Design Dashboard
EV charging is where continuous-load treatment stops being theoretical.
⚡ Code Check Summary
Every line traces to a section of the NEC edition selected in the header.
💡 Insight
Adjust the controls to see how EVSE sizing propagates from branch circuit to service.
📊 Service Capacity
🧩 Service Load Breakdown (kVA)
→ Branch → Feeder → Service
◯ Conductor Size Comparison
Relative cross-section (circular mils). Highlighted conductor is the selected branch conductor.
📏 Continuous-Load Sizing Comparison
Loading of a conductor sized only at 100% of nameplate (non-compliant) versus the selected conductor sized on the 125% continuous basis. This is a sizing comparison, not a thermal prediction.
Non-compliant — sized at 100% of nameplate
—load / ampacity
Compliant — sized at 125% continuous
—load / ampacity
Educational disclaimer: This tool is for learning and preliminary design estimation only. It is not a substitute for a licensed Professional Engineer, manufacturer instructions, or your Authority Having Jurisdiction. Always verify against the published NEC edition enforced in your jurisdiction.
Results
Branch circuit · feeder aggregation · service entrance
⚡ Branch Circuit (per EVSE)
NEC 210.19(A), 210.20(A), 240.6(A), 625.40, 625.41
Nameplate / Rated Current
—Continuous Load (×1.25)
—Minimum OCPD
——
Min Required Ampacity
——
Selected Conductor
—Final Usable Ampacity
—After ambient & grouping derating, capped at terminal column
Equipment Grounding Conductor
—NEC 250.122 minimum, sized on OCPD rating
Voltage Drop
——
🔄 Feeder Aggregation
NEC 215.2(A)(1), 215.3, 220.40, 625.42
EVSE on Feeder
——
Aggregate EV Load
—Other Load on Feeder
—Feeder Design Load
—Feeder OCPD
——
Feeder Conductor
——
Total EV kVA
—🏢 Service Entrance
NEC 230.42(A), 220.40, 220.57 (2023) — demand on the service voltage/phase basis
Existing Demand
—EV Load Added
——
Total Calculated Demand
—Service Capacity
—Minimum Service Size
—Next standard rating ≥ calculated demand
Spare Capacity
—Utilization
—📈 Single vs. Aggregate EV Load
Amps on the EVSE voltage basis. Aggregate is the sum of each EVSE at 125%, before any load-management cap.
📉 Load Growth (kVA)
Dashed outline is service capacity. Bars turn red when demand exceeds it.
💰 Relative Conductor Cost Index
Relative index, not dollars. The window centers on the selected conductor; sizes with no published index show as N/A.
📏 Voltage Drop vs. Distance
Red dashed line marks the 3% branch-circuit recommendation from the NEC Informational Note. Voltage drop is advisory, not a code pass/fail.
🌡 Ampacity Derating Waterfall
How ambient correction, conductor grouping, and the terminal-temperature cap reduce usable ampacity for the selected conductor.
Calculation Trace
The code path matters. The answer did not appear by magic.
📋 Assumptions
🔢 Step-by-Step Calculation
| Step | Description | Calculation | Result |
|---|
Design Challenge
Design it. The system will tell you if you got it right.
✎ Your Design
Learn Mode
Article 625 in practice
Why EVSE sizing is different
Electric vehicle supply equipment runs at its maximum rating for hours at a time. The NEC classifies any load that operates for three hours or more as a continuous load, and Article 625 states outright that EV charging loads are continuous. That single sentence drives everything else on this page.
The 125% rule: Branch-circuit conductors and the overcurrent device must each be rated at not less than 125% of the continuous load (210.19(A) and 210.20(A) for branch circuits, 215.2(A)(1) and 215.3 for feeders). A 40A EVSE therefore needs a 50A circuit — not a 40A one.
- A single Level 2 charger adds 40–50A of continuous load — comparable to adding a second electric range that never turns off.
- The 125% margin exists for thermal headroom in conductor insulation and equipment terminations, not for future capacity.
- Multi-charger installations routinely overwhelm services that looked adequate on paper.
The sizing hierarchy
- Branch circuit: One dedicated circuit per EVSE (625.40). Size conductor and OCPD at 125% of the EVSE rated current.
- Feeder: Sum the connected EVSE loads at 125%, add any non-EV load, then size the feeder OCPD and conductors (215.2/215.3). The NEC provides no generic quantity-based diversity factor for EVSE.
