	260

Ampacity values shown are for conductors in raceway, cable, or direct buried based on NEC Table 310.16. Actual ampacity may vary based on installation conditions, number of current-carrying conductors, and ambient temperature. Always verify requirements using current NEC tables.

Aluminum Wire Specifications

Aluminum conductors are commonly used for larger installations where cost and weight are significant factors. Aluminum has about 61% of the conductivity of copper, requiring larger sizes to carry the same current. However, aluminum is lighter and less expensive per ampere of capacity, making it economical for service entrances and feeders.

AWG	Area (mm²)	Diameter (in)	Resistance (Ω/kft)	60°C Amp	75°C Amp	90°C Amp
12	3.31	0.081	3.25	15	20	25
10	5.26	0.102	2.04	25	30	35
8	8.37	0.128	1.28	35	40	45
6	13.3	0.162	0.808	40	50	55
4	21.2	0.204	0.508	55	65	75
3	26.7	0.229	0.403	65	75	85
2	33.6	0.258	0.319	75	90	100
1	42.4	0.289	0.253	85	100	115
1/0	53.5	0.325	0.201	100	120	135
2/0	67.4	0.365	0.159	115	135	150
3/0	85.0	0.410	0.126	130	155	175
4/0	107	0.460	0.100	150	180	205

Note that aluminum is generally not used in sizes smaller than 8 AWG for branch circuits due to connection reliability concerns. Aluminum requires special attention at terminations, including the use of anti-oxidant compound and proper torque values. All terminals must be rated for aluminum conductors (marked AL or AL/CU).

Temperature Ratings and Derating

Wire ampacity depends significantly on the insulation temperature rating and the ambient conditions where the wire is installed. The NEC requires derating (reducing) ampacity when conditions differ from the standard assumptions used in the ampacity tables.

Insulation Temperature Ratings

Common insulation types and their temperature ratings include:

TW (60°C): Basic thermoplastic insulation, wet locations
THW, THWN (75°C): Heat-resistant thermoplastic, common for building wire
THHN, XHHW (90°C): High-heat resistant insulation, highest ampacity

Higher temperature ratings allow higher ampacity because the wire can safely operate at higher temperatures. However, the wire must be used with terminals rated for that temperature—most equipment terminals are rated at 75°C, limiting the usable ampacity even with 90°C wire.

Ambient Temperature Correction

NEC ampacity tables assume an ambient temperature of 30°C (86°F). When ambient temperature exceeds this, ampacity must be reduced. The following correction factors apply:

Ambient Temp (°C)	60°C Wire	75°C Wire	90°C Wire
21-25	1.08	1.05	1.04
26-30	1.00	1.00	1.00
31-35	0.91	0.94	0.96
36-40	0.82	0.88	0.91
41-45	0.71	0.82	0.87
46-50	0.58	0.75	0.82
51-55	0.41	0.67	0.76

Multiply the table ampacity by the correction factor for your ambient temperature. For example, 12 AWG copper THHN in a 40°C environment has ampacity of 30A × 0.91 = 27.3A.

Conduit Fill Derating

When multiple current-carrying conductors share a conduit, heat dissipation is reduced, requiring ampacity reduction. NEC Table 310.15(C)(1) specifies adjustment factors based on the number of conductors. Three current-carrying conductors require no adjustment, but more conductors require the following derating:

4-6 conductors: 80% of table ampacity
7-9 conductors: 70% of table ampacity
10-20 conductors: 50% of table ampacity
21-30 conductors: 45% of table ampacity

Wire Size Selection Process

Selecting the proper wire size requires a systematic approach that considers both ampacity and voltage drop. Follow these steps to ensure your conductor selection meets all requirements.

Step 1: Determine Load Current

Calculate or measure the expected load current. For motors, use the full-load ampere values from NEC Table 430.248 (single-phase) or 430.250 (three-phase) rather than nameplate values. For resistive loads, use P = V × I to find current. For continuous loads (operating 3+ hours), multiply the load current by 1.25.

Step 2: Select Wire for Ampacity

Using the appropriate ampacity table, find the smallest wire size with ampacity equal to or greater than your load current. Consider the insulation temperature rating and apply any derating factors for ambient temperature or conduit fill. The selected wire must have adequate ampacity after all adjustments.

Step 3: Calculate Voltage Drop

Using the wire size from Step 2, calculate voltage drop using the appropriate formula for your circuit type. Use our voltage drop calculator for quick results. If voltage drop exceeds 3% for branch circuits (or your design target), you'll need a larger wire.

Step 4: Increase Wire Size if Needed

If voltage drop exceeds your target, select the next larger wire size and recalculate. Repeat until voltage drop is acceptable. The final wire size may be larger than required by ampacity alone—this is normal for long circuit runs.

Step 5: Verify with Code Requirements

Check that your selected wire size meets all NEC requirements including minimum sizes for the circuit type, grounding conductor sizing, and any special requirements for the equipment being served. Document your calculations for permit applications and inspections.

Frequently Asked Questions

For a 20 amp circuit, 12 AWG copper is the minimum size required by the NEC. This wire has an ampacity of 20A at 60°C or 25A at 75°C. However, for long runs exceeding about 50 feet, you may need 10 AWG to keep voltage drop within acceptable limits. Use the voltage drop calculator to verify your specific situation.

Modern aluminum wire is safe when properly installed with appropriate terminations. Use only terminals rated for aluminum (marked AL or AL/CU), apply anti-oxidant compound, and follow manufacturer torque specifications. Aluminum is commonly used for service entrance conductors and large feeders. The issues with aluminum wiring in the past were related to improper connections, not the wire itself.

AWG to mm² conversion doesn't follow a simple formula. Common equivalents are: 14 AWG = 2.08 mm², 12 AWG = 3.31 mm², 10 AWG = 5.26 mm², 8 AWG = 8.37 mm², 6 AWG = 13.3 mm². The wire size charts in this article include both AWG and mm² for easy reference. International standards typically use mm² rather than AWG.

Wire insulation has a maximum operating temperature. When current flows through a conductor, it generates heat due to resistance. Higher temperature-rated insulation can withstand more heat, allowing more current before reaching its limit. A 90°C rated wire can safely run hotter than a 60°C wire of the same size, permitting higher current flow.

Solid wire consists of a single conductor, while stranded wire contains multiple smaller conductors twisted together. Both have the same ampacity for the same AWG size. Solid wire is easier to terminate at devices but is stiffer and more prone to fatigue from movement. Stranded wire is more flexible and preferred for pulling through conduit, especially in larger sizes. The resistance values in NEC Table 8 apply to both types.