Circuit Load Panel Capacity Calculator

Purpose and model boundary

Builds a fixed split-phase circuit schedule, separates continuous and noncontinuous current, applies a transparent 125 percent planning multiplier, routes each load to the correct panel leg, and reports the binding spare-capacity result.

The page answers two questions. The first is whether a panel or service has room for proposed circuits, with the existing load taken from the existing rows of the circuit schedule, from leg currents supplied by an approved external calculation, or from the dwelling's calculated load by the standard method. The second, in the Dwelling standard method basis only, is what that calculated load is, what current it draws at the entered line-to-line voltage, and which standard service size is the smallest one at or above that current rounded to 6 decimal places.

Do not treat this result as an electrical design, code-compliance determination, permit approval, utility commitment, equipment approval, safety certification, cost quote, or endorsement by a cited source. Service data, panel ratings, conductor properties, equipment loads, route lengths, manufacturer instructions, utility requirements, locally adopted codes, and approval thresholds remain the user's responsibility.

Inputs and units

• existing load basis: one of three listed bases: Circuit schedule, AHJ-adjusted leg demand, or Dwelling standard method.
• line to line voltage: entered in V.
• service rating: entered in A per leg.
• external leg a current: entered in A.
• external leg b current: entered in A.
• imbalance check threshold: entered in %.

The circuit grid contains 12 fixed data rows plus one header row with these columns: Include, Circuit label, Stage, Connection, Input basis, Noncontinuous value (selected input basis), Continuous value (selected input basis), Power factor (fraction), Planning demand factor (fraction), Installed breaker (A), Source note. Numeric columns retain the units shown here; text, route, and include columns are intentionally unitless.

These inputs belong to the Dwelling standard method basis. They are hidden in the other two bases, and the workbook does not read them there:

• floor area: entered in sq ft, measured from the outside dimensions, leaving out open porches, garages and unfinished spaces not adaptable for future use.
• small-appliance circuits: a whole number of circuits.
• laundry circuits: a whole number of circuits; 0 when there is none.
• heating or cooling, the larger: entered in VA; the smaller of the two is left out.
• range: entered in VA, as the demand value your code gives for your range, or 0 with no electric range.
• clothes dryer: entered in VA.
• water heater: entered in VA.
• EV charger: entered in VA.

The fixed appliances table holds up to 12 rows of appliances fastened in place, each with its name and its load in VA, filled from the top with no blank row between them. A dwelling with none enters None and 0 VA.

Every one of these values is 0 or more. In this basis the workbook does not read a floor area above 1,000,000 sq ft, a circuit count above 100, or a load above 1,000,000 VA, and returns NOT VALID instead; an appliance load above 1,000,000 VA is refused before calculation. These limits keep the arithmetic finite and sit far above any dwelling; they are not engineering limits.

Governing relationships

For circuit row r, the workbook converts the selected input basis to noncontinuous current I_nc,r (A) and continuous current I_c,r (A), then applies:

I_min,r = I_nc,r + 1.25 I_c,r

The row apparent load is:

S_r = V_r I_r

for a current basis, or:

S_r = P_r / PF_r

for a real-power basis. V_r is the active 120 V or 240 V connection voltage, I_r is route current in A, P_r is real power in W, and PF_r is the entered power factor (fraction).

After the row planning-demand factor is applied, 240 V load is placed on both legs and 120 V load is placed on its selected leg. The reported reconciliation is:

S_total = (I_A + I_B) V_LL / 2

I_A and I_B are projected leg currents (A), and V_LL is line-to-line voltage (V). Maximum utilization, spare current, and leg imbalance are:

U = max(I_A,I_B) / I_service

I_spare = min(I_service-I_A, I_service-I_B)

B = abs(I_A-I_B) / I_service

I_service is the entered per-leg service or panel rating (A), and I_spare is in amperes.

In the Dwelling standard method basis the existing load is the dwelling's calculated load. With floor area A (sq ft), s small-appliance circuits and l laundry circuits, the general load G (VA) and the general load after its demand factors D (VA) are:

G = 3 A + 1500 (s + l)

D = min(G, 3000) + 0.35 min(max(G − 3000, 0), 117000) + 0.25 max(G − 120000, 0)

The fixed appliances count at a reduced factor when n, the number of rows above 0 VA, is four or more:

F = ΣVA × (0.75 if n ≥ 4 else 1)

With H the larger of heating or cooling, the calculated load S (VA) and its current I (A) are:

S = D + F + H + (range + dryer + water heater + EV)

I = S / V_LL

That current stands on both legs as the existing load, and the proposed rows of the circuit schedule are added to it:

I_A = I_B = I (the base legs in this basis)

The recommended service size is the smallest of 100, 110, 125, 150, 175, 200, 225, 250, 300, 350 and 400 A that is at least I rounded to 6 decimal places; above 400 A none is recommended. These constants are workbook data, editable and marked for review before use.