- Service: Add the EV demand to the rest of the calculated load (Article 220) and compare with the service rating. Service conductors follow 230.42(A) — 125% of continuous plus 100% of non-continuous.
Load management is the sanctioned shortcut: Where a listed load-management / energy-management system limits the maximum EV load, the feeder and service may be sized to that controlled maximum instead of the raw sum (625.42). It requires listed equipment and AHJ acceptance — it is not an assumption you can make on paper.
Ampacity is not one number
Table 310.16 gives a base ampacity in three insulation columns. Two corrections and one cap stand between that number and what you may actually use:
- Ambient correction (310.15(B)): Applied in the insulation column. A 75°C conductor at 40°C ambient keeps only 88% of its rating.
- Grouping adjustment (310.15(C)): More than three current-carrying conductors in a raceway — 80% at 4–6, 70% at 7–9, 50% at 10–20.
- Terminal temperature cap (110.14(C)): 90°C wire terminating on 75°C lugs is limited to the 75°C column. The 90°C column is useful for derating, not for final ampacity.
Common mistakes
- Sizing at nameplate: A 48A EVSE on a 50A breaker with 8 AWG copper fails 210.19(A), 210.20(A), and 625.41. It needs 60A and 6 AWG.
- Inventing diversity: "They won't all charge at once" is not a code provision. Without a listed load-management system there is no reduction.
- Ignoring the service: The breaker fits; the service does not. Check both.
- Mixing materials carelessly: Aluminum needs a larger conductor for the same ampacity and drops about 64% more voltage per circular mil.
- Forgetting the terminal cap: Selecting from the 90°C column without checking 110.14(C).
- Skipping voltage drop on long runs: A 200 ft run at 40A on 6 AWG copper is already past 3%.
⚡ EV Charging Levels
| Level | Supply | Typical Current | Power | Typical Use |
|---|---|---|---|---|
| Level 1 | 120V 1φ | 12–16A | 1.4–1.9 kW | Overnight, cord-and-plug |
| Level 2 | 208–240V 1φ | 16–80A | 3.3–19.2 kW | Home, workplace, fleet |
| DC Fast | 480V 3φ input | 60–400A+ | 50–350+ kW | Highway, depot |
DC fast charging equipment is sized here on its AC input rating — the DC output side is internal to the listed equipment.
🧠 Knowledge Quiz
Score: 0 / 0
NEC References & Code Logic
Educational paraphrase — verify against your adopted edition
Reference Panel
📜 Article 625 — EV Power Transfer System
| Section | What it requires (paraphrased) |
|---|
🛠 Code Logic Implemented by This Tool
- Continuous load (Article 100, 210.19(A), 210.20(A), 215.2(A)(1), 215.3, 625.41): EVSE is a continuous load; conductors and OCPD at ≥125% of rated current.
- Standard OCPD ratings (240.6(A)): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400…
- Conductor protection (240.4): OCPD may not exceed conductor ampacity, except where 240.4(B) permits the next standard size up (≤800A, ampacity does not correspond to a standard rating, not a multi-outlet receptacle circuit).
- Small conductors (240.4(D)): Cu 14/12/10 AWG capped at 15/20/30A; Al 12/10 AWG capped at 15/25A.
- Ampacity (Table 310.16): 60/75/90°C columns, copper and aluminum.
- Ambient correction (310.15(B)) and grouping adjustment (310.15(C)), applied in the insulation column.
- Terminal temperature (110.14(C)): final usable ampacity capped at the terminal column.
- Equipment grounding conductor (250.122): minimum EGC from the OCPD rating.
- Feeder / service (215.2, 215.3, 220.40, 230.42(A)): aggregate EV load plus other load, sized on the continuous basis.
- Load management (625.42): where a listed system limits the maximum EV load, the feeder/service may be sized to that controlled maximum.
- Voltage drop (Informational Notes to 210.19(A) and 215.2(A)): 3% branch / 5% total recommendation — advisory, not enforceable.
🧪 Self-Test Suite
Verifies the calculation engine against hand-computed NEC examples. Also runnable from the console with
runSelfTests(), or by loading this page with #selftest.
Copyright notice: NEC® and National Electrical Code® are registered trademarks of the National Fire Protection Association. This tool paraphrases requirements for educational purposes and reproduces no copyrighted text. Verify every result against the published edition adopted by your AHJ.