Wire Size for 100 Amp Electric Service: Complete Guide

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You must use the proper gauge wire to install a 100 amp electrical service for a detached garage or new room addition. To complete the skilled work, you’ll need a permit. Check with your local building department to see if you can get the permit or if you’ll require an electrician’s assistance.

Installation of 100 amp electric service or subpanel needs a #4 AWG copper wire or #2 AWG aluminum or copper-clad wire inside a minimum 1.25 inch, schedule 40 or 80 PVC electric conduit for underground service. Wiring 100 amp service for a garage or room addition does require an electric permit and professional installation by a qualified electrician following local electric codes.

Homes today have a 200 amp meter box connecting to the main feeder breaker disconnect and then to a secondary panel (subpanel). A 100 amp secondary circuit breaker panel has individual circuit breakers bringing additional circuits to a designated area, like a garage, shop, shed, pool house, or addition.

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Size Wire for 100 Amp Service

AWG, American Wire Gauge, is the US standard for sizing electrical wiring.

A 100 amp breaker panel requires either #4 copper wiring or #2 aluminum or copper-clad wiring. Which wire used is determined by the voltage drop and length of the wiring.

The following table reflects wiring size, length, and voltage drop:

Service or Feeder RatingCopper WireAluminum or
Copper-Clad Aluminum Wire
Minimum Conduit Size
100 Amps#4 AWG#2 AWG1.25 inch
110 Amps#3 AWG#1 AWG1.25 inch
125 Amps#2 AWG#1/0 AWG1.25 inch
150 Amps#1 AWG#2/0 AWG1.25 inch
175 Amps#1/0 AWG#3/0 AWG1.5 inch
200 Amps#2/0 AWG#4/0 AWG1.5 inch
225 Amps#3/0 AWG250 kcmil1.5 inch
250 Amps#4/0 AWG300 kcmil2 inch
300 Amps250 kcmil350 kcmil2.5 inch
350 Amps350 kcmil500 kcmil3 inch
400 Amps400 kcmil600 kcmil3 inch
Source: National Electrical Code

Many licensed electricians use #2 copper-clad or aluminum wire size. Because aluminum wiring has a lower conductivity than copper, it requires a larger diameter wire. You’ll need a #2 gauge wire for both aluminum and copper-clad aluminum wire.

To resist corrosion, the connection terminals of aluminum wiring require an antioxidant coating. At the connection terminals, copper-clad aluminum does not need an antioxidant layer.

Aluminum wiring appeals to many since it is less expensive, lighter, and more flexible than copper. Even though copper is considered a superior conductor, it is still cheaper, lighter, and more flexible than aluminum.

For copper wiring, you’ll need a #4 AWG copper wire, which is commonly used for electrical wiring since it is more heat resistant than aluminum. Copper doesn’t rust and is unaffected by oxidation.

Why Copper Wiring is Preferred over Aluminum

Copper conductors are popular among electricians and homeowners alike for branch circuits. Copper wiring has several advantages, including:

  • Conductivity: Copper wire is highly conductive, second only to silver. Copper can handle a greater electrical demand so that you may use a lower gauge wire. For the same level of conductivity, copper wire is around half the diameter of aluminum wire.
  • Heat Resistant: Copper wire is more heat-resistant than aluminum wire since it has better heat-conductive properties. The connections on the lugs stay tight because the copper wire does not expand and contract like aluminum.
  • Corrosion Resistant: Copper is resistant to corrosion—the color of the outer surface of copper changes when it patinas. Oxidation causes patina, which does not affect conductivity.
  • Malleability: Copper wire is more malleable and conductive than aluminum wire. Because copper is highly conductive, the wire strands are thinner, allowing for greater malleability without breaking.
  • Distance: Copper may be used over greater distances owing to its highly conductive characteristics and heat tolerance.

Pros of Aluminum or Copper-Clad Aluminum Wire

Stranded aluminum or copper-clad wire is more popular on service entry cable for a variety of reasons, including:

  • Budget Friendly: Due to its lower cost, aluminum wiring is popular. Aluminum copper-clad wire is superior in price but less expensive than aluminum wiring. Aluminum wiring is a lot more affordable than copper-clad and copper wires.
  • Installation is Quicker: Aluminum is lighter than copper and much more malleable regarding wire gauge. Electricians enjoy working with it because it is lightweight and readily passes through the electrical conduit, resulting in faster installation and lower material and labor costs.

