Adding a Level 2 EV charger (7,200W), a hot tub (7,500W), or a workshop subpanel without checking the service can trip the main breaker on the first simultaneous run — or worse, overheat a panel that was already at 85% of its continuous rating. Electrical load is the total power (watts) all appliances and circuits draw, converted to amps to compare against the panel's capacity (e.g., 100A ×240V =24,000W) and each branch circuit's limit (e.g., 20A ×120V =2,400W, 1,920W continuous). For a typical 3-bedroom home with central AC (3,500W), electric dryer (5,000W), range (8,000W), lights and receptacles (3,000W), and an EV charger (7,200W continuous →9,000W for sizing), the connected load is 26,700W but the demand load per the National Electrical Code is 12,400W →7,440W at 0.6 diversity →31A at 240V, or 31% of a 100A service with 69% headroom. An electrical load calculator that sums appliance watts, applies NEC Article 220 demand and continuous-load factors, and shows panel and circuit percentages per NFPA 70 (NEC) 2023 turns the guess of "will the panel handle it?" into a number with a clear headroom for the next large load — before the electrician is called or the permit is pulled.
This complete, in-depth guide explains how to calculate electrical load for a home, a panel, or a single circuit — what load is per NEC Article 220 (Standard Method), the watts/volts/amps and volt-amps relationships, connected vs demand vs continuous, the NEC 80% rule (continuous loads at 125%), how to calculate step-by-step with a free tool, panel and circuit capacity checks (100A vs 200A, 15A vs 20A vs 30A), common home loads per NEC Table 220.14, safety and code notes (when to call a licensed electrician), and the next steps after the calculation — with references to NFPA 70 (NEC) 2023, NEC Article 210.19 (continuous), NEC Article 230.42 (service), NIST, and ESFI (Electrical Safety Foundation).
What Is Electrical Load? — Connected vs Demand vs Continuous
Electrical load is the power drawn at a moment, measured in watts (real power) or volt-amps (apparent power). For homes, three related numbers are used, and confusing them is the top cause of undersized or oversized estimates:
- Connected load: The sum if everything were on at once — e.g., AC 3,500W + dryer 5,000W + range 8,000W + lights 1,200W + receptacles 1,800W + EV charger 7,200W = 26,700W in the full example, or 12,400W in the trimmed example used for the panel calc. This is the worst-case ceiling and is always higher than the design load.
- Demand load: The realistic load with diversity — not all loads run simultaneously — per NEC Article 220 Standard Method. General lighting and receptacles at 3VA/sq ft with demand factors (100% first 3,000VA, 35% next 117,000VA, 25% remainder per 220.42), HVAC at the largest of heating or cooling (not both), and range/dryer per tables. For the example, 12,400W connected at 0.6 diversity + continuous and HVAC rules yields 7,440W demand. Demand is the number compared to the panel and is always less than connected — designing to connected would oversize the service by 40-60%.
- Continuous load: Any load running 3+ hours — EV charger, water heater, space heater, and many lighting loads — per NEC 210.19(A)(1) is calculated at 125% for sizing, or equivalently limited to 80% of the circuit/panel rating. A 7,200W EV charger is 9,000W for sizing (×1.25) and requires a 40A breaker for a 30A continuous draw (30×1.25=37.5A → next size 40A). Demand load already includes this 125% where applicable, so the panel comparison is demand vs 80% of panel rating for the continuous portion, or more simply demand (with continuous already at 125%) vs the panel's full rating where the demand calc was per NEC.
Per NIST and the NEC, power, voltage, and current are linked by Watts = Volts × Amps (for resistive loads; for inductive like motors/AC, Watts = Volts × Amps × Power Factor, PF≈0.8, so 1,000VA AC ≈800W). Rearranged: Amps = Watts ÷ Volts, which is how the calculator converts the demand watts to the amps that the panel and breakers are rated in.
Electrical Load Basics — Watts, Volts, Amps, VA, and the NEC 80% Rule (In Depth)
Watts (W) is real power — what does work and what the utility bills (kWh). Volt-amps (VA) is apparent power — Volts × Amps without power factor. For resistive loads (heaters, incandescent lights), Watts ≈ VA (PF≈1). For inductive loads (motors, AC compressors, some LED drivers), Watts = VA × Power Factor (PF≈0.8 for many motors), so a 1,000VA AC is ~800W and 1,250VA. The NEC uses VA for general lighting (3VA/sq ft) and for some appliance tables, so the calculator shows both and uses VA where the code does.
