Panel Schedule Calculator

Method

How the calculator sizes each branch circuit, adds up the panel load, and checks the panel against NEC 2023. Every result in the circuit editor lists its steps and the sections that apply.

Equipment and positions

Product lines

Lighting and appliance panelboards (Eaton PRL1X and PRL2X, Square D NQ and NF, Siemens P1 and P2, ABB ReliaGear RQ and RE/RS), power panelboards (Eaton PRL4X, Square D I-Line HCJ and HCP, Siemens P4/P5, ABB ReliaGear neXT) and switchboards (Square D QED-2, Eaton Pow-R-Line Xpert, Siemens SB, ABB ReliaGear SB), plus generic equipment. Each line lists its systems, bus ratings, circuit counts or branch space, and the breaker families it accepts, from the manufacturers' catalogs (sources below).

Panelboard positions

Odd circuits are on the left and even circuits on the right; each row is one pole space. Phases follow the rows A, B, C (A, B on single-phase panels), so a 2-pole breaker at circuits 1 and 3 connects phases A and B. A breaker uses one space per pole, more where the manufacturer says so (for example, a 150 A QO-VH 2-pole breaker uses 4 spaces). In power panelboards a row is the manufacturer's mounting unit (1.5 in for I-Line, 1.25 in for Siemens, 1X for Eaton and ABB), and large frames use several rows or the full width.

Switchboards

Each distribution device has a section, a side of the vertical bus and a height from the top of the section. Frames that mount two across take one side; larger frames take the full width. A new device goes to the highest free place, left side first, and a new section is added when no section has room. Drag a device on the drawing, or use the Section, Side and Move buttons of the editor, to move it; devices below it in that column move down to make room. Devices are numbered in mounting order, section by section, top to bottom, left before right. 1- and 2-pole devices connect to the least-loaded phases unless a phase is chosen.

Circuit loads

Load and current

Loads are entered in VA, W, kW, kVA or amperes and treated at unity power factor. I = VA ÷ V for 1-pole and single-phase loads, VA ÷ (√3 × V) for three-phase loads, at the nominal voltage (220.5(A)). Receptacle outlets are 180 VA each (220.14(I)); sign outlets not less than 1,200 VA (220.14(F)); dwelling small-appliance and laundry circuits not less than 1,500 VA (220.52).

Motors

Full-load current from Table 430.248 (single-phase) or Table 430.250 (three-phase), not the nameplate (430.6(A)(1)). The load is FLC × V.

HVAC units

Nameplate MCA and MOCP (440.4(B)). The load is MCA × V, which is conservative because the MCA includes 125 % of the largest motor.

Continuous loads

Loads operating 3 hours or more are sized at 125 % (210.20(A), 215.3). Space heating, storage water heaters and EVSE branch circuits are sized at 125 % in every case (424.4(B), 422.13, 625.41); EV charging is always a continuous load (625.42).

Breakers

The breaker family is the smallest frame of the product line that covers the rating, the poles and the circuit voltage. Slash-rated breakers (120/240 V, 480Y/277 V) are used only on solidly grounded systems where the voltage to ground does not exceed the lower value, so they are excluded from delta systems and from the high leg (240.85). The interrupting version is the lowest one not less than the available fault current when the panel is fully rated (110.9). GFCI, equipment ground-fault, AFCI and dual-function breakers are chosen by the protection field.

Conductors, grounding and raceway

Ampacity

Copper (or aluminum) THHN/THWN-2 from Table 310.16. The ampacity at the terminal temperature, 60 °C for circuits of 100 A or less and 75 °C above unless the equipment is listed for 75 °C (110.14(C)(1)), is not less than the noncontinuous load plus 125 % of the continuous load (210.19(A)(1), 215.2(A)(1)). The 90 °C ampacity corrected for ambient temperature (Table 310.15(B)(1)(1)) is not less than the load (210.19(A)(2)). Each circuit is assumed to be in its own raceway, so no more than three current-carrying conductors.

Protection

The conductor is protected by the breaker (240.4); the next standard rating above the ampacity is permitted up to 800 A except for receptacle circuits (240.4(B)); 14, 12 and 10 AWG copper are limited to 15, 20 and 30 A (240.4(D)). Motor and HVAC conductors follow 430.22 (125 % of FLC) and the MCA instead (240.4(G)).

Parallel sets

Above the largest size chosen in the design basis, conductors are run in parallel sets of 1/0 AWG and larger (310.10(G)).

Grounding conductor

Table 250.122 by the breaker rating, increased in proportion to the circular mils per phase where the circuit conductors are larger than the code minimum (250.122(B)), and not larger than the circuit conductors. One in each raceway of a parallel run (250.122(F)).

