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Voltage Drop Chart
NEC Formula for Long Runs

📅 March 2026 ⏱ 8 min read 🔌 Electrical / Wire Sizing

You ran 150 feet of 12 AWG to the garage subpanel and now the lights dim every time the compressor kicks on. That's voltage drop — and by the time you feel it, the wire is already in the wall. Run the formula below, or skip to the voltage drop calculator for any AWG and length.

Skip the Math — Use the Calculator

Single-phase, three-phase, and DC with NEC 3%/5% pass/fail and the next wire size up. This page is the formula and circular-mil chart.

Open Voltage Drop Calculator →

What Is Voltage Drop and Why Does It Matter?

Every conductor has resistance. When current flows through that resistance, some electrical energy converts to heat — and the voltage available at the far end of the run is lower than what left the panel. That loss is voltage drop, and it gets worse the longer the run and the higher the current.

For short runs under 50 feet or so, voltage drop is usually insignificant and not worth calculating. But run that same circuit 150 feet to a garage, barn, or shop subpanel, and suddenly you might be losing 6 or 8 volts before a tool even plugs in.

The real-world consequences aren't subtle:

Motors run hotter and wear out faster. An induction motor operating at 90% of rated voltage produces roughly 81% of rated torque — it has to work harder to do the same job, which means more heat and shorter lifespan.
Electronics reset or malfunction. Computers, PLCs, VFDs, and smart devices have tight voltage tolerance windows. Consistent undervoltage can cause lockups, data loss, and unpredictable behavior.
Lights dim noticeably. Incandescent and halogen fixtures are especially sensitive — 5% voltage drop produces a visible, annoying flicker every time a load cycles.
You waste energy. Power lost to conductor resistance turns into heat in the wire, not work at the load. You're paying for electricity that never reaches the equipment.

⚡ How Voltage Drop Works — Panel to Load

The NEC 3% and 5% Guidelines — What They Actually Mean

You'll see the "3 percent" and "5 percent" rules thrown around constantly in electrical work. Here's the honest version: these are recommendations, not requirements — at least for most installations.

NEC 210.19 Informational Note No. 4 states that branch circuit conductors should be sized to prevent voltage drop exceeding 3% at the farthest outlet, and that the combined feeder plus branch circuit drop shouldn't exceed 5%. NEC 215.2 says the same thing for feeders.

ℹ NEC Code Clarification Per NEC 90.5(C), Informational Notes are explanatory material and are not enforceable as code. An inspector cannot fail you solely for a 3.5% voltage drop — unless your local jurisdiction has adopted those notes as law, or unless the equipment manufacturer specifies a minimum voltage at its terminals (in which case NEC 110.3(B) makes it mandatory). Fire pump circuits under NEC 695.7 and sensitive electronic equipment under Article 647 have mandatory voltage drop limits.

So why follow them? Because the equipment cares, even if the inspector doesn't. Motors, VFDs, EVSE chargers, and most modern electronics have tight voltage windows. Operating consistently below those windows shortens service life and voids warranties. The 3%/5% guideline exists because it's where equipment starts to be affected — not because someone picked a random number.

Professional electricians generally target:

3% or less on the branch circuit alone
2% or less on feeders when the branch circuit needs the remaining 3%
Total 5% or less from the panel to the furthest outlet
2% or less for sensitive electronics, motors, and anything with a tight voltage rating

Skip the Math — Use the Calculator

Our free voltage drop calculator handles single-phase, three-phase, and DC circuits with NEC pass/fail results and next-size-up wire recommendations.

⚡ Open Voltage Drop Calculator →

How to Calculate Voltage Drop

The formula for single-phase and DC circuits is straightforward once you understand what each variable means:

📐 Voltage Drop Formula — Single Phase & DC

The "2" in the formula accounts for the round trip — current travels out through the hot conductor and returns through the neutral, so the effective conductor length is doubled. For three-phase circuits you use 1.732 (√3) instead, because the three phases share the return path.

