Phase Academy Interactive Power Systems Lab

Dwelling Unit Load Calculator

-- A live
⚡ Active Method Summary
What the selected method includes, step by step
📊 Three Methods, Same Dwelling
Every method runs on every input change — the active one drives pass/fail

💡 Insight

Adjust the inputs to see how demand factors reshape the calculated load.

⏱ Service Utilization
Calculated load vs the selected service size
--A
of -- A service
<85% 85–100% >100%
🧩 Load Composition
Where the volt-amperes live (active method)
📈 Method Comparison
Standard vs Optional vs Existing-Dwelling totals, against the selected service capacity
⚖ Method Comparison Summary
Which method produces the lower calculated load — and whether it is permitted
📏 Service Size Evaluation
Selected service utilization (active method)
0%
0 A used-- A capacity
🧱 Service Size Ladder
Standard service/OCPD sizes (240.6(A)) and where this load falls

📖 Reading these results

The code criterion is simple: the service must be at least the calculated load. Everything above that is design headroom, not a code requirement. The Recommended size is the smallest standard rating in 240.6(A) that still covers the calculated load — but a one-family dwelling service disconnect is never smaller than 100 A, 3-wire (230.79(C)).

Standard Method — Step-by-Step
Each load category carries its own demand rule. Every number has a reason.
Optional Method — Step-by-Step
Fewer steps, different demand logic — new dwellings only.
Existing-Dwelling Method — Step-by-Step
Used when adding load to an existing dwelling — not interchangeable with the new-dwelling optional method.
📝 Assumptions
🎯 Design Challenges
Pick a mode. The baseline scenario loads into the sidebar automatically, then your design is graded live.
Your Score
—
☆☆☆☆☆
Adjust inputs to begin…
Criteria

📝 How scoring works

Each challenge has its own criteria and you earn points for meeting them. Oversizing is penalized — the NEC asks for a service that fits the calculated load, not one twice as big. Design headroom is advisory, never a code pass/fail criterion.

Why This Matters

An undersized service is not just an inconvenience — it is a safety problem and a failed inspection. The NEC calculation methods exist so designers, electricians and inspectors arrive at the same number from the same house. Dwelling service sizing is also one of the most frequently misapplied calculations in the code, because Article 220 offers three legitimate methods that produce meaningfully different results for the same home.

“Connected load is what you own. Calculated load is what the code says you will actually use at the same time.”

The Three Methods

  • Standard Method (Art. 220, Part III): Every category is calculated separately — general lighting at the unit load per ft², small-appliance and laundry circuits at 1,500 VA each, fastened-in-place appliances (75% when four or more), range demand from Table 220.55, dryer demand from 220.54, and heating/cooling under the noncoincident rule of 220.60. Methodical, granular, and always permitted.
  • Optional Method (220.82): Groups the general loads and appliance nameplates together, takes the first tier at 100% and the remainder at 40%, then adds the largest applicable heating/cooling selection from 220.82(C). Simpler, but it requires a single dwelling on one 120/240 V or 208Y/120 V 3-wire service or feeder of 100 A or more.
  • Existing Dwelling (220.83): A separate calculation for adding load to an existing dwelling. Other loads take the first 8 kVA at 100% and the remainder at 40%; added air-conditioning or space heating is included at 100% under 220.83(B).
The Optional Method usually wins for small to medium all-electric homes, because heat and cooling get category factors instead of a straight 100% of the larger one.

Standard vs. Optional — Conceptual Summary

The Standard Method demands each category on its own terms: the general lighting/small-appliance/laundry subtotal runs through the 3,000 / 120,000 VA tier table, appliances get their own 75% rule, and cooking equipment gets Table 220.55. The Optional Method throws the general loads and every appliance nameplate into one pile, takes the first 10 kVA at 100% (8 kVA in the 2026 NEC) and 40% of everything past it, then adds HVAC separately.

Neither method is “the right answer.” They are two permitted paths, and a designer is allowed to run both and use the smaller result — provided the optional method's qualifying conditions are met.

Common Mistakes

  • Using connected load as the calculated load — forgetting demand factors entirely
  • Double-counting HVAC: only the larger of heating or cooling is included, because they are noncoincident (220.60)
  • Using fewer than two small-appliance branch circuits — 210.11(C)(1) sets the floor at two
  • Missing the 75% demand factor when four or more fastened-in-place appliances are present (220.53)
  • Counting ranges, dryers, space heating or A/C inside that appliance group — 220.53 explicitly excludes them
  • Assuming the optional method always wins — it depends on the load profile, and on qualifying under 220.82(A)
  • Applying a single dryer at less than 5,000 VA in the Standard Method regardless of nameplate (220.54)
  • Using 8,000 VA for a small (≤ 8¾ kW) range instead of the Column A/B 80% demand factor
  • Adding 25% of the largest motor on top of an MCA value that already contains it

What to Notice

  • How far demand factors pull the calculated load below the connected load — often 40–50% on an all-electric house
  • That range demand (Table 220.55) is usually far below nameplate: a 12 kW range calculates at 8 kW
  • That HVAC dominates all-electric dwellings and is usually what drives the service size
  • That the Optional Method's advantage grows as the appliance pile grows, because more of it lands in the 40% tier
  • That the 2026 NEC moved load calculations from Article 220 to Article 120 and dropped the dwelling feeder/service unit load from 3 to 2 VA/ft² — branch-circuit counts still use 3 VA/ft² (120.13)
Educational Disclaimer: This calculator paraphrases NEC logic for teaching and does not reproduce copyrighted code text. It models a single range and a single dryer, and it does not replace a full load study, plan review, or AHJ interpretation. Always verify against the NEC edition adopted by your jurisdiction.
🧪 In-File Test Harness
Runs the pure calculation functions plus DOM and accessibility checks. Also callable from the console as window.NEC.runTests(), or by loading the page with #test in the URL. Running tests never clobbers your inputs — the form is snapshotted and restored.