100 Amp Wire Size Calculator

Calculator Inputs

Load Current (A)

Continuous Load

Conductor Material

System Type

System Voltage (V)

One Way Distance (ft)

Maximum Voltage Drop (%)

Wire Temperature Rating

Terminal Temperature Rating

Ambient Temperature (C)

Current Carrying Conductors

Extra Safety Margin (%)

Formula Used

Design amps: load amps x continuous factor x safety margin.

Corrected ampacity: table ampacity x ambient factor x conductor derating factor.

Voltage drop: current x conductor resistance x distance multiplier x one way distance / 1000.

Drop percentage: voltage drop / system voltage x 100.

Single phase and DC use a multiplier of 2. Three phase uses 1.732. The calculator picks the first conductor that passes both ampacity and voltage drop checks.

How to Use This Calculator

Enter the expected circuit current. Use 100 for a standard 100 amp planning check.
Choose whether the load is continuous. Continuous loads are sized with a higher design current.
Select copper or aluminum. Match the material to the wire you plan to install.
Enter voltage, phase type, and one way route distance.
Set voltage drop, temperature ratings, ambient heat, and conductor count.
Press calculate. The result appears above the form and below the header.
Use the CSV or PDF button to save your calculated summary.

Example Data Table

Example	Material	Load	Voltage	Distance	Drop Limit	Typical Suggested Size
Short feeder	Copper	100 A	240 V	50 ft	3%	3 AWG or larger
Long feeder	Copper	100 A	240 V	150 ft	3%	1/0 AWG or larger
Aluminum run	Aluminum	100 A	240 V	100 ft	3%	1/0 AWG or larger
Three phase	Copper	100 A	208 V	100 ft	3%	1 AWG or larger

100 Amp Wire Size Planning Guide

A 100 amp circuit needs careful wire sizing. The conductor must carry the load safely. It must also keep voltage drop within a useful range. Long runs need more attention because resistance rises with distance. Material matters as well. Copper carries more current than aluminum at the same size. Aluminum can still be correct when it is sized and terminated properly.

Why Ampacity Matters

This calculator combines ampacity and voltage drop checks. It starts with your design current. It can apply a continuous load factor when the circuit may run for three hours or more. It then checks common conductor ampacity values. Temperature rating and ambient heat can reduce safe ampacity. Extra current carrying conductors can also require derating. These corrections help the suggestion stay realistic.

Why Voltage Drop Matters

Voltage drop is a performance limit, not only a safety issue. A wire may be large enough for heat, yet still waste energy on a long run. Low voltage can make motors run hot. Lights may dim. Equipment can act weak during startup. The tool estimates drop from resistance, current, length, and system type. It compares the result with your chosen percentage limit.

Use Results Carefully

Use the result as a planning guide. Always confirm the final design with local electrical rules. Local codes may require different conductor types, insulation ratings, conduit fill, grounding conductors, or terminal temperature limits. Breaker size, equipment labels, and installation conditions can change the answer. Outdoor, wet, buried, and rooftop runs need special review. A licensed electrician should approve permanent work.

Compare Practical Choices

The example table shows how distance changes the suggested size. Short copper runs can often use smaller conductors than long aluminum runs. The final choice should balance safety, cost, installation space, and future expansion. Oversizing may reduce heat and voltage loss. It can also make pulling wire harder. This calculator gives a clear starting point before you buy cable or plan conduit.

Good Inputs Matter

Good inputs matter. Measure the full one way route, not only straight line distance. Include rises, bends, and panel offsets. Pick the material you will actually install. Select the voltage used by the equipment. Set a voltage drop target that fits the load. Three percent is common for sensitive branch circuits. Five percent may be acceptable for some feeders. Review labels carefully.

FAQs

What size wire is commonly used for 100 amps?

Many installations use 3 AWG copper or 1 AWG aluminum at suitable ratings. Longer runs may need larger wire to limit voltage drop.

Does distance change the wire size?

Yes. Longer distance increases resistance. The wire may need to be larger even when ampacity is already enough.

Why does copper need a smaller size than aluminum?

Copper has lower resistance and higher ampacity for the same gauge. Aluminum can work well when sized and terminated correctly.

What voltage drop limit should I use?

Three percent is a common planning target for branch circuits. Five percent may be used for some feeder and branch combinations.

What is a continuous load?

A continuous load may run for three hours or more. It is often planned at 125 percent of the actual load.

Why do terminal ratings matter?

Terminals can limit the ampacity column used. A 90 C wire may still need 60 C or 75 C terminal sizing.

Can this replace local electrical code?

No. It is an estimating tool. Local code, equipment labels, conductor type, and inspection rules control the final design.

Does conduit heat affect the result?

Yes. High ambient heat and many current carrying conductors can reduce usable ampacity. The calculator includes derating fields.