Conduit sizing for a 100-amp service is the calculation of the minimum internal diameter of a protective raceway required to safely house the service entrance conductors without exceeding the National Electrical Code's 40% fill capacity limit. For a standard modern 100-amp residential service using three 4/0 AWG aluminum THHN/THWN-2 conductors (two hots, one neutral) and one #2 AWG aluminum ground, you need a minimum 2-inch PVC Schedule 40 or 2-inch EMT (Electrical Metallic Tubing). If your local code requires PVC Schedule 80 for physical protection, you must step up to 2.5-inch.

The Direct Answer: Use 2-inch PVC Schedule 40 for standard underground runs, or 2-inch EMT for above-ground mast/wall runs. Never force 4/0 aluminum into a 1.5-inch pipe.

The Core Sizing Table for 100-Amp Services

Before pulling any wire, you must match your conductor material to the specific conduit type. The NEC dictates that when pulling three or more current-carrying conductors (plus a ground) into a single raceway, the wires cannot occupy more than 40% of the conduit's internal cross-sectional area. The table below maps the exact minimum trade sizes based on NEC Chapter 9, Table 4 fill capacities.

Wire Configuration (Hots+Neu+Gnd) Conduit Material Min Trade Size Max 40% Fill Area Actual Wire Area
3x 4/0 Al + 1x #2 Al PVC Schedule 40 2 inch 1.153 sq in 1.087 sq in
3x 4/0 Al + 1x #2 Al PVC Schedule 80 2.5 inch 1.482 sq in 1.087 sq in
3x 4/0 Al + 1x #2 Al EMT (Steel) 2 inch 1.349 sq in 1.087 sq in
3x 2/0 Cu + 1x #4 Cu PVC Schedule 40 1.5 inch 0.794 sq in 0.778 sq in
3x 2/0 Cu + 1x #4 Cu EMT (Steel) 1.5 inch 0.814 sq in 0.778 sq in

Worked Numeric Example: Calculating the Fill

Let's break down the math for the most common 100-amp service configuration today: 4/0 AWG Aluminum THHN. We rely on NEC Chapter 9, Table 5 for the exact cross-sectional area of the insulated wires.

  1. Calculate the Hot and Neutral Wires: A single 4/0 AWG THHN wire has an area of 0.3237 square inches. Multiply this by three (two hots, one neutral) to get 0.9711 sq in.
  2. Calculate the Ground Wire: A #2 AWG THHN aluminum ground has an area of 0.1158 square inches.
  3. Total Wire Area: 0.9711 + 0.1158 = 1.0869 sq in.
  4. Apply the 40% Rule: Because we have four wires in the pipe, NEC Chapter 9, Table 1 limits us to 40% fill.
  5. Select the Conduit: Looking at Table 4, a 2-inch PVC Schedule 40 pipe has a 40% fill capacity of 1.153 sq in. Since 1.0869 is less than 1.153, the 2-inch pipe is legally and physically compliant.
The Schedule 80 Trap: If your utility or local AHJ requires Schedule 80 PVC where the conduit emerges from the trench (for physical damage protection), a 2-inch Schedule 80 pipe only has a 40% fill of 0.916 sq in. Your 1.0869 sq in of wire will not fit. You must use a 2.5-inch Schedule 80 sweep at the bottom and a 2.5-inch to 2-inch reducer, or run 2.5-inch all the way to the meter base.

Where You Meet This in Practice

Understanding conduit fill isn't just about passing an inspection; it fundamentally alters the physics of your installation. What it changes in a real installation is the pulling tension and the thermal dissipation profile of the circuit. When you pull 4/0 aluminum through a 2-inch pipe, the wires have enough air gap to shed heat, and the sidewall pressure on the wire insulation remains low. If you attempt to undersize the conduit, the pulling compound strips off, the THHN insulation micro-tears against the PVC edges, and you risk a dead short the moment the utility energizes the meter. Furthermore, tightly packed wires trap heat, which forces you to apply NEC Article 310.15 ampacity derating factors, potentially dropping your 100A wire below its required capacity.

What people commonly confuse it with is the difference between nominal 'trade size' and actual internal diameter. In the electrical trade, a '2-inch' pipe does not have a 2.000-inch internal diameter. The term 'trade size' is a historical plumbing nominal measurement. A 2-inch PVC Schedule 40 pipe has an actual ID of roughly 2.047 inches, while a 2-inch Schedule 80 has a much thicker wall, yielding an ID of only 1.913 inches. This is exactly why the NEC provides fill tables based on square inches rather than simple diameter ratios—you cannot eyeball it based on the outside of the pipe.

In the field, you will meet this calculation when trenching a 24-inch deep line from a utility transformer to a house meter base, or when bending an above-ground EMT mast through a roof eave. Always use a fiberglass fish tape and a high-quality pulling lubricant like Polywater J when feeding 4/0 aluminum; the stiffness of the wire requires mechanical advantage, and a properly sized 2-inch conduit gives you the physical room to maneuver the wires around the 90-degree sweeps.

Installation Edge Cases and Code Caveats

Can I use 1.5-inch PVC if I only pull 3 wires and use the conduit as the ground?

No. First, using PVC (a non-metallic raceway) as an equipment grounding conductor is impossible. If you are using metallic conduit (like Rigid Metal Conduit) as the ground path, you still have three 4/0 AWG wires. Three wires equal 3 x 0.3237 = 0.9711 sq in. A 1.5-inch PVC Schedule 40 pipe maxes out at 0.794 sq in for 40% fill. Even with just three wires, 4/0 aluminum will not legally fit inside a 1.5-inch pipe. You must use 2-inch.

What if I am using SER cable instead of individual THHN wires?

SER (Service Entrance Round) cable is typically used for above-ground, indoor feeder runs (like from a main panel to a subpanel) and is not pulled through long underground conduit runs. However, if you are sleeving SER cable through a short section of conduit for physical protection, the NEC treats the entire cable as a single conductor. For a single conductor, you are allowed 53% fill. You must measure the outer diameter of the specific SER cable brand you bought, calculate its area, and size the sleeve accordingly. Usually, a 2-inch or 2.5-inch sleeve is required for 100A SER cable due to the bulky outer jacket.

Do I need to account for the 90-degree sweeps at the panel and meter?

Yes. NEC Chapter 9 limits you to no more than 360 degrees of total bends between pull points. For a 100-amp service, the factory 90-degree sweeps included on meter bases and panel knockouts count toward this limit. If your run is complex, use large-radius sweeping elbows rather than standard tight bends to reduce the pulling tension on the 4/0 aluminum. Always verify your local utility's specific metering requirements, as some utilities mandate a specific conduit trade size for their service lateral regardless of the NEC minimums.

For further verification on your specific wire brand and insulation type, cross-reference your calculations with the Southwire Conduit Fill Calculator or consult the latest edition of the NFPA 70 National Electrical Code. Proper sizing ensures your 100-amp service remains safe, cool, and upgradeable for decades.