Common Voltages and Panel Voltages in U.S. Homes
| Circuit / Service | Volts | Capacity | Continuous Max (80%) | Typical Loads |
|---|---|---|---|---|
| 15A branch | 120V | 1,800W | 1,440W | Lights, receptacles |
| 20A branch | 120V | 2,400W | 1,920W | Kitchen, bath, microwave |
| 30A dedicated | 240V | 7,200W | 5,760W | Dryer, EV (30A) |
| 50A range | 240V | 12,000W | 9,600W cont. | Electric range |
| 100A service | 240V | 24,000W | 19,200W cont. | Small home, apartment |
| 200A service | 240V | 48,000W | 38,400W cont. | Most modern homes |
The service is 120/240V split-phase — two 120V legs that combine to 240V for large appliances. The panel's amp rating is per leg at 240V for the total, but branch circuits are 120V (one leg) or 240V (both legs). The calculator handles 120V vs 240V per appliance and sums correctly to the 240V panel total.
The NEC 80% Rule — Why Continuous Matters Most
Per NEC 210.19(A)(1) and 230.42(A)(1), continuous loads — defined as maximum current for 3+ hours — must be calculated at 125% for conductor and overcurrent sizing, which is equivalent to limiting the load to 80% of the circuit or service rating. A 20A×120V circuit is 2,400W, but the maximum continuous load is 1,920W; a 30A×240V EV circuit is 7,200W, but the continuous max is 5,760W, so a 30A continuous EV charger actually requires 30×1.25=37.5A → next standard breaker 40A. For the panel, a 100A×240V service is 24,000W, but the continuous max is 19,200W. The example demand of 7,440W includes the 125% where applicable (EV 7,200→9,000), so comparing 7,440W to 24,000W already reflects the continuous rule; comparing without the 125% would understate the load by 1,800W for the EV alone. This is why generic "add up watts" calculators that ignore continuous overstate headroom by 20-25% for EV and water heater homes.
How to Calculate Electrical Load — 3 Steps (Per NEC Article 220 Standard Method)
Step 1: List Appliances and Watts — Connected Load (Nameplate or NEC Table 220.14)
Enter each load's watts and volts from the nameplate (e.g., AC nameplate: 3,500W, 240V; dryer: 5,000W, 240V; range: 8,000W, 240V) or, if the nameplate is not accessible, from the calculator's appliance list which is pre-filled per NEC Table 220.14 and manufacturer data: general lighting and receptacles at 3VA per sq ft (per 220.14(J)), small-appliance and laundry circuits at 1,500VA each, dryer at 5,000W, range at 8-12kW per Table 220.55, EV charger at nameplate (7,200W for 30A Level 2), water heater at 4,500W, and HVAC at the larger of heating or cooling (per 220.50, not both, since they don't run simultaneously). For the example home (1,500 sq ft, AC 3,500, dryer 5,000, range 8,000, lights 3VA×1,500=4,500VA, two small-appliance 1,500VA each, EV 7,200, water heater 4,500), the connected sum is ~26,700W, but the trimmed example for the panel calc uses 12,400W to show a typical non-HVAC general load plus EV.
Connected load is the ceiling: 12,400W in the trimmed example is the worst case if every general load and the EV ran at once — it is always higher than the design load and should not be compared directly to the panel without demand factors, or the service will appear overloaded when it is not.
Step 2: Apply Demand and Continuous Factors — Demand Load (The Realistic Number)
Demand is connected with diversity and continuous handling per NEC Article 220 Standard Method:
- Continuous (3+ hours) ×1.25: EV charger 7,200W ×1.25 = 9,000W for sizing; water heater 4,500W ×1.25 = 5,625W if it runs 3+ hours (storage type often does). Non-continuous dryer 5,000W stays 5,000W.
- General lighting and receptacles demand: Per NEC 220.42, first 3,000VA at 100%, next 117,000VA at 35%, remainder at 25% — or the simpler 0.6 factor the calculator uses for the general portion when the detailed 220.42 table is not needed for a quick estimate. For the example, general loads at 0.6 and HVAC at largest only (heat or cool, not both, per 220.50) yield 7,440W demand from 12,400W connected.
- HVAC — largest only: If heating is 8,000W and cooling is 3,500W, only 8,000W is counted — they don't run together.
- Range and dryer tables: Per Table 220.55, a single range at 8kW is 8kW demand; multiple ranges have demand factors. The calculator applies the table for the entered range/dryer count.
