How ChargeRight Calculates Your NEC Load: The Public Calculation Spec
The same math, reproducible. Every method, every input, every exclusion, every rounding rule — and worked vectors you can check with a calculator.
Most load calculators are a black box: you answer questions, a number appears, and you have no way to tell whether the number is right or which code section produced it. That is a bad deal when the answer decides whether you spend money on a service upgrade.
I'm Jason Walls, Master Electrician, IBEW Local 369. This page is the specification for ChargeRight's calculation engine, written so an electrician, an inspector, or a homeowner with a calculator can reproduce a report line by line and disagree with it in specific terms. Everything below describes what the shipped engine actually does — the methods it implements, the rule that picks which one binds, what it includes, what it deliberately leaves out, and how it rounds.
How does ChargeRight calculate whether a panel can support an EV charger?
ChargeRight runs five NEC load calculations on the same input set — 220.82, 220.83(A), 220.83(B), the Standard Method of Article 220 Part III, and a 2026 NEC preview — then names the one that binds for your situation. The calculated load in VA is divided by 240 to get amps and compared to the service rating under NEC 230.79. The load must be at or under 100% of the rating to pass; the 80% line is a separate advisory comfort band, not the pass/fail rule.
Showing all five methods is the transparency move: methods can disagree, and when they do the report says so and binds the verdict to the method written for your situation rather than to the friendliest number. The output is a preliminary calculation with organized inputs your electrician can check — not a permit, not a final design, and not an inspection.
NEC References:
- NEC 220.82
- NEC 220.83(A)
- NEC 220.83(B)
- NEC 220 Part III
- NEC 230.79
Last updated: August 2026
Edition scope, and the local-amendment limit
The engine is written to the 2023 NEC. The 2026 row is a clearly labelled preview of the upcoming treatment and is not yet adopted in most jurisdictions — it is shown for comparison, and it never binds the verdict.
What this calculation cannot know
States adopt NEC editions on their own schedules, and the authority having jurisdiction (AHJ) can amend the edition it adopts — a local amendment can change a demand factor, add a required input, or set a minimum the national text does not. ChargeRight does not model local amendments. It also cannot see your actual conductors, grounding, breaker fill, panel condition, or anything else that only a person standing in front of the panel can assess. The calculation is preliminary: your AHJ and your licensed electrician make the call, and this is meant to arrive in their hands as organized inputs rather than as a verdict that competes with them.
More on where the engine stops: what ChargeRight can and cannot tell you and the methodology page.
Can your panel handle an EV charger?
Find out in minutes with a professional NEC Article 220 load calculation. J.D. Power found only 28% of EV owners needed one — even though 65% were told they did (ESFI). Skip the $300 electrician visit.
1. The five methods that are implemented
All five run on every assessment, from the same input set. They differ in the demand factor structure, the VA-per-square-foot figure, and how the EV charger and motor loads are treated.
| Method | General lighting | Demand factor | HVAC | EV charger |
|---|---|---|---|---|
| 220.82 Optional Method | 3 VA/ft² | First 10 kVA at 100%, remainder at 40% | Larger of heating or cooling; electric space heat at 65% | Amps × 240 V, at 100%, outside the demand pool |
| 220.83(A) Existing dwelling | 3 VA/ft² | First 8 kVA at 100%, remainder at 40% | Existing HVAC, larger of heating or cooling, at 100% | Amps × 240 V, at 100%, outside the demand pool |
| 220.83(B) Existing dwelling + new HVAC | 3 VA/ft² | First 8 kVA of all other loads at 100%, remainder at 40% | Larger of air conditioning or space heating, at 100%, on top of the pool | Amps × 240 V, at 100%, outside the demand pool |
| Standard Article 220 Part III | 3 VA/ft² | Table 220.42 tiers on lighting + small-appliance circuits; 220.53 on fixed appliances | Larger of heating or cooling at 100%, plus 25% of the largest single cooling unit | Amps × 240 V × 1.25 (continuous) |
| 2026 preview Not yet adopted | 2 VA/ft² | First 8 kVA at 100%, remainder at 40% | Larger of heating or cooling; electric space heat at 65% | Amps × 240 V, at 100%, no demand factor permitted |
The 25% largest-motor surcharge belongs to exactly one method
It is applied in the Standard Method only, at 25% of the largest single cooling unit. NEC 220.82 contains only its general-load list and its largest-of-six heating/air-conditioning selection; 220.83(A) and (B) contain their load lists and demand tiers. None of them contains a 25% motor step, so the engine does not add one there. Putting it in an optional-method calculation inflates the load and pushes homeowners toward upgrades the code does not require.