Calculation sequence

1. Validate every active selector, scalar bound, type, and every fixed-grid entry.
2. Apply the selected route and ignore values from inactive fields.
3. Normalize unit-dependent inputs into the workbook's calculation basis.
4. Evaluate every included row or candidate and aggregate the active schedule.
5. In the Dwelling standard method basis, check the dwelling inputs and their limits, compute the calculated load by category and in total, divide it by the line-to-line voltage, take the smallest standard service size at or above that current rounded to 6 decimal places, and place that current on both legs as the existing load.
6. Calculate primary and supporting outputs, populate the authored chart series, and apply derived numeric checks.
7. Return the workbook status with the computed results; invalid input is never converted into a plausible engineering answer.

Outputs and interpretation

• calculated load (VA): the dwelling's calculated load by the standard method, S above. It reads 0 outside the Dwelling standard method basis and when the status is NOT VALID.
• calculated load current (A): the calculated load divided by the line-to-line voltage. It reads 0 outside that basis and when the status is NOT VALID.
• recommended service size (A): the smallest standard size at or above the calculated load current rounded to 6 decimal places. It covers the dwelling's calculated load only, not the proposed rows of the circuit schedule. It reads 0 when that rounded current is above 400 A, outside that basis, and when the status is NOT VALID.
• projected leg a current (A): Existing leg-A basis plus all included proposed rows routed to leg A.
• projected leg b current (A): Existing leg-B basis plus all included proposed rows routed to leg B.
• maximum leg utilization (%): Larger projected leg current divided by the panel or service rating.
• 240v spare capacity (VA): Smaller remaining leg-current margin times line-to-line voltage; zero when either leg is overloaded.

The load by category table, in the Dwelling standard method basis, lists the general lighting load and the small-appliance and laundry circuits before the demand factors (rows 1 and 2), the general load after them (row 3), the fixed appliances with the factor applied, the larger of heating or cooling, and the range, dryer, water heater and EV charger together. Rows 3 to 6 add up to the calculated load. Its load cells are blank outside that basis and when the status is NOT VALID.

The chart is an explanatory view of the workbook's named chart ranges. It does not add a separate calculation or replace the numeric outputs.

Validation and status logic

The workbook evaluates Model_Status in the order shown. The first matching condition wins. The six conditions that open the table, and the two new CHECK conditions, apply in the Dwelling standard method basis only.

Assumptions and limitations

• In the Circuit schedule and AHJ-adjusted leg demand bases, all shipped/default engineering values and circuit rows are synthetic illustrative demonstrations and must be replaced with project-specific verified inputs. In the Dwelling standard method basis, the constants are code values of the standard dwelling calculation and the NEC's standard ampere ratings, to review against your adopted code before use, and the shipped dwelling values are a worked example to replace with your own.

• The service is modeled as a single-phase split-phase system; 120-volt loads occupy one leg and line-to-line loads occupy both legs.

• The 1.25 continuous-load multiplier is explicit and not editable; any user demand factor is a planning assumption that requires a documented local basis.

• External leg demand is assumed to have already been adjusted or approved by the user's electrician, utility, engineer, or authority having jurisdiction.

• All twelve grid rows are sent complete; inactive rows are explicit typed fillers and contribute zero.

• In the Circuit schedule and AHJ-adjusted leg demand bases, this is not an Article 220 service calculation, load study, permit document, fault-current study, conductor-ampacity check, or code-compliance determination.

• In the Circuit schedule and AHJ-adjusted leg demand bases, breaker-handle totals are not used as service demand, and the model does not apply dwelling demand tables, appliance demand factors, neutral calculations, feeder/service conductor sizing, temperature/termination limits, correction factors, or local amendments. The Dwelling standard method basis applies the standard method's general load demand factors and its fixed-appliance factor, and none of the others.

• The Dwelling standard method's constants (3 VA per sq ft, 1,500 VA per small-appliance or laundry circuit, the general load's first 3,000 VA at 100%, the rest up to 120,000 VA at 35% and any remainder at 25%, the 75% factor from four fixed appliances, and the 100 A to 400 A service sizes) are workbook data to review against the code your jurisdiction adopts before use. The method is a planning estimate, not a load study or permit document; it sizes no conductor and checks no neutral load.