For the example, the math is: general 4,500W (lights/receptacles) at 0.6 =2,700W, plus dryer 5,000W at 0.6? Actually dryer is at 100% per table for one, plus range 8,000W at 8,000W, plus EV 9,000W (continuous), plus the largest HVAC 3,500W — but the trimmed 12,400W example simplifies to general 12,400×0.6=7,440W as a blended demand for illustration. The calculator does the per-category math accurately; the 0.6 blended is for the quick example.
Demand < Connected — and that is correct: Demand is the realistic design load per NEC; connected is the worst-case sum that never occurs because not all loads run at once (diversity). Designing to connected would oversize the service by 40-60% and suggest a 200A upgrade where 100A is sufficient. The calculator shows both so the conservatism is visible: connected 12,400W, demand 7,440W — the 31% panel load is demand, not connected, and is the code-compliant number.
Step 3: Check Panel and Circuit — Demand Amps vs Capacity
Convert demand watts to amps: Amps = Watts ÷ Volts. At 240V panel voltage, 7,440W ÷240V =31A. On a 100A×240V=24,000W service, that's 31% loaded with 69% headroom; on a 200A×240V=48,000W service, 15% with 85% headroom. For a single branch circuit, check the same: a microwave at 1,200W on 120V is 10A; on a 20A×120V=2,400W circuit, that's 50% (or 12.5A and 62% as continuous at 1.25×). The calculator shows both the panel percentage and the per-circuit percentage for the selected appliance, so a new microwave can be checked against the 20A kitchen circuit before it is added to the panel total.
Add the next load and re-check: Add the EV charger at 7,200W (9,000W continuous) to the 7,440W demand → new demand ~12,000W? Actually the EV was already in the 12,400W connected, so the demand with EV as continuous is 7,440W already including the 9,000W EV portion weighted by diversity — the incremental EV addition to a home without an EV is roughly 7,200W connected and ~5,760W demand (if general diversity applies) or 9,000W if the EV is the dominant continuous load. The calculator recomputes: a home at 4,500W general demand plus a new 9,000W EV continuous → 13,500W demand → 56A at 240V → 56% of 100A — still safe but now over half. Without the demand factor, the same addition would appear as 11,700W and 49% — similar, but the demand calc is code-correct.
Panel and Circuit Capacity — Is There Headroom for the Next Load?
Panel Capacity (Whole Home) — 100A vs 200A
| Service | Capacity | Continuous Max (80%) | Example Demand 7,440W | Headroom |
|---|---|---|---|---|
| 100A ×240V | 24,000W | 19,200W | 31% loaded | 69% — safe for EV, not for EV + hot tub + workshop together |
| 200A ×240V | 48,000W | 38,400W | 15% loaded | 85% — ample for any single addition |
A 100A service is sufficient for many homes without electric heat and with gas dryer/range — 31% demand headroom is comfortable. A 200A service is standard for new construction and for all-electric homes with EV, electric range, dryer, and heat pump. The decision to upgrade from 100A to 200A ($2,000-4,000 including riser, panel, and permit) should be based on demand, not on "100A sounds small" — many 100A homes have 50%+ headroom and do not need an upgrade for a single 30A EV charger.
Circuit Capacity (Per Circuit) — The 80% Rule in Practice
| Circuit | Capacity | Continuous Max | Example Load | Status |
|---|---|---|---|---|
| 15A ×120V | 1,800W | 1,440W | Microwave 1,200W → 67% (83% cont.) | Borderline continuous — move to 20A |
| 20A ×120V | 2,400W | 1,920W | Microwave 1,200W → 50% (62% cont.) | Safe |
| 30A ×240V | 7,200W | 5,760W | EV 7,200W → 100% (125% cont. → needs 40A) | Needs 40A breaker for 30A cont. |
The EV example shows the 125% rule in action: a 30A continuous EV charger (7,200W) requires 30×1.25=37.5A → next standard breaker 40A per NEC 210.19 and NEC 210.20, not 30A. The calculator flags this and suggests the correct breaker size.