Where the EV charger sits relative to the demand factor
A literal reading of 220.83(A)(3)(a) — appliances fastened in place, permanently connected, or on a specific circuit — could place an EVSE inside the 8 kVA / 40% pool. The engine instead adds the new charger at 100%, outside the pool, consistent with the 2023 NEC EVSE load provision at 220.57. That is the conservative side of the interpretation: it counts more load, not less. An already-installed charger is different — it is treated as an existing fixed appliance at nameplate (amps × 240 V) inside the pool, and the new charger is added on top.
2. How the binding method is chosen
All five results are printed. One is named as the method that binds the verdict, by this rule, in this order:
- You told the wizard you are planning a service change (or the job is new construction) → 220.82 binds. That is the section that sizes a dwelling from scratch.
- Otherwise, you are adding new air-conditioning or electric space-heating equipment alongside the charger → 220.83(B) binds.
- Otherwise — including when those questions were never answered → 220.83(A) binds.
Why "no answer" defaults to 220.83(A) and not to 220.82
Every ChargeRight customer is, by definition, an existing dwelling asking about an existing panel — so the absence of a signal is not evidence of new construction, and the default lands on the section titled for the question actually being asked. 220.82 stays on the report as a cross-check.
Neither section is uniformly the bigger number
It is tempting to say the bound method is always the conservative one. That is not true, and the engine's own outputs say so. On general loads, 220.83(A)'s smaller 8 kVA first tier pushes more load into the 40% remainder, so it can land lower than 220.82 — in Vector 1 below it does, by 5 A. But the engine takes existing electric space heating at 100% under 220.83 where 220.82 permits 65%, so on an electric-heat home the order flips: the same 2,200 ft² house with a 10 kW electric-heat nameplate calculates to 133.8 A under 220.83(A) and 124.3 A under 220.82. Which section applies is a question about your house, not a lever to pull. That is exactly why all five rows are printed.
When the methods disagree, the report says so explicitly rather than showing one number. If four of the five say an upgrade is needed and only one says the service is sufficient, the headline takes the conservative side and names the dissenter. If four say it fits and one recommends an upgrade, the headline reflects that but still names the method that disagrees. On a true split, the report says the methods disagree and points the question at your electrician. The verdict itself stays bound to the method for your situation — never reassigned to whichever row reads best.
3. Required inputs
These are the fields the calculation consumes. Nothing else in the assessment changes the load number.
| Input | How it is used |
|---|---|
| Service / panel rating (A) | The denominator of the NEC 230.79 comparison |
| Home square footage | × 3 VA/ft² (× 2 VA/ft² in the 2026 row) |
| Heating type | Electric, gas, heat pump, or other — selects the space-heating term |
| Air conditioning | Tonnage × 1,200 VA per ton; multiple units sum when they can run together, otherwise only the largest counts |
| Electric range / dryer / water heater | Added at nameplate into the fixed-appliance bucket |
| Pool pump / hot tub / well pump | Added at nameplate into the fixed-appliance bucket |
| Small-appliance & laundry circuits | 1,500 VA each; two small-appliance circuits minimum (210.11(C)(1)), one laundry circuit assumed if not stated (210.11(C)(2)) |
| Charger amps | × 240 V. If not given, derived from daily miles, available charging hours and vehicle efficiency, floored at 24A and snapped up to the next standard size (24 / 32 / 40 / 48 / 64 / 80) |
| Subpanel + feeder amps | Optional. When present, the binding rating becomes the smaller of the panel rating and the feeder |
| Adding new HVAC? Planning a service change? | Selects which method binds (see section 2) |
| Nameplate reads (optional) | A confidently read condenser, water heater, pool, hot tub or well pump nameplate overrides that appliance's default estimate. A read label beats a guess |
| Existing EV charger amps (optional) | × 240 V into the fixed-appliance bucket, before the demand factor |
Defaults when a field is missing. Small-appliance circuits default to the code minimum of two and laundry to one. If the heating type is electric or a heat pump and no nameplate wattage was captured, the engine assumes 10 kW of resistance/strip heat at the 65% factor — 6,500 VA — and takes the larger of that and the compressor estimate. That default is deliberately biased to overstate, because the unsafe direction is undercounting.
4. Major exclusions — what is deliberately not in the number
- Bathroom circuits are not a separate load line. The 1,500 VA per-circuit allowance covers small-appliance and laundry circuits (210.11(C)(1) and (C)(2)). Bathroom receptacle circuits are 210.11(C)(3) and are already covered by the 3 VA/ft² general load, so counting them again would double-count. They still appear in the circuit visualization, because they exist — they just are not a second load line.
