How to Size Wire for a Subpanel: Distance and Load Guide

How to Size Wire for a Subpanel: Distance and Load Guide

Sizing wire for a subpanel requires calculating both the total electrical load and the distance from your main panel to ensure safe operation and code compliance. The two critical factors are amperage demand and conductor length, which together determine the minimum wire gauge needed to prevent voltage drop and overheating. This guide walks you through the process step-by-step so you can get it right the first time.

Understanding Subpanel Load Calculation

Before you can size your wire, you need to determine how much current your subpanel will draw. This is measured in amperes (amps) and depends on all the circuits and appliances it will serve.

Start by listing every circuit that will connect to the subpanel. Add up the wattages of all devices that might run simultaneously. For example, if you’re running a workshop subpanel, you might have:

• One 240V welder (50 amps)
• One 20-amp 240V air compressor circuit
• Multiple 120V circuits for tools and lighting (60 amps total)

Convert watts to amps using this formula: Amps = Watts ÷ Volts. Once you’ve calculated the total amperage, add 25% to account for future expansion and simultaneous usage patterns. This gives you your demand amperage.

For example, if your calculated load is 100 amps, your actual sizing basis becomes 125 amps. Always use the larger number when choosing your wire and breaker size. The National Electrical Code (NEC) requires this safety margin, and it protects your investment.

Accounting for Distance and Voltage Drop

Distance matters more than most homeowners realize. The longer the wire run from your main panel to the subpanel, the more resistance the copper encounters, which causes voltage drop. Voltage drop reduces the effectiveness of your electrical system and can damage sensitive equipment.

The NEC recommends voltage drop not exceed 3% on branch circuits or 5% combined (feeder plus branch). To calculate voltage drop, you need to know:

• The amperage of the circuit
• The one-way distance in feet
• The wire gauge material (copper or aluminum)

A 100-amp subpanel fed with #2 copper wire can safely travel about 100 feet with acceptable voltage drop. If your run is 150 feet, you’d need to step up to #1 wire. A 200-amp subpanel at 200 feet might require 3/0 copper.

This is why distance is as important as amperage in your calculation. Two identical 100-amp loads at different distances will require different wire sizes. Aluminum wire requires one gauge larger than copper for the same ampacity, so a #1 copper job becomes a 1/0 aluminum job.

Wire Sizing Table and Selection Process

Here’s the practical process for selecting your wire:

Step 1: Determine Your Breaker Size
Based on your load calculation, select the appropriate main breaker for the subpanel. Common sizes are 100 amps, 125 amps, 150 amps, or 200 amps. This breaker size becomes your amperage baseline for wire selection.

Step 2: Measure Your Run Distance
Measure the actual wire run from the main panel to the subpanel location. Count all the bends and vertical rises. Use the total one-way distance, not the straight-line distance.

Step 3: Check Wire Ampacity at Your Size
A 100-amp subpanel normally uses #2 copper or 1/0 aluminum. A 150-amp subpanel uses 1/0 copper or 3/0 aluminum. A 200-amp subpanel uses 2/0 or 3/0 copper, or 4/0 aluminum.

Step 4: Verify Voltage Drop Won’t Exceed 5%
For a 100-amp circuit at 150 feet, #2 copper experiences approximately 2.8% voltage drop—acceptable. If you exceeded 5%, you’d upsize the wire.

Step 5: Confirm Local Code Requirements
Your local authority having jurisdiction (AHJ) may have stricter requirements than the NEC. Always check with your local electrical inspector before purchasing materials.

Remember that wire size also affects your conduit size. Larger wire requires larger conduit. Don’t forget to include the ground wire (same size as your hot conductors) and the neutral wire in your conduit fill calculation.

How to Use the Subpanel Wire Sizing Calculator

Rather than performing manual calculations, you can use the subpanel wire size calculator to instantly determine the correct gauge for your specific distance and load requirements. Simply enter your breaker amperage, the one-way distance to your subpanel, and whether you’re using copper or aluminum conductor. The calculator accounts for voltage drop and NEC requirements automatically, giving you immediate results you can trust.

Frequently Asked Questions

What happens if my subpanel wire is undersized?

Undersized wire creates excessive resistance, causing voltage drop that damages equipment, reduces tool performance, and generates heat that can melt insulation and cause fires. The wire heats up, the breaker may not trip properly at overload conditions, and you risk catastrophic failure. Always follow code requirements—they exist because people got hurt.

Can I use the same wire size for aluminum and copper?

No. Aluminum has higher resistance than copper, so it requires stepping up one gauge size. If copper requires #2, aluminum requires 1/0. If you use aluminum wire rated the same as copper, you’re actually undersizing the circuit.

Do I need a separate ground wire for my subpanel feeder?

Yes, if the subpanel is in a separate building or structure. If the subpanel is in the same building, bonding rules are different, but you always need an equipment grounding conductor. Check your local code or consult your electrical inspector for your specific situation.