Wire Size for 100 Amp Sub Panel: Complete Guide 2026

Installing a 100-amp sub-panel requires #3 AWG copper or #1 AWG aluminum wire per NEC Table 310.16 for most residential installations. However, distance matters significantly — runs longer than 50 feet may require upsizing to #2 AWG copper or #1/0 aluminum to maintain acceptable voltage drop under 3%. Getting the wire size wrong creates safety hazards (undersized) or wastes money (oversized). This guide covers NEC-compliant wire sizing, voltage drop calculations, conduit sizing, and practical installation tips for every common 100-amp sub-panel scenario.

Wire Size Requirements for 100 Amp Sub Panel

NEC Table 310.16 rates #3 AWG copper THHN at 100A for the 75°C column, which applies when terminated at standard residential equipment rated for 75°C. This is the minimum wire size for a 100-amp sub-panel feeder up to approximately 50 feet. The 75°C column is standard for residential because most modern breaker panels and breaker lugs are rated for 75°C terminations.

For runs exceeding 50 feet, voltage drop becomes the determining factor rather than ampacity. The NEC recommends (Article 210.19 Informational Note No. 4) a maximum 3% voltage drop on feeders and 5% total from service entrance to the farthest outlet. At 100 amps on #3 AWG copper over 75 feet at 240V: voltage drop = (2 × 75 × 100 × 0.000259) ÷ 1 = 3.89V (1.62%) — acceptable for the feeder alone but leaves limited headroom for branch circuits.

Aluminum wire is the cost-effective choice for 100-amp sub-panel feeders. #1 AWG aluminum costs approximately 40% less than #3 AWG copper while providing equivalent 100A ampacity. Aluminum requires AL-CU rated breakers and connectors, anti-oxidant compound on all connections, and proper torque values. Modern aluminum wire (AA-8000 alloy) is significantly safer and more reliable than the AA-1350 aluminum used in problematic 1960s-70s wiring.

The feeder to a sub-panel requires four conductors: two hot conductors (carrying 240V split-phase power), one neutral conductor, and one equipment grounding conductor. The neutral and ground must be separate and isolated in the sub-panel — the neutral bus and ground bus must NOT be bonded at the sub-panel. This is different from the main panel where neutral and ground are bonded together.

SER (Service Entrance Rated) cable is a popular choice for residential sub-panel feeders. SER cable bundles 3 insulated conductors plus a bare ground in a flat configuration that is easy to route through walls and along joists. #1 AWG aluminum SER cable for a 100A sub-panel costs approximately $3-$5 per foot versus $7-$12 per foot for #3 AWG copper THHN in conduit.

Voltage Drop Calculator for 100 Amp Feeders

Voltage drop on a 100-amp feeder is calculated using: VD = (2 × Distance × Current × Resistance per foot) for single-phase 240V circuits. For #3 AWG copper at 100A over 100 feet: VD = 2 × 100 × 100 × 0.000259 = 5.18V on 240V = 2.16%. This is within the 3% NEC recommendation for feeders.

At the same 100-foot distance with #1 AWG aluminum: VD = 2 × 100 × 100 × 0.000266 = 5.32V on 240V = 2.22%. Aluminum has slightly higher resistance per foot than copper of equivalent ampacity, so voltage drop is comparable but marginally worse. For most residential distances, the difference is negligible.

The 3% feeder voltage drop recommendation becomes binding at longer distances. At 150 feet on #3 AWG copper: VD = 7.77V (3.24%) — exceeds 3%. Upsizing to #2 AWG copper: VD = 6.16V (2.57%) — acceptable. At 200 feet, even #2 AWG copper exceeds 3%: VD = 8.22V (3.42%), requiring #1 AWG copper: VD = 6.52V (2.72%).

The total voltage drop from the utility meter to the farthest outlet should not exceed 5%. If the feeder uses 2% of the budget, only 3% remains for branch circuits in the sub-panel. This is important because branch circuits in a detached garage or workshop may run an additional 50-75 feet from the sub-panel to the farthest outlet. The combined voltage drop must remain under 5% for adequate appliance performance.

Our voltage drop calculator at voltflow.net/voltage-drop-calculator handles these calculations automatically. Enter the wire size, distance, load amps, and voltage to get instant voltage drop results. The calculator also recommends the minimum wire size needed to stay under 3% for any given distance and load combination.

Conduit Sizing for 100 Amp Sub Panel Wire

When running individual THHN/THWN conductors, NEC Chapter 9 Table 4 specifies maximum conduit fill. For four #3 AWG THHN conductors (each 0.0973 sq in), total fill = 0.389 sq in. A 1-inch EMT conduit has 0.346 sq in available at 40% fill — too small. A 1-1/4 inch EMT (0.598 sq in at 40% fill) provides adequate space with room for conductor pulling.

For four #1 AWG aluminum THHN conductors (each 0.1562 sq in), total fill = 0.625 sq in. A 1-1/4 inch EMT (0.598 sq in at 40%) is marginal. A 1-1/2 inch EMT (0.814 sq in) provides comfortable fill. When in doubt, upsize the conduit — the incremental cost of larger conduit is minimal compared to the labor cost of pulling wire through a tight conduit.

PVC conduit (Schedule 40) is the standard for underground runs from the main panel to a detached building sub-panel. Minimum burial depth for PVC conduit containing electrical conductors is 18 inches under NEC Table 300.5. The conduit must include a proper draining sweep at each end to prevent water accumulation. Use PVC cement rated for electrical conduit — not plumbing PVC cement.

Liquidtight flexible metal conduit (LFMC) is useful for transitions between rigid conduit and equipment connections. A 2-3 foot section of LFMC at the sub-panel allows adjustment during installation without precise conduit stub-up measurements. LFMC is also required where vibration or movement is expected, such as on a separate structural foundation from the main building.