The circular mils (CM) value comes from NEC Chapter 9, Table 8. Common values:

📋 Common Wire Sizes — Circular Mils (NEC Table 8)

Wire Size	Circular Mils	Max Amps (75°C)	Common Use
14 AWG	4,110	20A	15A lighting circuits
12 AWG	6,530	25A	20A general circuits
10 AWG	10,380	35A	30A circuits, long runs
8 AWG	16,510	50A	Dryers, ranges, feeders
6 AWG	26,240	65A	Subpanels, EVSE
4 AWG	41,740	85A	Large feeders
2 AWG	66,360	115A	Service entrance, large feeders
1/0 AWG	105,600	150A	Service entrance
2/0 AWG	133,100	175A	200A service (aluminum)
3/0 AWG	167,800	200A	200A service copper

Worked Example — 20A Circuit, 120 Feet to the Shop

Say you're running a 20-amp, 120V single-phase circuit to a detached shop 120 feet away. You plan to use 12 AWG copper.

Step 1 — Gather your variables: K = 12.9 (copper), I = 20A, L = 120 feet (one-way), CM = 6,530 (12 AWG)

Step 2 — Apply the formula: VD = (2 × 12.9 × 20 × 120) / 6,530 = 61,920 / 6,530 = 9.48 volts

Step 3 — Calculate percentage: 9.48 / 120 = 7.9% — way over the 3% recommendation

That's nearly 8 percent voltage drop on a circuit the code says should be under 3. Your tools will struggle and your lights will be visibly dim.

Upsize to 10 AWG: VD = (2 × 12.9 × 20 × 120) / 10,380 = 61,920 / 10,380 = 5.96 volts = 4.97%

Still over 3% but under the combined 5% threshold. Acceptable for most applications.

Upsize to 8 AWG: VD = (2 × 12.9 × 20 × 120) / 16,510 = 61,920 / 16,510 = 3.75 volts = 3.1%

Right at the 3% guideline. This is the wire to use.

📊 Wire Size Comparison — 20A, 120 Feet, 120V

The Electrician's Rule of Thumb (And Why It's Wrong)

You'll hear experienced electricians say "upsize one gauge every 100 feet" as a quick-and-dirty rule. It shows up constantly on Mike Holt forums and ElectricianTalk — some guys say every 75 feet, some say every 100.

Here's the honest reality: this rule only holds roughly true for 120V, 15A and 20A circuits. It completely falls apart for 240V circuits (where voltage drop is half the percentage for the same wire size and distance), for three-phase systems, for higher-current feeders, and for aluminum conductors. The 100-foot rule for a 120V/20A circuit doesn't apply at all to a 240V/30A circuit — which needs the calculation.

💡 Pro Tip — When to Always Run the Calculation Any time the one-way run length in feet approaches or exceeds the system voltage — 120 feet on a 120V circuit, 240 feet on a 240V circuit — do the math. It takes 30 seconds with a calculator and saves you from pulling the wrong wire. Long runs to detached garages, workshops, agricultural buildings, and outdoor panels are where this catches people most often.

Voltage Drop Quick Reference — Common Scenarios

📋 Maximum One-Way Run Length at 3% — Copper, 120V Single Phase

Wire Size	15A Load	20A Load	30A Load	Result
14 AWG	57 ft	43 ft	29 ft	Very limited range
12 AWG	92 ft	69 ft	46 ft	Short to moderate runs only
10 AWG	147 ft	110 ft	73 ft	Better — still limited at 20A+
8 AWG	234 ft	175 ft	117 ft	Solid range for detached buildings
6 AWG	370 ft	278 ft	185 ft	Long runs, subpanels
4 AWG	590 ft	442 ft	295 ft	Agricultural, large buildings

⚠ These Numbers Change Significantly for 240V Circuits On a 240V circuit, the same wire size can run roughly twice the distance for the same voltage drop percentage. A 12 AWG 20A circuit at 240V can run about 138 feet before hitting 3% — versus 69 feet at 120V. Always recalculate for your actual voltage.

