Electrical Panel Schedule Builder: Calculate Circuit Totals, Balance Loads & Avoid Overloads

This guide explains electrical panel schedule calculation principles for safe, balanced circuit distribution and compliance.

Engineers will learn load balancing, circuit totals, feeder sizing, and overload prevention with examples included.

Electrical Panel Load Balancing and Main Breaker Sizing Calculator

System type Three-phase 4-wire wye Single-phase 2-wire

Nominal line voltage (V)

Main breaker rating (A)

Phase A total connected load (kVA)

Phase B total connected load (kVA)

Phase C total connected load (kVA)

Advanced options

Demand factor (%)

Max recommended main loading (% of rating)

Max acceptable phase unbalance (%)

You may upload a nameplate or single-line diagram photo to suggest typical values for this panel schedule.

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Enter the panel data to calculate main current, loading, and phase balance.

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Formulas used by this panel load calculator

• Total connected load per phase (kVA): kVA_phase,connected = user input for each phase.
• Demand load per phase (kVA): kVA_phase,demand = kVA_phase,connected × (Demand factor % ÷ 100).
• Single-phase line current (A): I_phase = (kVA_phase,demand × 1000) ÷ V_line.
• Three-phase phase-to-neutral voltage (V) from line-to-line voltage: V_phase-neutral = V_line-line ÷ 1.732.
• Three-phase line current per phase (A): I_phase = (kVA_phase,demand × 1000) ÷ V_phase-neutral.
• Average phase current (A): I_avg = (I_A + I_B + I_C) ÷ 3.
• Maximum phase current (A): I_max = max(I_A, I_B, I_C) for three-phase systems (for single-phase, I_max = I_A).
• Main breaker loading (% of rating): Loading % = (I_max ÷ Main breaker rating) × 100.
• Phase unbalance (% of average current, three-phase only): Unbalance % = (max(|I_A − I_avg|, |I_B − I_avg|, |I_C − I_avg|) ÷ I_avg) × 100.

Typical reference values for panel scheduling

Parameter	Typical values	Notes
Three-phase voltages	400/230 V (50 Hz), 208/120 V, 480/277 V (60 Hz)	First value is line-to-line, second is line-to-neutral.
Main breaker loading target	80% of rating	Common practice for continuous loads to avoid nuisance tripping.
Phase unbalance limit	≤ 10% (general), 2–5% (motor-intensive)	Higher unbalance increases neutral current and heating.
Demand factor range	60–90%	Based on occupancy type and applicable installation code tables.

Technical FAQ about this panel schedule calculator

Does this calculator size the main breaker using connected load or demand load?

The main current and loading are calculated from the phase demand loads, which are obtained by multiplying the connected loads per phase by the demand factor (%). This allows you to reflect diversity according to your design criteria or code tables.

How should I interpret the phase unbalance percentage?

The phase unbalance is the maximum deviation of any phase current from the average current, expressed as a percentage of the average. Values below about 10% are usually acceptable for general-purpose panels, while motor-intensive installations typically require tighter limits.

What happens if the calculated loading exceeds the main breaker rating?

If the maximum phase current exceeds 100% of the main breaker rating, the panel is considered overloaded and the result will flag this condition. You should then either reduce the connected load, increase the breaker and busbar rating, or split the load to additional panels.

Can I use this tool for both single-phase and three-phase panel schedules?

Yes. For single-phase panels select “Single-phase 2-wire” and enter the total connected kVA on the single phase. For three-phase panels select “Three-phase 4-wire wye” and enter the connected kVA on phases A, B, and C. The tool then evaluates total demand, per-phase currents, phase balance, and main breaker loading.

Scope and objectives of a panel schedule builder

An electrical panel schedule builder is a procedural and computational tool engineers use to:

• Calculate total circuit currents and power demands for every panel and feeder.
• Assign circuits to phases to minimize neutral currents and unbalance.
• Verify compliance with applicable codes and manufacturer ratings to avoid overloads.
• Provide a definitive schedule for commissioning, maintenance, and future expansion.