Expansion fittings are required by the NEC for PVC conduit runs exceeding 50 feet to accommodate thermal expansion and contraction. PVC expands approximately 4 inches per 100 feet over a 100°F temperature range. Without expansion fittings, the conduit can buckle, crack, or pull free from connections. Place expansion fittings every 50-75 feet on long outdoor or underground runs.

Sub Panel Location and Installation Requirements

NEC Article 408 governs panelboard installation requirements. The sub-panel must be mounted in an accessible location with a minimum 30 inches of width, 36 inches of depth (working space), and 78 inches of headroom in front of the panel. The panel should be mounted with the top breaker no higher than 79 inches (6 feet 7 inches) from the floor.

Sub-panels in detached buildings (garages, workshops, barns) require specific grounding. NEC Article 250.32 mandates a grounding electrode system at each separate building served by a feeder. This typically means driving a ground rod (or two ground rods 6+ feet apart) at the detached building and connecting them to the sub-panel grounding bus with #6 AWG copper.

The sub-panel must have a main breaker or main disconnect to serve as the disconnecting means for the building. While a main lug panel (no main breaker) is acceptable when the feeder breaker at the main panel serves as the disconnect, the NEC requirement for a disconnect within sight of the building entrance makes a main breaker panel the better choice for detached buildings.

Panel scheduling is essential for balanced loading and future expandability. A 100-amp sub-panel should have 20-24 spaces to accommodate current circuits plus future additions. Distribute loads across both bus bars (legs) to balance the phases. A well-planned panel leaves 4-6 spare spaces for future circuits such as EV chargers, additional tools, or lighting.

Feeder disconnect at the main panel uses a 100A two-pole breaker in the main panel. Ensure the main panel has physical space for the breaker and sufficient bus bar capacity. If the main panel is a 200A panel already heavily loaded, verify that the total connected load (including the sub-panel) does not exceed the panel rating. A load calculation per NEC Article 220 determines whether the main panel can support the additional sub-panel load.

Cost Breakdown for 100 Amp Sub Panel Installation

The total cost of a 100-amp sub-panel installation ranges from $400-$1,200 for DIY to $900-$2,800 for professional installation, depending primarily on the wire run distance and local labor rates. Wire is the largest material cost — #3 AWG copper THHN costs $7-$12 per foot for 4 conductors, while #1 AWG aluminum SER cable costs $3-$5 per foot.

DIY sub-panel installation is legal in many jurisdictions for homeowners working on their own property, but a permit and inspection are required. The inspection verifies proper wire sizing, grounding, neutral-ground separation, working clearances, and code compliance. Failed inspections require corrections before the panel can be energized — plan for a re-inspection fee of $50-$100 if needed.

Professional electrician rates for sub-panel installation range from $75-$150 per hour, with most installations taking 4-8 hours depending on complexity. The electrician handles permit acquisition, load calculations, installation, and inspection coordination. For detached building installations requiring trenching and underground conduit, additional labor of 2-4 hours is typical.

Cost-saving tips: Use aluminum SER cable instead of copper (saves 40-60% on wire cost). Purchase the sub-panel and materials yourself from a home improvement store (saves 15-25% versus contractor supply house markup). Get quotes from 3 electricians and compare labor rates. Schedule during shoulder season (spring/fall) when electricians are less busy and more competitive on pricing.

Avoid false economies that create problems: Do not undersize wire to save money — undersized wire is a fire hazard. Do not skip the grounding electrode at detached buildings — it is a code requirement and safety essential. Do not use a panel with insufficient spaces — adding more circuits later requires replacing the panel entirely. Pay the incremental cost for quality components and proper installation.

When to Choose 100A vs 200A Sub Panel

A 100-amp sub-panel is appropriate for: detached garages with lighting, receptacles, and 1-2 power tools; workshops with moderate tool loads; guest houses or in-law suites with standard appliances; pool houses with pump, lighting, and GFCI receptacles; home additions with 3-5 circuits. Most residential detached building needs are well-served by 100A.

A 200-amp sub-panel is needed for: large workshops with multiple 240V tools (welder, compressor, dust collector); buildings with EV charging plus other heavy loads; properties with electric heating in the sub-panel building; commercial or agricultural buildings; any building where total load calculation exceeds 80A continuous. The wire cost increase from 100A to 200A is significant — #2/0 copper versus #3 copper.

Future-proofing considerations often favor 100A even when current loads are modest. Installing a 100A sub-panel when only 40A is needed today costs only marginally more than a 60A panel but provides significant headroom for future additions. Adding an EV charger (40A), welder (50A), or additional circuits later is possible without a panel upgrade.

If the total potential load in the detached building exceeds 60A (including future planned loads), install a 100A sub-panel. If it exceeds 120A, install a 200A panel. The panel itself costs only $50-$100 more for 200A versus 100A — the real cost difference is in the wire. Make the panel decision based on future needs, and the wire decision based on current and near-term loads.

For the main panel feeding the sub-panel, verify available capacity. A 200A main panel with 150A of existing load has only 50A of spare capacity — insufficient for a true 100A sub-panel without demand diversity calculations per NEC Article 220. A load calculation determines the actual demand versus connected load, which is typically 40-60% of connected load due to demand diversity. Most 200A panels can support a 100A sub-panel when existing loads are reasonable.

Consult a licensed electrician for a formal load calculation if you are unsure whether your main panel can support a 100A sub-panel. The electrician calculates existing demand, adds the sub-panel demand, and determines whether the total falls within the main panel and service entrance capacity. This calculation is part of the permit process in most jurisdictions and costs $100-$200 as a standalone service.

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