You’re running a 100 amp subpanel to a garage, workshop, or ADU and need to know exactly what gauge wire to buy. This calculator checks both NEC ampacity and voltage drop for your distance — so you don’t end up with wire that passes the ampacity table but still causes voltage sag at 150 feet.
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Formulas: NEC 2023 Table 310.16 & Chapter 9 Table 8 — April 2026
Enter your run distance, voltage, and conductor material. The calculator checks NEC ampacity and 3% voltage drop — and recommends the larger of the two.
Feet from main panel to subpanelEnter distance (1–500 ft).
Most subpanels are 240V
Aluminum costs ~55% less at this size
Continuous loads need 125% ampacity per NEC 210.19
3% = 7.2V on a 240V circuit
Already have a gauge in mind? Enter it below to see the exact voltage drop percentage at your run distance.
Select the gauge you plan to useSelect a gauge.
One-way feet from panelEnter distance (1–500 ft).
Aluminum has higher resistance
240V for most subpanelsSelect voltage.
Recommended Wire
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NEC 2023 Table 310.16
Voltage Drop0%
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⚠️ Safety notice: This calculator is for reference and planning only. Wire sizing for 100 amp service involves life-safety considerations. Always verify against your locally adopted NEC edition, obtain required permits, and have work inspected by your Authority Having Jurisdiction (AHJ). Hire a licensed electrician for panel work and service upgrades. NEC is a registered trademark of NFPA.
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Sources & Methodology
All wire sizing values and formulas in this calculator are derived from the following authoritative sources:
NFPA 70: National Electrical Code (NEC) 2023 — Table 310.16 (ampacity of insulated conductors), Table 250.122 (minimum size of equipment grounding conductors), Article 210.19(A)(1) (125% continuous load rule), Chapter 9 Table 8 (conductor properties for voltage drop).
nfpa.org
Cerrowire Ampacity Charts — Ampacity reference tables based on NEC Table 310.16, verified against manufacturer published data for THHN/THWN conductors.
cerrowire.com
WireRef — NEC 2023 Wire Size for 100A Circuit — Pre-computed max-distance table from NEC Chapter 9 Table 8 resistance values; used to cross-check voltage drop outputs.
wireref.com
Formulas used: Ampacity check from NEC Table 310.16 at 75°C (standard termination limit). Continuous load adjusted to 125% per NEC 210.19(A)(1). Voltage drop: VD% = (2 × K × I × L) / (CM × V) × 100, where K=12.9 copper / 21.2 aluminum (per NEC Ch 9 Table 8 K-values), I=100A, L=one-way feet, CM=circular mil area, V=system voltage. Recommended AWG is the larger of: ampacity-required gauge and voltage-drop-required gauge.
What Size Wire Do You Actually Need for 100 Amp Service?
Most people searching this question expect a single answer: “3 AWG copper, done.” That’s correct for a short run — but the moment your subpanel is 100 feet away, that answer gets you wire that is technically legal for ampacity but fails the voltage drop test at full load. Getting this wrong shows up as dim lights in the garage, tools that don’t run right, and tripped breakers under loads that should be fine. It’s not dangerous in the same way that undersized ampacity is, but it’s expensive and frustrating to rewire after the fact.
The NEC approach to 100 amp wire sizing has two separate requirements that both have to be satisfied:
Ampacity check (NEC Table 310.16): The conductor must be rated to carry at least 100 amps continuously — 125 amps if the load is continuous per NEC 210.19(A)(1).
Voltage drop check (NEC Chapter 9 FPN): The voltage lost across the conductor at full load should not exceed 3% for feeders (7.2V on a 240V circuit).
The right wire gauge is whichever one is larger after you run both checks. At short distances, ampacity drives the size. At longer distances, voltage drop takes over and forces a bigger gauge than the ampacity table alone would suggest.
The Worked Example First: 100A Subpanel 150 Feet Away
Let’s size a 100 amp copper feeder to a workshop 150 feet from the main panel at 240V — a very common real-world scenario. The load is continuous (workshop tools running for hours).
Step 1 — Ampacity requirement: Continuous load, so 125% rule applies. Required ampacity = 100A × 1.25 = 125 amps minimum. From NEC Table 310.16 at 75°C: 3 AWG copper = 100A (too small), 2 AWG = 115A (still too small), 1 AWG = 130A (passes). So ampacity alone says you need 1 AWG copper.
Step 2 — Voltage drop check: Using VD% = (2 × 12.9 × 100 × 150) / (83,690 × 240) × 100. For 1 AWG copper (83,690 CM): VD% = 387,000 / 20,085,600 × 100 = 1.93%. That’s well under 3%. So 1 AWG copper works for both requirements.
If the load were non-continuous and we only had to meet ampacity, 2 AWG (115A) would be the minimum. The continuous load rule pushes it up one size. Use the calculator above — it runs both checks automatically and shows you which constraint is driving the size.