- No 25% motor surcharge outside the Standard Method. See section 1.
- Small fixed appliances are not itemized. Dishwashers, disposals, and microwaves are not asked about, so they are not in the fixed-appliance bucket. A hand worksheet that itemizes them will land slightly higher than the report on the same house.
- Heating and cooling are never summed. The engine takes the larger of the two, treating them as noncoincident loads (NEC 220.60). Where a heat pump's supplemental strips are interlocked so the compressor cannot run with them, only the larger of the two is counted rather than the compressor plus a factored strip term.
- No metered-demand method. NEC 220.87 lets a calculation use a year of actual metered demand data. The engine does not implement it — it works from the questionnaire and photos, not from your utility's interval data.
- No load-management credit. The 2023 NEC permits the maximum load on a service to be the maximum permitted by an automatic load management system (625.42). The engine does not model one, so a home with an EVSE load-management setup is calculated conservatively — higher than it may need to be.
- No neutral load calculation. The engine calculates the ungrounded service load; the neutral calculation (220.61) is a design step for your electrician.
- No branch-circuit or conductor design. The engine reports the load and the comparison. Sizing the charger's overcurrent protection at not less than 125% of the maximum load (NEC 625.41 — EV charging loads are continuous loads under 625.42), the conductors, the grounding, and the physical space in the panel are all the installing electrician's work.
5. Rounding rules
Rounding is where two honest calculations drift apart by a couple of amps, so here is exactly where it happens:
- The 220.83(A) and 220.83(B) demand results are rounded to the nearest whole VA. So are the Table 220.42 result and the 220.53 appliance subtotal in the Standard Method.
- The 220.82 and 2026 demand tiers are not rounded — they carry full precision into the total.
- Amps are total VA ÷ 240, carried at full precision. Nothing is rounded before the NEC 230.79 comparison, so the verdict is never decided by a rounding artifact.
- Utilization percent and headroom amps are rounded to one decimal for display only, after the verdict is decided.
- Circuit counts round up (a partial circuit is a whole circuit).
- A derived charger size snaps up to the next standard amperage, never down.
- When an upgrade is recommended, the suggested service size is the smallest standard rating — never below the 100A one-family dwelling minimum of NEC 230.79(C) — that leaves the calculated load under 80% of the new rating. That is a sizing target for new equipment, not the pass/fail test for your existing panel.
6. The verdict: NEC 230.79 at 100%, plus an 80% advisory band
The pass/fail test is NEC 230.79. The service disconnecting means must have a rating not less than the calculated load. So the test is: calculated amps ≤ the service rating. On a 200A service that line is 200A. Not 160A.
The 80% figure is real, but it is a different thing: it comes from the continuous-load rules, where a circuit serving a continuous load is loaded to 80% of its overcurrent device. It is the headroom electricians like to keep. ChargeRight reports it as an advisory band rather than folding it into pass/fail, because treating 80% as the code limit tells homeowners to buy service upgrades the NEC does not require — and hiding it entirely tells them a 99%-loaded panel is comfortable. Both are dishonest. So the report carries three states:
| State | Condition | NEC 230.79 | What it means |
|---|---|---|---|
| Comfortable | Load ≤ 80% of the rating | Passes | Typically fits, with continuous-load headroom left over |
| Tight | Above 80% and at or below 100% of the rating | Passes | Passes the book test but uses the last 20% — worth confirming details, or a lower charger setting, before the install |
| Exceeds | Load > the rating | Fails | The calculation says this configuration does not fit as specified |
Two headroom figures appear on a report and they are not interchangeable: headroom to the rating (rating minus load, the 230.79 margin) and available capacity to the 80% line, which floors at zero once you are in the tight band. In the tight band the second reads 0 while the first is still positive. That is correct, not a bug.
The feeder can only lower the ceiling
If the charger lands on a subpanel, the binding rating is the smaller of the panel rating and the feeder breaker supplying it. A 100A feeder into a 200A-rated subpanel means 100A is the real ceiling, and the verdict is judged against 100A. Taking the smaller of the two can only ever turn a "fits" into "calculation required" — never the reverse.
7. Worked vectors — reproduce these by hand
These input sets are the fixtures from the engine's own test suite, reproduced field for field. The VA and amp figures are what the shipped calculation functions return when those exact fixtures are run through them. Nothing here is illustrative: if you work the arithmetic below and get a different number, one of us is wrong and it is worth an email.