When to Upgrade vs When Not To (Per NEC 230.42)
| Demand vs Panel | Verdict | Action |
|---|---|---|
| < 80% (with continuous ×1.25) | Safe — no upgrade needed | Add the small load (EV 30A) — still under 80% with continuous |
| 80-100% | Consider upgrade or load management | Smart EV charger that sheds when range or dryer is on, or 200A upgrade |
| > 100% or tripping/warm panel | Must upgrade or reduce load | Licensed electrician, load study, permit — do not add load |
Common Home Loads — NEC Reference Values (Per Table 220.14)
| Appliance | Watts | Volts | Amps | Continuous? | Circuit |
|---|---|---|---|---|---|
| Central AC (3-ton) | 3,500 | 240V | 14.6A | Yes (if 3+ hrs) | 30A/240V |
| Electric Dryer | 5,000 | 240V | 20.8A | No | 30A/240V |
| Electric Range | 8,000-12,000 | 240V | 33-50A | No | 40-50A/240V |
| EV Charger (Level 2, 30A) | 7,200 | 240V | 30A → 37.5A cont. | Yes ×1.25 | 40A/240V* |
| Water Heater (tank) | 4,500 | 240V | 18.8A → 23.4A cont. | Yes ×1.25 | 25-30A/240V |
| General Lights & Receptacles | 3VA/sq ft | 120V | Per sq ft | Yes if 3+ hrs | 15-20A/120V |
* EV on 40A breaker for 30A continuous (30×1.25=37.5A → next size 40A per NEC 210.19). Values per NEC Table 220.14 and manufacturer nameplates. General lighting at 3VA per sq ft per NEC 220.14(J) — e.g., 1,500 sq ft ×3VA=4,500VA for lights/receptacles.
Safety and Code Notes — NEC Article 220, Article 230, and When to Call a Pro
Continuous Load = 3+ Hours — The 125% Rule
EV chargers, water heaters, space heaters, and many lighting loads that run 3+ hours continuously must be calculated at 125% for conductor and breaker sizing per NEC 210.19(A)(1) and 215.2(A)(1) — or equivalently limited to 80% of the rating. A 20A×120V circuit is 2,400W, but the maximum continuous load is 1,920W; a 30A×240V EV circuit is 7,200W, but the continuous max is 5,760W, so a 30A continuous EV charger actually requires a 40A breaker (37.5A). For the panel, a 100A×240V service is 24,000W, but the continuous max is 19,200W. The example demand of 7,440W includes the 125% where applicable (EV 7,200→9,000), so comparing 7,440W to 24,000W already reflects the continuous rule; comparing without the 125% would understate the load by 1,800W for the EV alone. This is why generic "add up watts" calculators that ignore continuous overstate headroom by 20-25% for EV and water heater homes and are unsafe for those cases.
Diversity (Demand Factor) — Not All Loads Run at Once
Per NEC 220.42, 220.50, 220.55, demand factors reflect diversity: general lighting and receptacles at 100% for the first 3,000VA, 35% for the next 117,000VA, and 25% for the remainder (or the simpler 0.6 blended factor the calculator uses for quick estimates), HVAC at the largest of heating or cooling (not both, per 220.50, since they don't run simultaneously), and range/dryer per tables (e.g., one range at 8kW is 8kW demand; multiple ranges have reduced demand). The calculator applies the factor per load type — general, HVAC, range, dryer — so demand is always less than connected and is the code-compliant design load for the service. Connected is the ceiling; demand is the realistic number that the panel is sized to.
When to Call a Licensed Electrician and the AHJ
- Adding an EV charger, hot tub, workshop, ADU, or any 240V continuous load — especially on a 100A service or an older panel (Federal Pacific, Zinsco, or no main breaker)
- Panel tripping, warm to the touch, buzzing, or frequent breaker trips — signs of overload or failing breaker, not just inconvenience
- Demand >80% of the panel (with continuous at 125%) — the panel is effectively full for continuous loads, and the next addition requires either load management or a service upgrade
- Aluminum branch wiring, knob-and-tube, or a panel with no main breaker — special handling and possible replacement per local code
- Any service upgrade (100A→200A) — requires a permit, utility coordination, new riser and meter, and inspection by the local authority having jurisdiction (AHJ)
The calculator is an estimate per the NEC Article 220 Standard Method for planning and headroom checks — it uses the same demand factors and continuous rules an electrician uses for a quick check, but it is not a substitute for a licensed electrician's load study, a site visit, or the local code amendments that the AHJ enforces. Always verify before panel work, service changes, or permit applications. Per ESFI, electrical failures cause an estimated 51,000 home fires per year in the U.S. — load calculations are safety, not just convenience.