⚡ NEC Wire Sizing Formulas — 100 Amp Service (Verified NEC 2023)
Ampacity Required (non-continuous) = 100AAmpacity Required (continuous, NEC 210.19) = 100A x 1.25 = 125AVoltage Drop % = (2 x K x I x L) / (CM x V) x 100 K = 12.9 (copper) | 21.2 (aluminum) per NEC Ch.9 Table 8 I = load current (100A) | L = one-way feet CM = circular mil area of conductor | V = system voltageMax VD = 3% of V = 7.2V on 240V | 3.6V on 120V (NEC recommended)
The table below gives you the minimum AWG recommended at common run distances for a 100 amp 240V feeder — accounting for both the 125% continuous load rule and the 3% voltage drop limit. These are starting points. Always run the full calculation for your specific installation, especially if ambient temperature exceeds 30°C or you have more than 3 conductors in a conduit.
Run Distance
Copper (THHN/THWN)
Copper Ampacity
VD% at Full Load
Aluminum (THHN/XHHW)
Aluminum Ampacity
0–50 ft
2 AWG
115A
under 1%
1/0 AWG
120A
50–100 ft
2 AWG
115A
1.3–2.6%
1/0 AWG
120A
100–125 ft
1 AWG
130A
2.4–3.0%
2/0 AWG
135A
125–175 ft
1 AWG
130A
under 3%
2/0 AWG
135A
175–225 ft
1/0 AWG
150A
under 3%
3/0 AWG
155A
225–275 ft
2/0 AWG
175A
under 3%
3/0 AWG
155A
275–350 ft
2/0 AWG
175A
verify VD
4/0 AWG
180A
The orange rows mean voltage drop is becoming the controlling factor — you’re going bigger than the ampacity table alone would require. The red rows are long runs where it’s worth questioning whether a 100 amp service is the right solution at all (a closer subpanel or a separate service might be more cost-effective than paying for 2/0 copper).
Aluminum wire for 100 amp runs costs roughly 50–65% less than copper at these gauges. The downside is that you need AL/CU rated lugs at every termination, and you must apply anti-oxidant compound to every connection. Skip those two steps and aluminum corrodes at the terminations, which is how aluminum wiring got its bad reputation in the 1960s. Done correctly, aluminum feeders are completely reliable — they’re standard in commercial construction.
Ground Wire Size for 100 Amp Service
The equipment grounding conductor (EGC) is sized separately from the current-carrying conductors. Per NEC Table 250.122, a 100 amp circuit requires a minimum 8 AWG copper or 6 AWG aluminum grounding conductor. This doesn’t change based on the distance — the EGC doesn’t carry load current, so voltage drop isn’t a factor. For detached structures, NEC 250.32(A) requires a separate grounding electrode system at the building: two 8-foot ground rods driven at least 6 feet apart, or a single rod supplemented by another electrode type.
Conduit Size for 100 Amp Wire
Conduit fill can’t exceed 40% of the conduit’s cross-sectional area (NEC Chapter 9, Table 1) when running three or more conductors. For a 4-conductor 100 amp feeder (2 hots + neutral + ground) using 2 AWG THHN copper, a 1-inch EMT conduit is sufficient. If you step up to 1 AWG or 1/0, use 1.25-inch EMT. For 2/0 or larger, 1.5-inch conduit. Underground runs typically use Schedule 40 PVC with USE-2 or XHHW-2 rated conductors — direct burial without conduit is also allowed with appropriate cable types like UF-B, but conduit makes future replacements far easier.
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Detached garage rule: Running 100 amp service to a detached garage or workshop requires four conductors — two hots, one neutral, and one dedicated ground. The neutral and ground are not bonded at the subpanel (they’re only bonded at the main service panel). The detached building needs its own grounding electrode system. Many homeowners and some older electricians skip this step, which creates a shock hazard if there’s ever a fault on the feeder. Per NEC 250.32(B)(1), failing to isolate the neutral at the subpanel is one of the most common residential electrical code violations.
Copper vs Aluminum — Which Should You Actually Choose for 100 Amps?
Here’s the honest comparison most guides bury: aluminum wins on cost for 100 amp feeders if you’re using a licensed electrician who knows how to terminate it. Copper wins if you’re doing it yourself and you’re not completely confident about torquing every connection to spec. The stakes are different than they were with the thin aluminum branch circuit wiring of the 1960s and 70s — 1 AWG or larger aluminum for a feeder is much more forgiving than #12 aluminum was in bedroom outlets.
When Copper Makes More Sense
Short runs under 75 feet where the cost difference is small (<$100)
DIY installation where you want maximum forgiveness at connections
Local code or utility requirements specifying copper (some jurisdictions still do)
Connections to devices without clear AL/CU rating
When Aluminum Makes More Sense
Long runs over 100 feet where cost difference is $200–$400+
When the panel lugs and breakers are already AL/CU rated (most modern panels are)
When a licensed electrician is doing the work and will torque connections properly
Underground feeders where lighter weight is a handling advantage
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What most people get wrong: They size the wire based on ampacity alone and ignore voltage drop. A 100 amp circuit “should” work with 3 AWG copper per NEC Table 310.16. But at 100 feet on a 240V circuit, 3 AWG produces about 3.2% voltage drop at full 100A load — slightly above the 3% NEC recommendation. Your tools will still work, but you’ve built in a performance margin you could have avoided for the cost of going up one wire size. The calculator above does this check automatically, which is why it exists.
