This voltage drop calculator helps you size conductors and check percent voltage drop before you pull wire. Enter the supply voltage, load current, one-way run length, and conductor material (copper or aluminum) and get the drop in volts (V) and percent drop (%). You’ll also see a quick wire size suggestion as a starting point.
Common field practice in the U.S. follows the “3% per branch, 5% total” guideline (NEC informational note). Always verify with the code and local requirements.
Contents
Table of Contents
Voltage Drop Calculator (Single / Three-Phase)
Enter voltage, current, one-way length and material to see voltage drop (%), end voltage, and a suggested wire size.
Result
- Wire size: !
- Next size up:
- Voltage drop: V
- Percent: %
- End voltage: V
- Loop resistance: Ω
- Power loss: W
- Max distance:
Heads-up: current exceeds a typical 75°C ampacity for this size.
Info: Recommended voltage drop: ≤3% branch, ≤5% total (NEC 210.19(A)(1), 215.2(A)(1) Informational Notes (3% branch, 3% feeder, 5% total)).
Education only. Verify against field conditions. © Engineer Tsai
What is voltage drop and why it matters
Voltage drop happens because every conductor has resistance; as a result, the longer the run and the higher the current, the more voltage you lose before the load. Consequently, lights may dim, motors may struggle to start, and EV chargers may ramp down. In practice, keeping the drop within targets protects equipment and improves performance. Therefore, most U.S. designers follow the “3% per branch, 5% total” guideline from NEC’s informational note. Even so, always confirm with your adopted code and local amendments.
How to use Voltage Drop Calculator
First, choose Single-phase or Three-phase. Next, set the supply voltage—120/240 V for residential or 208/240/277/480 V for commercial jobs. Then, enter the expected load current (A). After that, type the one-way length from source to load; the tool handles loop length internally. Meanwhile, select Copper (Cu) or Aluminum (Al). Additionally, pick an allowed percent drop—3% is a common branch target, while 5% covers feeder plus branch. Finally, click Calculate to get Voltage drop (V) and Percent drop (%). If the result exceeds your limit, try a larger AWG, shorten the run, raise system voltage where allowed, or split the load. For example, a 120-V, 15-A lighting run at 140 ft often needs #12 Cu even if #14 passes ampacity.
When to upsize wire or adjust the design
- Near the limit – if the result shows 2.8–3.2%, bump the conductor up one AWG or consider three-phase where available.
- Motor/EV/heavy start – keep branch drop under 3% (or even lower) to protect starting torque and reduce flicker.
- Long exterior runs – driveway lighting, detached garages, docks: upsizing wire often beats fighting callbacks.
- Feeder + branch combo – check both stages; a “good” branch can still fail if the feeder already used up the margin.
Design & safety notes with Voltage Drop Calculator
Passing a voltage-drop check doesn’t guarantee ampacity or temperature rise compliance. Always verify: installation method, ambient temperature, grouping/derating, termination ratings, and OCPD coordination. Aluminum terminations require listed lugs and correct torque. Three-phase uses a different coefficient than single-phase; this calculator applies standard engineering approximations for a fast check.
Examples you’ll see in the field about Voltage Drop Calculator
- 120 V, 15 A lighting run, 140 ft – #14 may pass ampacity but miss 3% drop; #12 often fixes it.
- 240 V, 40 A EV charger, 90 ft – borderline on #8 Cu? Upsize to #6 Cu or shorten the route.
- 277/480 V fan at 300 ft – three-phase geometry helps, but you’ll still likely bump a size for 3% targets.
Extended reading & internal resources
- Panel upgrades & breaker selection
- AFCI/GFCI protection basics →
/afci-gfci-guide - Copper vs Aluminum wire size & ampacity →
/wire-size-ampacity
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References about Voltage Drop Calculator
- Voltage drop
- Engineering Toolbox — Copper/Aluminum resistivity tables
- NEC “3%/5%” design guidance (informational note; check your adopted code)
Disclaimer: This Voltage Drop Calculator is for education and pre-design checks. Final conductor sizing must follow the NEC and local amendments, equipment listings, and the project engineer/authority having jurisdiction.about Voltage Drop Calculator
Next steps: turn this check into a real-world plan
If this voltage drop check saved you some guesswork, the next step is to turn it into a repeatable checklist.
👉 Download the free “Home circuit quick check” (Move-in Lite) – a 30-minute walk-through you can print or keep on your phone.
When you’re ready for full-room coverage and documentation templates, you can upgrade to the Move-in Pro kit.
FAQ
Q1. What does “3% branch, 5% total” really mean?
A. It’s a common design guideline: try to keep voltage drop on any single branch circuit within about 3%, and keep the total drop from service to the farthest outlet (feeder + branch) within about 5%. It’s an informational note, not a hard code rule—always follow your adopted electrical code and local amendments.
Q2. Should I enter one-way distance or round-trip distance?
A. This calculator expects one-way run length from source to load. It automatically accounts for the round-trip path of the current when computing loop resistance, so you don’t need to double the distance manually.
Q3. Which voltage do I enter for three-phase circuits?
A. For single-phase, just enter the nominal line voltage (120 V, 240 V, 230 V, etc.).
For three-phase, choose the correct option in the “3φ load type” field:
Line-to-line (L-L) for 208/240/480 V loads
Line-to-neutral (L-N) for 120/230 V three-phase four-wire systems
The tool adjusts the geometry factor internally based on this choice.
Q4. How do I choose a good “allowed % drop” value?
A. For most lighting and receptacle circuits, designers often target around 2–3% on the branch circuit. Long feeders or sensitive equipment may need even tighter limits. Some heavy-duty loads (motors, EV chargers, welding machines) may tolerate a bit more drop, but check the manufacturer’s recommendations and your local code first.
Q5. Can I select wire size directly from this result and skip ampacity tables?
A. No. The suggested size is based mainly on voltage drop, with a friendly ampacity warning. Final conductor selection must still respect ampacity, insulation rating, temperature, installation method, correction factors, and any local rules. Treat this calculator as a pre-design check, not a replacement for official tables.
Q6. How accurate is the “include reactance & PF” option?
A. With the option turned off, the tool uses a resistive approximation that’s usually fine for short runs and typical branch circuits. Turning it on adds a simplified reactance model plus power factor, which is more representative for long runs and motor loads. It’s still an engineering estimate—always verify with detailed design when you’re close to limits.
Q7. Can I use this calculator for aluminum conductors or underground cable?
A. Yes, you can choose Copper (Cu) or Aluminum (Al) in the material field; the resistivity values are adjusted accordingly. However, underground runs and special cable types may have different thermal conditions and installation rules, so always cross-check with the cable manufacturer’s data and local code.
Q8. Does this tool support both AWG and mm²? How should I read the result?
A. You can either let the tool auto-suggest a size or enter a specific mm² or AWG value. Results are shown in a dual format like “12 AWG (≈ 3.3 mm²)” so you can match either U.S. or metric conventions. When in doubt, choose the next larger standard size and verify ampacity from your preferred table.