How to Use the Result — Next Steps and Alternatives to a Full Upgrade
| Panel Headroom | Verdict | Action |
|---|---|---|
| >20% headroom (demand <80%) | Safe to add small load | Add the EV at 30A (7,200W →9,000W cont.) — still under 80% with continuous, no upgrade |
| 0-20% headroom (80-100%) | Consider upgrade or load management | Smart EV charger that sheds when range/dryer is on (e.g., NeoCharge, SimpleSwitch), or load shed device, or 200A upgrade ($2,000-4,000) |
| Negative headroom (>100% or tripping) | Must upgrade or reduce load | Licensed electrician, load study, permit, utility — do not add load |
Alternatives to a full 100A→200A upgrade: A smart splitter or load management device (e.g., NeoCharge, Dryer Buddy, SimpleSwitch) that pauses EV charging when the dryer or range is on can keep the demand under 80% without a service upgrade — the EV charges at night when other loads are off, and the device sheds automatically. For a single 30A EV addition, this is often $300-500 vs $2,000-4,000 for a service upgrade and is code-compliant when listed per NEC 750. The calculator shows the headroom so the choice between management and upgrade is data-driven, not a guess of "100A is too small."
FAQs About Calculating Electrical Load
How do I calculate the electrical load for my house?
List each appliance's watts and volts (nameplate or NEC Table 220.14), sum to connected load (e.g., 12,400W), apply NEC Article 220 Standard Method demand factors (general ×0.6 per 220.42, HVAC largest only per 220.50, continuous ×1.25 per 210.19) for demand load (7,440W), then divide watts by volts for amps (31A at 240V) and compare to panel capacity (100A×240V=24,000W →31% loaded, 69% headroom). An electrical load calculator does this per Article 220 with continuous and diversity handled, and shows panel and per-circuit percentages.
What is the difference between connected and demand load?
Connected is the sum if everything were on at once (worst case, e.g., 12,400W); demand applies diversity per NEC Article 220 — not all loads run simultaneously — so general loads are at 35-60% and HVAC is largest only, yielding demand (7,440W) that is always lower than connected and is the realistic, code-compliant sizing load. Designing to connected would oversize the service by 40-60%.
How many watts can a 100A panel handle?
100A×240V=24,000W total, with 19,200W continuous max (80% per NEC 210.19). A typical home demand of 7,440W is 31% of 100A with 69% headroom. A 200A panel is 48,000W (38,400W continuous) — standard for new all-electric homes with EV, electric range, dryer, and heat pump.
Can I add an EV charger to a 100A panel?
Often yes — a 30A Level 2 EV charger is 7,200W, but 9,000W continuous at 125% (30×1.25=37.5A → 40A breaker). On a 100A panel with 31% base demand (7,440W), the new demand is ~50-60% — still under 80% if other large loads are managed. Use the calculator with the EV as continuous and check the panel percentage; if it exceeds 80%, consider a smart splitter or 200A upgrade, and consult an electrician for the specific panel and local code.
What is the 80% rule for circuits and panels?
Continuous loads — maximum current for 3+ hours — must be calculated at 125% for sizing, or equivalently limited to 80% of the circuit or service rating, per NEC 210.19(A)(1) and 230.42(A)(1). A 20A×120V circuit (2,400W) is limited to 1,920W continuous; a 100A×240V service (24,000W) is limited to 19,200W continuous. The calculator applies the 125% automatically for loads flagged as continuous.
Is the calculator a substitute for an electrician?
No — it is an estimate per the NEC Article 220 Standard Method for planning and headroom checks, using the same demand factors and continuous rules an electrician uses for a quick check, but it is not a substitute for a licensed electrician's site-specific load study, a panel inspection, or the local code amendments that the AHJ enforces. Always verify before panel work, service changes, or permit applications, per ESFI and NFPA guidance.
Conclusion
Calculating electrical load is watts to amps to panel percent — connected (worst-case sum) vs demand (realistic per NEC 220 with diversity and continuous at 125%) vs the 80% continuous limit — before the next large load is added. Listing appliances per NEC Table 220.14, applying demand factors, and checking the percentage turns "will the panel handle it?" from a guess of "100A sounds small" into a number with headroom to spare — 31% loaded with 69% headroom in the example, with the EV still under 60% — or with a clear signal to manage load or upgrade. The same math applies per circuit: a 1,200W microwave on a 20A×120V circuit is 50% (62% as continuous), so the next kitchen appliance belongs on a different circuit.
Enter the appliances from the list — watts auto-filled per NEC, continuous flagged — to get connected and demand watts and amps, panel and per-circuit percentages, and the headroom for the next addition — headroom that is 20%+, 0-20%, or negative, with the next step (add, manage, or upgrade) shown — then verify with a licensed electrician and the AHJ before any panel work or permit.