When calculating wire size, you must know the length of the service line from the meter base to the electrical panel. The maximum voltage drop permitted is 3%, and we will discuss it in greater depth shortly. Choosing between aluminum or copper wire depends on how far your service entry wire needs to travel.

The Length Your Service Wire Runs Matters

When calculating wire size, you must know the length of the service line from the meter base to the electrical panel. The maximum voltage drop permitted is 3%, and we will discuss it in greater depth shortly. Choose between aluminum or copper wire, depending on how far your service entry wire needs to travel.

Depending on the location, distances may vary. Check with your local building department for regulations.

You can use aluminum or copper for a 100 amp service entry wire. The aluminum wire’s reduced conductivity and heat resistance will increase the gauge. You can use copper wiring for greater distances.

What is Voltage Drop?

Voltage drop is the loss of voltage that occurs over the wire due to natural resistance. The greater the length of a wire, the more voltage is lost. Increase the wire gauge to offset voltage loss.

The voltage at an individual circuit breaker should be 120 volts. Take a reading at the appliance or the farthest device using a voltage meter. Voltage drop shouldn’t exceed 3% (117 volts on a 120-volt circuit or 233 volts on a 240-volt circuit).

How Many Wires You Need for 100 amp Service 

Your electrical current load requirements determine the correct gauge wire you’ll need. The three-wire cable from the meter base to the main disconnect box or service panel consists of two hot wires and one neutral wire. Electric bonding joins the bars on the neural and ground bars.

The feeder cable is a 4-wire, comprising two hot feeder wires, one neutral wire, and one ground wire. You don’t bond the neutral and ground bars in subpanels.

What Size Conduit You Need for 100 Amp Service

A 100 amp service will need a minimum of 1.25 inches, schedule 40 or 80 PVC grey electric conduit. You can use a larger conduit to maintain the inner fill of 40% or less. A 1.5 or 2-inch conduit is best if running three or more electrical wires within the same electric conduit.

Chapter 9, Table 5A of the 2020 NEC, #2 Aluminum-stranded insulated conductors take up 112.9mm2 of fill space. Multiplying 112.9mm2 by three gives you 338.7mm2 of fill space used by the entire feeder.

According to Chapter 9, Table 4 of the 2020 NEC, a 1.5″, Schedule 80 PVC conduit has 442mm2 usable fill in rigid Schedule 80 PVC conduit.

For example, you may use a 1.5-inch conduit or go larger to pull the wires more easily. If two or more wires are present, the area of the cables must not exceed 40% of the pipe’s internal volume.

You’ll need to bury the conduit 24 inches deep in most areas. Some areas may require a deeper trench. A trenching tool like a ditch witch can speed the trenching process.

When choosing an electrical conduit, Schedule 80 PVC is thicker and can handle more pressure than Schedule 40 PVC. The outer diameter is the same for both pipes. However, because Schedule 80 PVC is thicker, you’ll have less inner fill space.

Check Local Electrical Codes & Permit Requirements

The National Electrical Code (NEC) is the authority for electrical codes but may differ by location. Using a licensed electrician familiar with your city’s local rules and requirements, such as 100 amp electrical service, ensures that it meets them.

The 100 amp wire size mentioned in this post is from the NEC code and may differ from your local building code. The NEC standards periodically change or are enhanced to meet safety guidelines.

100 Amp Electrical Service Installation Costs

The cost of installing a 100 amp entry service in a home is nearly challenging to estimate. We must consider several variables, such as the installation’s location and whether additional services are required.

Installing a 100 amp electrical service usually entails the following elements: a 100 amp feeder wire, conduit, and a 100 amp sub panel.

Based on HomeAdvisor, the national average cost of a complete 100 amp electric installation is $1500 to $4,500, with average costs of $3,000, including the following:

  • 100 Amp Electric Panel: Average price ranges from $500 to $1500, with a median price of $1000.
  • 100 Amp Feeder Wire: Average cost ranges from $1000 to $3,000, with a median price of $2,000.

Related reading: 80 Amp Wire Size: Breaker & Wiring Gauge Guide


Installing a 100 amp electric service or subpanel requires professional installation by a qualified electrician. Installing a 100 amp sub panel needs a #4 AWG copper wire or #2 AWG aluminum or copper-clad power feeders inside a minimum 1.25 inch, schedule 80 PVC conduit for underground service. It requires an electrician’s professional installation following local electrical codes.

Also, see our 200 Amp Wire Size: Residential Guide.


Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.