Wire Size for 100 Amp Electric Service: Complete Guide

If you want to install a 100 amp electrical service for a detached garage or a new room addition, you must use the proper gauge wire. 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 whether you’ll require the assistance of an electrician.

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.

Size Wire for 100 Amp Service

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

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 it has a lower conductivity than copper, aluminum wiring 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 acknowledged to be a superior conductor, it is still cheaper, lighter, and more flexible than aluminum.

For copper wiring, you’ll need a #4 AWG copper wire that 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 a highly conductive material, second only to silver. Copper can handle a greater electrical demand, so 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 due to the fact that 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 risk of 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 terms of 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 in terms of 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. Using a voltage meter, take a reading at the appliance or the farthest device. 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 run from the meter base to the main disconnect box or main 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 if necessary 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.

In Chapter 9, Table 5A of the 2020 NEC, #2 Aluminum stranded insulated conductors take up 112.9mm2 of fill space. If you multiply 112.9mm2 by three gives you a total of 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 if you want 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 they 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 location of the installation and whether there are any additional services 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.com, the national average cost of a complete 100 amp electric installation costs $1500 to $4,500, with average costs of $3,000, includes the following:

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

Conclusion

Installing a 100 amp electric service or subpanel requires professional installation by a qualified electrician. Installing a 100 amp subpanel 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.

Photo of author

Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started HomeInspectionInsider.com to help people better understand the home inspection process and answer questions about homeownership and home maintenance.
DISCLAIMER: The content published on HomeInspectionInsider.com is not professional advice. You should consult with a licensed professional and check local permit requirements before starting any project.
HomeInspectionInsider.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com. We also participate in other affiliate programs with other affiliate sites. We are compensated for referring traffic and business to these companies.