Recommended Tools for Voltage Drop Work

🔧 What You Actually Need for This Job

Calculate first, verify after — here's what makes it happen

Top Pick

Klein Tools MM400 Auto-Ranging Multimeter

Verify actual voltage at the load after installation
Auto-ranging — no manual range setting
AC/DC voltage, current, continuity
Klein — trusted brand on job sites

View on Amazon → Field Essential

Klein Tools ET310 Circuit Breaker Finder

Trace circuits before pulling wire
Works through walls without cutting power
Transmitter + receiver included
Essential for long-run troubleshooting

View on Amazon → Wire Up

Southwire 12/2 NM-B Indoor Wire — 100ft

12 AWG — most common branch circuit size
SIMpull coating — easier pulling
UL listed, NEC compliant
Southwire — industry standard brand

View on Amazon →

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When Upsizing Wire Is the Right Call

There are scenarios where a professional electrician will upsize wire not because code requires it, but because it's the right installation. Here's when to reach for the next gauge up automatically:

Any run over 100 feet at 120V. Even at modest loads, these runs are fighting voltage drop from the start.
EV charger circuits. A Level 2 EVSE running at 48A continuous for hours on end needs every volt. Skimping on wire here shows up as slower charging and potential equipment lockouts.
Motor circuits. Every motor has a minimum voltage rating. Running a motor at 90% of rated voltage costs you in motor torque, temperature, and longevity.
Sensitive electronics rooms. Server rooms, CNC machines, medical equipment — anything with a tight voltage window. Design for 2% or less total drop.
Long agricultural runs. Barn circuits, irrigation pumps, poultry house equipment — these often run 200, 300, even 500 feet. This is where the calculation isn't optional.
Any run where future load increases are likely. If you're running wire in a new shop, run a size bigger than you calculate for today. Adding load later won't require a rewire.

FAQ Is voltage drop a code violation? For most general wiring, no. NEC 210.19 and 215.2 contain Informational Notes recommending 3% on branch circuits and 5% combined, but per NEC 90.5(C) these notes are not enforceable code requirements. An inspector cannot fail your work solely because voltage drop exceeds 3%. However, if equipment manufacturers specify minimum terminal voltage in their listing instructions, that becomes enforceable under NEC 110.3(B). Fire pump circuits (NEC 695.7) and sensitive electronic equipment circuits (Article 647) have mandatory voltage drop limits. What gauge wire for 100 feet at 20 amps on 120V? 12 AWG gives you 4.7% voltage drop at 100 feet — over the 3% branch circuit guideline. 10 AWG drops that to 2.96%, just under 3%. For a 20A circuit at 120V running 100 feet, use 10 AWG for voltage drop compliance. If the circuit is 240V, 12 AWG at 100 feet drops to 2.35% — well within limits. Does voltage drop affect breaker sizing? No. Breaker size is determined by conductor ampacity and the load, not by voltage drop. Upsizing wire for voltage drop is common — a 10 AWG wire on a 20A breaker is perfectly fine and legal. The breaker protects the conductor, and 10 AWG can handle well over 20A at standard conditions. How is voltage drop different for 240V versus 120V? The drop in volts is identical for the same wire, length, and current — but the percentage is half on a 240V circuit. Lose 4 volts on a 120V circuit and that's 3.3%. Lose the same 4 volts on a 240V circuit and that's only 1.67%. This is why 240V circuits can run significantly farther than 120V before hitting the percentage limits. Does aluminum wire change the voltage drop calculation? Yes. Aluminum has higher resistance than copper — the K factor is 21.2 for aluminum versus 12.9 for copper. This means aluminum wire of the same AWG has about 64% more voltage drop than copper over the same distance at the same current. Aluminum feeders need to be upsized (typically two AWG sizes larger than the equivalent copper) to keep voltage drop in check, on top of the standard upsize for ampacity. Can I use a larger wire to fix voltage drop after it's already installed? If the conduit is big enough, you can pull larger wire through an existing conduit. Run the conduit fill calculation first. If you're working with Romex in walls, that's a rewire — which is why getting it right before the drywall goes up matters. The correct approach for long-run jobs is to calculate before purchasing wire, not after the lights are dim.

Run Your Own Numbers

Our free voltage drop calculator handles single-phase, 3-phase, and DC circuits. Enter your wire size, run length, and load — get your drop percentage, pass/fail, and next-wire-size recommendation.

⚡ Open Voltage Drop Calculator → Data per NEC 210.19, 215.2, 90.5, Table 8, and Table 9. Always verify with current NEC and local code requirements. TestTalkHQ.com — free tools for tradespeople.