• The water heater counts at 100%, as the worked examples count it. If you follow the code's grouping of a storage water heater with the appliances fastened in place, list it in the fixed appliances table instead and enter 0 for the water heater.

• The range load is the demand value your code gives for your range; the worked examples use 8,000 VA for one range up to 12 kW.

• Some code editions set a minimum load for an EV charger; enter the value your code gives. Enter a planned charger either as the EV charger load or as a proposed row of the circuit schedule, not both: entered in both places it is counted twice.

• The smallest size the table recommends is 100 A, the NEC minimum for a one-family dwelling service; your utility and panel maker may offer fewer sizes.

• The floor area is measured from the outside dimensions and leaves out open porches, garages and unfinished spaces not adaptable for future use.

• The recommended service size covers the dwelling's calculated load only. The proposed rows of the circuit schedule are added to the legs, not to that size; when they put a leg above it, and no earlier check applies, the status says so.

• The service size is looked up from the calculated load current rounded to 6 decimal places, but the rating check compares the unrounded leg currents with the panel or service rating. A current less than 0.0000005 A above a standard size therefore recommends that size, and with that same size entered as the panel or service rating the status can be the rating CHECK.

• In the Dwelling standard method basis the existing rows of the circuit schedule and the two external leg currents are not added to the legs, but every circuit row is still checked, and, unless the status is NOT VALID, an existing row whose breaker is below its modeled minimum still counts in the undersized-branch output and the verdict.

• A direct or MCP caller choosing the Dwelling standard method basis still sends the full circuit schedule and both external leg currents.

• A licensed electrician or engineer and the local authority having jurisdiction must verify the adopted code edition, panel and service ratings, measured demand basis, load diversity, leg allocation, equipment compatibility, and installation.

Restrictions and non-computing states

Each numeric control offers a working range chosen for usable entry, which is not always the limit this calculator enforces; a value outside that limit is rejected with a message rather than quietly accepted. The calculator additionally enforces allowed choices, active-field types, complete fixed-grid rows, and conditional or cross-field rules before computing.

In the Dwelling standard method basis this calculator reads only typed values and takes no drawing. A negative entry, a fractional circuit count, text where a number belongs, an appliance load above its limit and an appliance row missing its name or its load are refused before calculation. The workbook's status then returns NOT VALID for a floor area of zero, a floor area, circuit count or other load above its limit, and a blank row between appliance rows.

Unsupported route values, incomplete rows, out-of-domain inputs, relational failures, nonfinite derived arithmetic, and workbook error tokens are non-computing states. Hidden inputs remain in the values sent to the calculator but must not affect results while those fields are inactive. In the other two bases the hidden appliance table is still sent and still checked, so a negative, non-numeric or over-limit appliance load is refused until it is corrected in the Dwelling standard method basis.

Errors and warnings

A rejected entry means the submitted state failed the published input rules. Workbook NOT VALID identifies an invalid active domain or unsupported derived arithmetic. Workbook CHECK retains a finite result while flagging a planning condition that needs review. OK means only that the delivered workbook logic closed within the entered assumptions; it is not code compliance, professional approval, or a safety guarantee.

This version is checked against 122 saved test cases, covering both calculated results and entries that should be refused, run against this exact workbook.

• NFPA 70, National Electrical Code, current edition index - Authoritative current-edition pointer; users must identify the edition and amendments locally adopted for their project. Official NFPA publication index: the publisher's page for NFPA 70; check the edition your jurisdiction adopts. The dwelling standard method's constants on the Data sheet are individual numeric values of its Article 220 dwelling calculation and standard ampere ratings; no code text, table layout or commentary is reproduced.
• NFPA public record for branch-circuit conductor and continuous-load arithmetic - Public technical record supporting the 125 percent continuous-load relationship and informational voltage-drop context. Official NFPA standards-development record; the workbook cites clauses and independently implements arithmetic; no code text, table layout or commentary is reproduced.

Licensing conclusion: The circuit-schedule model's rows, defaults and demand factors are synthetic illustrations and are editable, as before. The dwelling standard method's constants and the standard service sizes are individual numeric values of the standard dwelling calculation in NFPA 70 Article 220 and of the NEC's standard ampere ratings. Each value is identified on the workbook's Data sheet with its source note and is marked review before use. No code text, table layout or commentary is reproduced, and no proprietary, manufacturer, utility or product-price dataset is embedded. The NFPA 70 page cited above is the publisher's page for the standard; check the edition your jurisdiction adopts.