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Category Engineering Systems

Current (Amperes)

System Voltage

Phase

One-Way Distance

Unit

Allowable Voltage Drop (%) NEC recommends max 3% for branch circuits.

Conductor Material

Insulation Temp Rating

Continuous Load (≥ 3 hours) Applies 125% sizing rule for ampacity.

Recommended Wire Size

-- AWG

Copper

Limiting Factor -

Required Ampacity - A

Wire Max Ampacity - A

Actual Voltage Drop - V

Voltage Drop % - %

Cross Sectional Area - cmil

Enter parameters and click calculate to view results compliant with NEC Table 310.16.

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About

Determining the correct conductor size for a 100 Amp circuit requires more than simply consulting a standard ampacity chart. Failure to account for voltage drop over distance or the thermal rating of terminal connections can result in severe overheating, insulation degradation, and potentially catastrophic fire risks. This tool adheres to the standards set by the National Electrical Code (NEC) to calculate both the thermal ampacity requirement and the impedance-based voltage drop.

By evaluating the specific resistance (R) of either Copper or Aluminum conductors at specified lengths, this calculator ensures the voltage drop remains within acceptable operational thresholds (typically 3% for branch circuits). It strictly applies NEC Table 310.16 for thermal limits and Chapter 9, Table 8 for specific conductor resistance, eliminating approximation errors common in generic spreadsheet calculations. Note: This tool assumes standard ambient temperatures (30°C) and does not apply localized ambient derating factors.

Formulas

The calculation operates in two phases. First, the required wire gauge is determined by matching the target amperage to the thermal limits of the conductor based on its material and temperature rating. If the circuit operates continuously (more than 3 hours), the load must be scaled by 125%:

I_req = I_load × 1.25

Second, the tool calculates the voltage drop over the specified distance using the standard direct-current/low-frequency alternating-current resistance method. For a Single-Phase system, the formula is:

V_D =

2 × L × R × I1000

For a Three-Phase system, the factor of 2 is replaced by the square root of 3:

V_D =

√3 × L × R × I1000

Variables:
V_D = Voltage drop in Volts
L = One-way length of the circuit in feet
R = Conductor resistance in Ohms per 1000 feet (Ω/1000ft)
I = Load current in Amperes

Reference Data

Wire Size (AWG/kcmil)	Area (cmil)	Resistance Cu (Ω/1000ft)	Resistance Al (Ω/1000ft)	Ampacity Cu 75°C	Ampacity Al 75°C
14 AWG	4,110	3.14	5.08	20 A	-
12 AWG	6,530	1.98	3.18	25 A	20 A
10 AWG	10,380	1.24	2.00	35 A	30 A
8 AWG	16,510	0.778	1.25	50 A	40 A
6 AWG	26,240	0.491	0.808	65 A	50 A
4 AWG	41,740	0.308	0.508	85 A	65 A
3 AWG	52,620	0.245	0.403	100 A	75 A
2 AWG	66,360	0.194	0.319	115 A	90 A
1 AWG	83,690	0.154	0.253	130 A	100 A
1/0 AWG	105,600	0.122	0.201	150 A	120 A
2/0 AWG	133,100	0.0967	0.159	175 A	135 A
3/0 AWG	167,800	0.0766	0.126	200 A	155 A
4/0 AWG	211,600	0.0608	0.100	230 A	180 A
250 kcmil	250,000	0.0515	0.0847	255 A	205 A

Frequently Asked Questions

The ampacity (current-carrying capacity) of a wire is limited by the temperature its insulation can withstand before degrading. 60°C insulation (like TW or UF) cannot dissipate heat as effectively as 75°C insulation (like THWN). To keep the temperature below 60°C while carrying 100 Amps, a larger wire mass (1 AWG) is required to reduce the electrical resistance and subsequent heat generation.

The NEC defines a continuous load as any load expected to run for 3 hours or more continuously. If your 100 Amp circuit serves a continuous load (e.g., an EV charger or commercial lighting), the conductor must be sized to carry 125% of the continuous load. Therefore, a 100 Amp continuous load requires wire rated for at least 125 Amps.

Yes, but aluminum has higher electrical resistance than copper. To carry the same amperage and maintain the same voltage drop, you must use a thicker wire. For example, while 3 AWG Copper handles 100A at 75°C, you would need 1 AWG Aluminum to achieve the same thermal rating safely.

While the NEC explicitly mandates wire sizing based on ampacity, it offers an Informational Note (not a strict requirement) suggesting a maximum voltage drop of 3% for branch circuits, and 5% combined for feeders and branch circuits. Exceeding this can lead to inefficient operation of equipment, flickering lights, and motor stalling.