Key outputs expected from the schedule

• Per-circuit load (VA and amperes).
• Per-phase and total panel currents.
• Feeder breaker size and conductor ampacity with adjustment and correction factors.
• Identification of continuous vs non-continuous loads and application of factors.
• Recommended phase assignment to achieve phase balance.

Fundamental definitions and measurement units

• Voltage (V) — line-to-neutral or line-to-line, depending on system (V).
• Current (I) — measured in amperes (A).
• Power (P) — real power in watts (W) or volt-amperes (VA) for single-phase and three-phase applications.
• Power factor (pf) — ratio of real power to apparent power (0–1).
• Continuous load — a load expected to run for three hours or more (code definitions apply).

Essential formulas and variable definitions

Use these formulas in plain text; variables are then defined with typical values.

Where:

• P = power (W or VA when pf = 1).
• V = line-to-neutral or line-to-line voltage, typical values: 120 V, 208 V, 240 V, 277 V, 480 V.
• I = current (A).
• pf = power factor (typical pf: lighting 0.9–1.0, motor loads 0.8–0.95).

Where:

• V_L-L = line-to-line voltage (typical values: 208 V, 480 V).
• I_L = line current per phase (A).
• √3 ≈ 1.732.

For conductor and breaker selection when continuous loads exist, apply the 125% rule:

Where I_continuous and I_non-continuous are currents for continuous and non-continuous portions respectively.

Neutral current for multi-wire branch circuits or unbalanced three-phase systems is:

Explanation of variables with typical values

• I_continuous typical examples: lighting circuits (if scheduled continuously) 5–10 A per circuit, HVAC compressors variable.
• pf typical: lighting 0.9–1.0, resistive loads 1.0, motors 0.8–0.95.
• V typical: single-phase branch circuits 120/240 V; three-phase branch circuits 208/120 V or 480/277 V.

Data required to build a precise schedule

1. Full list of branch circuits with load descriptions, connected load in watts or VA, and anticipated duty cycles.
2. Device/motor nameplate data: locked-rotor, full-load amps (FLA), service factor, and motor efficiency.
3. Lighting schedules with fixture wattage, quantity, and control type (occupancy sensor, DALI, etc.).
4. Heating and HVAC loads including diversity and standby factors.
5. Voltage, available short-circuit current, and system grounding scheme.

Common load categories and typical values

Stepwise method to calculate circuit totals and balance loads

1. Inventory and quantify each circuit’s connected load in watts or VA.
2. Convert wattage to current using the appropriate single-phase or three-phase formula and assumed power factor.
3. Mark loads as continuous or non-continuous; apply 125% to continuous loads for breaker/feeder sizing.
4. Group branch circuits into logical panels and begin phase assignment to minimize phase currents.
5. Sum per-phase currents and compute neutral currents if applicable.
6. Select main and feeder protective devices considering inrush (motors), voltage drop, and temperature correction factors.
7. Verify code-required demand factors and diversity allowances are applied where allowed.

Phase assignment strategies

• Alternate phase assignment for single-pole circuits: A, B, C, A, B, C to approximate balance.
• Group large single-phase loads across phases manually to counter imbalance caused by multiple heavy loads.
• For multi-wire branch circuits, place shared neutral circuits on correct phase pairs to avoid neutral overload.

Practical example 1 — Single-phase residential panel (detailed)

Scenario: 120/240 V single-phase service, 200 A main. Residential panel with the following branch circuits:

• Lighting circuits: 3 circuits, each 12 LED fixtures × 10 W = 120 W per fixture total 1440 W per circuit.
• Small appliance branch: two 20 A circuits, each with 1800 W continuous load.
• Electric water heater: 4500 W at 240 V (continuous).
• Range: 8000 W at 240 V (demand factor applies).
• Dryer: 5000 W at 240 V.