Vector 1 — 2,200 ft², 200A service, 32A charger
Inputs: 200A panel · 2,200 ft² · gas heat · 3-ton AC · electric range · electric dryer · no electric water heater · no pool, hot tub or well pump · no subpanel · 2 small-appliance circuits · 1 laundry circuit · 32A charger. No new HVAC and no service change signalled, so 220.83(A) binds.
| Step | 220.83(A) — binding | 220.82 — cross-check |
|---|---|---|
| General lighting (2,200 × 3) | 6,600 | 6,600 |
| Small-appliance circuits (2 × 1,500) | 3,000 | 3,000 |
| Laundry circuit (1 × 1,500) | 1,500 | 1,500 |
| Fixed appliances (range 8,000 + dryer 5,000) | 13,000 | 13,000 |
| Subtotal before demand factor | 24,100 | 24,100 |
| After demand factor | 14,440 8,000 + (16,100 × 0.40) | 15,640 10,000 + (14,100 × 0.40) |
| HVAC (3 tons × 1,200) | 3,600 | 3,600 |
| EV charger (32 × 240) | 7,680 | 7,680 |
| Total calculated load | 25,720 VA | 26,920 VA |
| In amps (÷ 240) | 107.2 A | 112.2 A |
| Utilization of the 200A rating | 53.6% | 56.1% |
| Headroom to the rating | 92.8 A | 87.8 A |
| State | Comfortable | Comfortable |
Both are well under the 160A advisory line and far under the 200A rating. Note the direction: on this gas-heat house the binding 220.83(A) result is the lower of the two, because the smaller 8 kVA tier pushes more load into the 40% remainder. Swap the gas heat for a 10 kW electric-heat nameplate and the order reverses, for the reason in section 2. Neither section is the pessimistic one by nature — which is why the report shows you both.
Vector 2 — 1,200 ft², 100A service, 24A charger (the tight case)
Same house profile, smaller everything: 100A panel · 1,200 ft² · gas heat · 2-ton AC · electric range · electric dryer · 2 small-appliance circuits · 1 laundry circuit · 24A charger. 220.83(A) binds. This is the fixture the engine's tests use specifically because it lands inside the 80–100% band, where the 230.79 rule and the 80% line give different answers.
| Step | 220.83(A) — binding |
|---|---|
| General lighting (1,200 × 3) | 3,600 |
| Small-appliance circuits (2 × 1,500) | 3,000 |
| Laundry circuit (1 × 1,500) | 1,500 |
| Fixed appliances (range 8,000 + dryer 5,000) | 13,000 |
| Subtotal before demand factor | 21,100 |
| After demand factor: 8,000 + (13,100 × 0.40) | 13,240 |
| HVAC (2 tons × 1,200) | 2,400 |
| EV charger (24 × 240) | 5,760 |
| Total calculated load | 21,400 VA |
| In amps (÷ 240) | 89.2 A |
| Utilization of the 100A rating | 89.2% |
| Headroom to the rating | 10.8 A |
| Available capacity to the 80A advisory line | 0 A |
| State | Tight — passes NEC 230.79 |
89.2A is under the 100A rating, so it passes the code test — and it is over the 80A advisory line, so the report says "fits, but it's tight" rather than a clean green. Under the old, wrong "80% is the limit" rule this homeowner would have been told to buy a service upgrade the NEC does not require.
The same fixture also shows what method disagreement looks like. Run the other rows and you get 94.2A under 220.82 (tight), 87.2A under the 2026 preview (tight), and 116.6A under the Standard Method — which exceeds a 100A service. The Standard Method is higher because it applies the Table 220.42 tiers to lighting and small-appliance circuits, keeps the range and dryer at 100%, adds 600 VA for the 25% largest-motor step, and takes the charger at 125% (7,200 VA rather than 5,760). Four methods say it fits; one says upgrade. The report prints all five, binds the verdict to 220.83(A), and says plainly that the methods disagree.
Vector 3 — the verdict boundary itself
These are the shipped boundary cases for the pass/fail rule, with the exact expected values from the test suite. They are the cheapest way to check whether any calculator — ours or anyone's — is using the 80% line as a pass/fail rule when it should not be.
| Load | Rating | Utilization | Headroom | Result |
|---|---|---|---|---|
| 79.9 A | 100 A | 79.9% | 20.1 A | Comfortable, passes |
| 80.0 A | 100 A | 80.0% | 20.0 A | Comfortable (exactly at the line), passes |
| 99.0 A | 100 A | 99.0% | 1.0 A | Tight, still passes NEC 230.79 |
| 100.0 A | 100 A | 100.0% | 0 A | Tight, passes — the load equals the rating |
| 100.1 A | 100 A | 100.1% | 0 A | Exceeds, fails |
| 123.456 A | 200 A | 61.7% | 76.5 A | Comfortable — utilization rounds to one decimal for display |
The same boundary set is checked on 100A, 125A, 150A and 200A services, so the rule cannot quietly drift back to an 80% pass/fail test on one panel size and not another.