Raceway

THHN areas from Chapter 9 Table 5 (insulated grounding conductor), 40 % fill for three or more conductors (Chapter 9 Table 1), trade size from Table 4, not smaller than the design basis.

Panel load (Article 220)

The calculated load adds 25 % of the continuous loads after demand factors (215.2(A)(1), 215.3) and 25 % of the largest motor (430.24). The current the bus and main must carry is the larger of the calculated load divided by √3 × V (three-phase) or V (single-phase), and the heaviest line current at the same ratio of calculated to connected load. The spare capacity of the design basis is added on top as a design choice: falling short of it gives a warning, not an NEC finding.

The neutral load is the maximum unbalanced load (220.61(A)): the largest line-to-neutral load on one phase, with ranges and dryers at 70 % (220.61(B)(1)). Feeders to other panels are not included; the 70 % reduction above 200 A (220.61(B)(2)) is not applied.

Phase loads and currents

The schedule shows each circuit's VA divided equally among its poles, the usual drawing convention. The phase and neutral currents are the phasor sums of the circuit currents at unity power factor: a 1-pole load draws VA ÷ V_LN from its phase and returns it on the neutral, a 2-pole load VA ÷ V_LL in both lines, and a 3-pole load is three equal loads across the lines. On a high-leg delta panel, phase B has 208 V to ground and cannot supply 120 V loads (110.15, 408.3(E)(1)).

Panel checks

Ratings

The bus is not less than the design current (408.30). The main breaker is between the design current and the bus rating (215.3, 408.36); a main-lug panel needs a supply-side device not larger than the bus. Circuits fit the spaces (408.54).

Short-circuit rating

Fully rated: the lowest interrupting rating of the main and every branch device is not less than the available fault current (110.9, 110.10). Series rated: the entered rating, which must be a tested and listed combination (240.86(B)) and marked (110.22(C)); the main is still rated for the available fault current, and the series rating is not used where the motor full-load currents exceed 1 % of the lowest branch interrupting rating (240.86(C)). Branch devices are 5 kA or more (240.16). The available fault current and the date are marked on the equipment except in one- and two-family dwellings (408.6, 110.24 at services).

Other

Branch connector pair limits from the manufacturer (Eaton PRL1X/PRL2X 140 A, Square D NF 170 A). Ground-fault protection of equipment for 480Y/277 V mains and service disconnects of 1000 A and more, and a note for feeder devices of 1000 A and more (230.95, 215.10, 240.13). Arc energy reduction for breakers of 1200 A and more (240.87). Arc-flash labels at 1000 A and more (110.16(B)). Source marking for feeder-supplied equipment other than one- and two-family dwellings (408.4(B)), circuit descriptions (408.4(A)), high-leg marking (408.3(F)(1)), GFCI (210.8), AFCI (210.12), EVSE receptacles (625.54) and fire alarm circuits (760.41(B)).

Excel workbook

Export to Excel writes three sheets. Schedule is the drawing layout; its VA cells point to the Circuits sheet. Circuits lists every circuit with its load, continuous share, breaker, device and conductors; the blue values are inputs. Load Calc repeats the Article 220 calculation with formulas, so changing a load or a design-basis value updates the totals and the currents. The heaviest-phase and neutral currents are copied from the calculator and are not recalculated in the workbook; breakers and conductors are not resized when a load is changed in the workbook.

Limits

• Device selections follow published catalog data; confirm frames, mounting heights, connector limits and series ratings with the manufacturer before ordering. Some mounting heights are estimated and are marked in the circuit editor.
• Hotel and motel lighting demand factors are taken as 60 %, 50 % and 35 % (2020 revision). If the 2023 text differs, the earlier values (50 %, 40 %, 30 %) are lower, so these are conservative.
• Not covered: the optional dwelling calculations (220.82 to 220.84), health care receptacle demand (220.110), existing loads (220.87), the 2023 sentence of 220.60 on 125 % of a smaller noncoincident motor or A/C load, fire pumps (Article 695), elevators (Article 620), voltage drop, and more than three current-carrying conductors in one raceway.
• Ranges below 8¾ kW use Column C; Columns A and B give lower values.

Checks of the calculator

Worked examples run each time this page opens.

Sources

NFPA 70, National Electrical Code, 2023 edition. Manufacturer data: Eaton catalog CA08100003E and design guides DG014001EN, DG014003EN and DG015001EN; Square D Digest Plus (NQ, NF, I-Line, QED-2); Siemens SPEEDFAX Section 11, P1 data sheet, P4/P5 and switchboard catalogs; ABB ReliaGear lighting panelboard catalog 1TQC173600E0001, neXT catalog 1SQC900001C0201 and SB design guide 1SQC088005C0201.