Step 1 — Convert loads to currents (single-phase where applicable):

Step 2 — Apply continuous load rule for breaker/feeder sizing:

Step 3 — Determine breaker sizing and conductor selection:

• Lighting circuits: loads 12 A, choose 15 A breakers with #14 Cu conductors (verify fixture count and code).
• Small appliance circuits: 15 A load, 20 A breaker with #12 Cu conductors.
• Water heater: adjusted current 23.44 A, select 30 A double-pole breaker with #10 Cu.
• Range: per code apply range demand factor (NEC Table 220.55 alternative method) — for demonstration choose 40 A double-pole breaker with #8 Cu (verify per code).
• Dryer: 20.83 A choose 30 A double-pole breaker with #10 Cu.

Step 4 — Panel load summary and main sizing check:

Total continuous-equivalent currents for feeder calculation (convert 240 V loads to equivalent 120 V branch currents for diversity analysis or use load calculations per NEC Article 220): perform simplified feeder check by summing maximum branch currents adjusted for duty.

Simplified apparent feeder current estimate (approximation):

I_total_approx = (Lighting: 3 × 12.0 A) + (Small appliance: 2 × 15.0 A) + (Water heater adjusted 23.44 A) + (Range 33.33 A) + (Dryer 20.83 A)

Compute numeric:

Result: a 200 A main provides margin for diversity and future load growth. For exact compliance, perform NEC Article 220 load calculation including dwelling unit occupant and demand factors.

Practical example 2 — Three-phase commercial panel (detailed)

Scenario: 480/277 V, three-phase panel feeding mixed loads: lighting, HVAC rooftop unit, three single-phase motors, and receptacle circuits. Goal: compute per-phase currents, apply motor starting considerations, and balance phases.

• Lighting: total 12 kW at 277 V (lighting circuits on 277 V line-to-neutral).
• Rooftop HVAC: 25 kW three-phase, pf 0.9.
• Motor A: 10 hp, nameplate FLA = 22 A at 480 V, pf 0.85.
• Motor B: 5 hp, nameplate FLA = 11 A at 480 V, pf 0.85.
• Motor C: 7.5 hp, nameplate FLA = 16 A at 480 V, pf 0.85.
• Receptacles and misc: 6 kW single-phase at 480 V three-phase balanced across phases (assume balanced allocation).

Step 1 — Calculate lighting current (lighting at 277 V L-N):

I_lighting = 12000 W / 277 V ≈ 43.32 A (this is total line current per phase if distributed; treat as single-phase L-N circuits possibly split across phases)

Assume lighting circuits are distributed evenly across three phases, so per-phase lighting current ≈ 43.32 A / 3 ≈ 14.44 A per phase.

Step 2 — HVAC three-phase current:

Step 3 — Motor currents (use nameplate FLA directly):

• Motor A: 22 A (per phase)
• Motor B: 11 A
• Motor C: 16 A

Step 4 — Receptacles: 6000 W balanced three-phase. Convert to per-phase current:

I_recepts = P_total / (√3 × V_L-L) = 6000 / (1.732 × 480) ≈ 7.22 A total line current per phase.

Step 5 — Assign loads to phases to achieve balance. Goal is per-phase total currents as close as possible. Distribute lighting equally and allocate motors to phases considering starting torques.

Step 6 — Per-phase running currents (sum all per-phase contributions):

Note: For strictly three-phase loads (motors, HVAC), the current is the same in all three phases when balanced. Lighting and receptacles were distributed equally.

I_phase_total ≈ lighting_per_phase + HVAC + motorA + motorB + motorC + recepts

Step 7 — Consider motor starting currents (inrush). Large motors may draw 4–7× FLA at starting depending on starting method. To avoid nuisance trips:

• Specify service/feeder protective device with short-time delay, or use soft starters/VFDs for motors.
• Conduct a diversity/inrush study for simultaneous starting scenarios.

Feeder design: choose conductor and breaker sized to handle continuous and non-continuous components. Apply any code-mandated demand factors for lighting or receptacles in commercial occupancies per authority having jurisdiction and local codes.