❓ Frequently Asked Questions
Minimum 3 AWG copper or 1 AWG aluminum for short runs under 50 feet per NEC 2023 Table 310.16 at 75 degrees C. For runs 50-100 feet, use 2 AWG copper or 1/0 AWG aluminum to satisfy both ampacity and voltage drop. For 100-175 feet, use 1 AWG copper or 2/0 AWG aluminum. The calculator above tells you the right size for your exact distance.
No. 4 AWG copper has only 85A ampacity at 75 degrees C per NEC Table 310.16, which is below the 100A requirement. It will overheat under sustained 100 amp loads and may cause the 100 amp breaker to trip or, worse, the wire insulation to degrade without tripping the breaker if the load is just under 100A. You need minimum 3 AWG copper, or 2 AWG if the load is continuous or the run is over 50 feet.
For a 100 amp subpanel 100 feet away at 240V, use 1 AWG copper or 1/0 AWG aluminum. At 100 feet, 3 AWG copper produces about 3.2% voltage drop at full load, which exceeds the NEC 3% limit. 2 AWG copper gives about 2.5% drop and passes, but if the load is continuous (125% rule), you need 1 AWG copper (130A ampacity) regardless. The calculator checks both constraints for you.
Yes - aluminum is widely used for 100 amp feeders and is standard in commercial construction. Use minimum 1 AWG aluminum (100A at 75 degrees C). You must use AL/CU rated lugs, apply anti-oxidant compound to all connections, and torque every termination to manufacturer spec. Done correctly, aluminum feeders are completely reliable and cost 50-65% less than copper at these sizes.
NEC 210.19(A)(1) requires that conductors for continuous loads (running 3+ hours) must have ampacity of at least 125% of the load. For a 100A continuous load, the conductor needs at least 125A ampacity - which rules out 3 AWG copper (100A) and requires minimum 2 AWG copper (115A) or better 1 AWG (130A). Most subpanel feeders serve loads that run continuously, so this rule almost always applies.
For 4-conductor 100 amp feeder using 2 AWG THHN copper, use 1-inch EMT or Schedule 40 PVC. For 1 AWG or 1/0 conductors, use 1.25-inch. For 2/0 or larger, use 1.5-inch. NEC Chapter 9 requires conductors occupy no more than 40% of conduit cross-sectional area. Underground runs use Schedule 40 PVC at 24-inch minimum depth, or rigid conduit at 18 inches per NEC 300.5.
For a detached garage with 100 amp subpanel, use 2 AWG copper or 1/0 AWG aluminum for runs 50-100 feet, which covers most garage installations. Install 4 conductors - two hots, one neutral, one ground. Bury direct burial rated conductors (USE-2, XHHW-2) in Schedule 40 PVC conduit at 24 inches deep minimum. Add a grounding electrode system at the garage: two 8-foot copper ground rods, 6 feet apart minimum.
The equipment grounding conductor (EGC) for a 100 amp circuit is 8 AWG copper or 6 AWG aluminum per NEC Table 250.122. The ground wire does not need to match the hot conductors in size. For detached structures, NEC 250.32(A) requires a separate grounding electrode at the building - two 8-foot ground rods at least 6 feet apart - in addition to the ground conductor in the feeder.
4 wires, always, under current NEC. You need two insulated hot conductors, one insulated neutral, and one equipment ground. Three-wire feeders to detached buildings were allowed under pre-2008 NEC but are no longer code-compliant. The neutral and ground must be isolated from each other at the subpanel - they are only bonded at the main service panel. Bonding neutral and ground at the subpanel creates a shock hazard and is one of the most common code violations found during home inspections.
For 100 amp service 200 feet away at 240V (continuous load), use 1/0 AWG copper or 3/0 AWG aluminum. At 200 feet, voltage drop becomes the controlling factor. 1/0 AWG copper (150A ampacity) at 200 feet produces about 2.5% voltage drop at 100A - within the 3% limit. At this distance and cost level, it's worth having an electrician run a proper load calculation to confirm 100 amps is actually what you need before you spend $800+ on conductors.
Service entrance conductors (meter to main panel) are sized per NEC Table 310.12, which allows slightly smaller conductors for dwelling units: 4 AWG copper or 2 AWG aluminum for 100A residential service entrance. Feeder conductors (main panel to subpanel) use NEC Table 310.16: minimum 3 AWG copper or 1 AWG aluminum. The distinction matters because some inspectors check which table applies to your specific conductor run.
Correct wire size doesn't prevent breaker trips - the breaker responds to current exceeding 100 amps, regardless of wire gauge. If it's tripping, your actual load is above 100 amps. Add up your connected equipment: a 240V EV charger uses 48A, a central AC unit 30-50A, an electric range 40-50A, an electric dryer 30A. Add those up and a 100A service fills up fast. A 100 amp service at 240V delivers 24,000 watts maximum - considerably less headroom than most people assume.