8. What this output is, and what it is not
A ChargeRight report is a preliminary NEC load calculation with its inputs shown. It is not a permit, not a final design, and not an inspection. It does not replace the electrician — it walks in ahead of them with the square footage, the appliance list, the HVAC selection, the charger size, the method that binds and the arithmetic already organized, so the site visit is spent on the things that actually require a person: the conductors, the grounding, the panel's physical condition and available spaces, the meter and service drop, and the local amendments only your AHJ can tell you about.
Outcome language on a report is deliberately careful for the same reason. A given configuration typically fits, or usually needs the calculation run — there is no honest way to declare pass or fail from a panel's amperage alone, because two 100A services with different square footage, heat, and appliances are not the same electrical question.
Related
The section that binds for most homeowners, walked through step by step: NEC 220.83 explained.
The method for new construction or a planned full service change: the NEC 220.82 walkthrough.
How the assessment is built end to end: methodology.
Where the engine stops and a human starts: limitations.
Jason Walls
Master Electrician · EVITP Certified · KY Electrical License EE642643
NEC 220.82 Specialist · ChargeRight Founder
"I built ChargeRight because I was tired of seeing homeowners pay $2,000 to $4,500 for panel upgrades that a $12.99 load calculation would have shown they didn't need. The math doesn't lie. Every homeowner deserves to see it before they write a check."
Frequently Asked Questions
Which NEC methods does ChargeRight actually run?
Five, on every assessment: NEC 220.82 (Optional Method, 10 kVA first tier), NEC 220.83(A) (existing dwelling, no new HVAC, 8 kVA first tier), NEC 220.83(B) (existing dwelling adding HVAC), the Standard Method of Article 220 Part III, and a preview of the 2026 NEC treatment (2 VA/ft², 8 kVA first tier, EVSE at 100%). All five appear on the report; one of them is named as the method that binds the verdict for your situation.
How does ChargeRight decide which method binds the verdict?
By one explicit rule fixed in advance, not by picking the friendliest answer. If you tell the wizard you are planning a full service change or new construction, 220.82 binds. Otherwise, if you are adding new air-conditioning or electric space-heating equipment alongside the charger, 220.83(B) binds. In every other case — including when a question is left unanswered — 220.83(A) binds, because every ChargeRight customer is by definition an existing dwelling asking about an existing service, which is the exact question 220.83 is titled for. The choice is made by your situation, not by which section produces the smaller number.
Is the pass/fail test 80% of the panel rating?
No. The pass/fail test is NEC 230.79: the service disconnecting means must have a rating not less than the calculated load, so the calculated load is compared to 100% of the service rating. The 80% line is an advisory comfort band that comes from the continuous-load rules, not from 230.79. ChargeRight reports it as its own state: under 80% is comfortable, between 80% and 100% the panel passes but is tight, and above 100% the calculation says the service is exceeded.
Does ChargeRight add the 25% largest-motor surcharge?
Only in the Standard Method (Article 220 Part III), where it is 25% of the largest single cooling unit. There is no 25% motor surcharge in 220.82, in 220.83(A) or (B), or in the 2026 preview — those sections do not contain that step, and adding it there would overstate the load.
What NEC edition does the calculation follow, and what about local amendments?
The engine is written to the 2023 NEC, with the 2026 treatment shown as a clearly labelled preview that is not yet adopted in most jurisdictions. States adopt NEC editions on their own timeline and local authorities having jurisdiction (AHJs) can amend the code they adopt, so a local amendment can change a required input, a demand factor, or a minimum. ChargeRight does not model local amendments. The result is a preliminary calculation — it is not a permit, not a final design, and not an inspection. Your AHJ and your licensed electrician decide.
Can I reproduce ChargeRight’s numbers by hand?
Yes — that is the point of publishing the spec. Every step is arithmetic: square footage times VA per square foot, 1,500 VA per small-appliance and laundry circuit, fixed appliances at nameplate, the demand factor for the bound method, HVAC as the larger of heating or cooling, the charger at amps times 240 volts, then divide the total VA by 240 to get amps and compare to the service rating. The worked vectors on this page are shipped test fixtures with the exact figures the engine produces from them.
About the Author
Jason Walls
Master Electrician, IBEW Local 369, EVITP Certified. Jason built ChargeRight to give homeowners the same NEC Article 220 load calculation licensed electricians run — and to show the work, so it can be checked.