Neutral current and harmonic considerations

When designing a panel schedule, especially with single-phase nonlinear loads (electronic ballast, VFDs, office equipment), account for harmonics which can increase neutral current beyond simple arithmetic sums.

• For three-phase, four-wire circuits with non-linear loads, compute neutral current as the vector sum of phase currents, not algebraic sum; harmonics (especially 3rd and multiples) add in the neutral.
• Consider oversizing neutrals or using harmonic mitigations (filters, K-rated transformers) where harmonic distortion exceeds equipment ratings.

Verification steps and safety margins

1. Cross-check all nameplate data and verify that calculated breaker settings do not exceed equipment OCPD ratings.
2. Confirm conductor ampacity after applying ambient temperature correction and conduit fill adjustment factors.
3. Verify voltage drop under maximum load conditions; keep feeder voltage drop ≤ 3% to point of utilization when possible.
4. Check coordination studies for selective tripping between branch devices and upstream protective devices.
5. Perform physical panel layout review: grouping, phase sequence, labeling, and arc-flash hazard analysis.

Documentation format for the panel schedule

A practical schedule must include the following columns at minimum:

• Circuit number and panel location.
• Load description and connected VA/W.
• Voltage and phase.
• Calculated continuous and non-continuous currents.
• Breaker size and type (single-pole, double-pole, three-pole).
• Conductor size and insulation rating.
• Notes for special conditions (e.g., motor inrush, continuous, essential load).

Normative references and authority sources

• NFPA 70, National Electrical Code (NEC) — key requirements for load calculations and overcurrent protection: https://www.nfpa.org/nec
• IEC 60364 series — electrical installations for buildings (international guidance): https://www.iec.ch
• IEEE Std 141 (Red Book) — Recommended Practice for Electric Power Distribution for Industrial Plants: https://standards.ieee.org/
• Manufacturer datasheets and UL listings for equipment and overcurrent protective devices.
• Local authority having jurisdiction (AHJ) documentation and amendments to national or international codes.

Checklist for avoiding overloads and ensuring reliability

1. Validate nameplate currents and warranty instructions prior to final sizing.
2. Model inrush conditions and stagger motor starting when necessary.
3. Apply continuous load multipliers correctly and document calculations.
4. Use thermal and ambient correction factors for conductor ampacity.
5. Confirm short-circuit currents and protective device interrupting ratings.
6. Implement labeling and documentation for maintenance and future changes.

Commissioning, testing, and maintenance considerations

• Perform megger testing and continuity checks on newly installed feeders and branch circuits.
• Record phase currents during initial commissioning under known loads to validate balance assumptions.
• Schedule thermographic inspection of panels under load to identify hotspots and overloaded terminations.
• Update the panel schedule when modifications occur and archive versions for traceability.

Final verification and commissioning considerations

Prior to energization, ensure that the calculated feeder and branch currents do not exceed the selected conductor ampacity and OCPD ratings after applying all correction factors. Check coordination curves to prevent undesired upstream trips during temporary inrush events.

Document acceptance tests including:

• Measured phase currents and voltages at full load.
• Voltage drop verification.
• Trip testing of protective devices and interlocks.
• Verification of labeling, schematics, and as-built schedule.

Summary of best practices for schedule builders

• Always use nameplate and manufacturer data as primary inputs.
• Apply code-prescribed demand and diversity factors only where permitted.
• Design with headroom for future expansion and realistic operating profiles.
• Consider motor starting and harmonic content early in design to prevent retrofit mitigations.
• Maintain clear, versioned documentation for safety and maintenance.

Additional authoritative resources

• NEC Handbook and Annexes for worked examples on load calculations: https://www.nfpa.org/NEC
• IEC 60364 explanations and harmonization guidance: https://webstore.iec.ch/
• IEEE application guides for power systems and distribution: https://www.ieee.org/

If you require a downloadable panel schedule template or an automated calculation spreadsheet tailored to your system voltage, phase count, and typical loads, provide the component list and voltage system and I will prepare a detailed worksheet and populated schedule for review.