Circuit Calculator

Method

Sizes the overcurrent device, conductors, equipment grounding conductor and raceway of a branch circuit or feeder of 1000 V and less by the 2023 NEC, and finds the voltage drop, the maximum load and the maximum length. Each result in the calculator opens to show its steps and the section that requires it.

Load

General load

Amperes, or volt-amperes or watts at the stated power factor, with I = VA ÷ V single-phase and I = VA ÷ (√3 × V) three-phase. A continuous load is the maximum current for 3 h or more (Article 100). On a 208Y/120 V 3-wire circuit the line-to-line voltage is used, which is conservative for line-to-neutral loads.

Motor

Full-load current from Table 430.248 (single-phase) or Table 430.250 (three-phase) at the motor voltage of the system (115, 200, 208, 230, 460 or 575 V), used for the conductors and the short-circuit and ground-fault device (430.6(A)(1)); synchronous motors from the unity power factor columns of Table 430.250 (× 1.1 at 90 % and × 1.25 at 80 % power factor). The nameplate current sizes the overloads, and is used where the tables give none (430.6(A)(2)).

Air conditioning

The nameplate minimum circuit ampacity and maximum overcurrent protection (440.4(B)), or the compressor rated-load current with the fan and other loads (440.32, 440.33, 440.22).

Heating, water heater, EV

Fixed electric space heating with its motors at 125 %, against both the allowable ampacity and the ampacity after correction and adjustment (424.4(B)); storage water heaters of 120 gal or less at 125 % (422.13); EV charging loads are continuous (625.42).

Feeder

Noncontinuous and continuous load, and motors (125 % of the largest plus the others, 430.24). A dwelling feeder of 100 through 400 A carrying the whole load may use 83 % of its rating (310.12(B), Table 310.12(A)), with 75 °C terminations as the table assumes.

Overcurrent device

Minimum rule

General loads, heating, water heaters, EV equipment and feeders without motors: not less than the noncontinuous load plus 125 % of the continuous load (210.20(A), 215.3, 625.41), the next standard rating of Table 240.6(A) up. A device and assembly listed for 100 % operation take the load at 100 %. Water heaters: not more than 150 % of the rated current, next size up permitted (422.11(E)). A branch circuit with more than one receptacle is rated 15, 20, 30, 40 or 50 A (210.3, 210.23(A)), with receptacles rated by Table 210.21(B)(3).

Maximum rule

Motors: the Table 430.52(C)(1) percentage for the motor type and device, next standard size up (430.52(C)(1)(a)); where the motor will not start, the maximums of 430.52(C)(1)(b) or (C)(3). A/C: the nameplate maximum, or 175 % of the rated-load current plus the other loads with the next size up (440.22(A) and (B)), 225 % where it will not start. Feeders with motors: the largest motor branch device plus the other loads, next standard size down (430.62(A), 430.63).

Ratings

Table 240.6(A), 2023, which includes 10 A; fuses also 1, 3, 6 and 601 A.

Conductors

Minimums

The noncontinuous load plus 125 % of the continuous load against the Table 310.16 value at the terminal temperature, before correction and adjustment (210.19(A)(1), 215.2(A)(1), 110.14(C)); and the load against the ampacity after correction and adjustment (210.19(A)(2), 215.2(A)(2)). Motors: ampacity not less than 125 % of the full-load current (430.22). A/C: not less than the MCA (440.35).

Protection

Ampacity not less than the device rating, or the device is the next standard rating above the ampacity up to 800 A (240.4, 240.4(B)); not for multioutlet receptacle circuits. Small conductors by 240.4(D). Motor and A/C conductors are protected under Articles 430 and 440 instead (240.4(G)).

Ampacity

Table 310.16 at the insulation rating, or Table 310.20 for conductors on a messenger (40 °C basis); times the ambient correction of Table 310.15(B)(1)(1) or (B)(1)(2), with 33 °C added where a raceway or cable is less than 3/4 in above a roof, except XHHW-2 (310.15(B)(2)); times the adjustment of Table 310.15(C)(1) for the current-carrying conductors in the raceway or bundle (neutral counted by 310.15(E), EGC not counted). Not more than the terminal value: by default 60 °C for circuits of 100 A or less and 75 °C above (110.14(C)(1)); 75 °C for three-phase squirrel-cage motors with design letter B, C or D ((a)(4)); with an instantaneous-trip breaker the 125 % conductor current, not the trip setting, decides. NM and UF cable are limited to 60 °C (334.80, 340.80); SE cable in thermal insulation likewise (338.10(B)(4)(a)). Type MC cable, 12 AWG copper, three or fewer current-carrying conductors and twenty or fewer in total: no adjustment (310.15(C)(1)(d)).

Parallel

Single conductors and MC or TC cable 1/0 AWG and larger, each set in its own raceway or cable (310.10(G)). The smallest number of sets, then the smallest size.

Grounding, raceway, voltage drop

EGC

Table 250.122 for the device; for an instantaneous-trip breaker, the dual-element fuse rating the motor table would permit (250.122(D)(2)). Increased in proportion where the circuit conductors are increased for any reason other than correction and adjustment (250.122(B)); not larger than the circuit conductors (250.122(A)); one in each parallel raceway or cable (250.122(F)). A cable’s own grounding conductor, or a multiplex messenger used as the EGC (250.118(1), 225.4 Exception), is checked against the required size.

Raceway

Conductor areas from Chapter 9 Table 5 (Table 5A for compact aluminum, Table 8 for bare); cables at their published diameter, the major diameter for flat cables, each counted as one conductor (Notes 5 and 9). 53, 31 or 40 % fill (Table 1), 60 % in a nipple of 24 in or less (Note 4).

Voltage drop

Ze = R cos θ + X sin θ from Chapter 9 Table 9 at 75 °C (PVC, aluminum or steel conduit; the PVC values for cables and multiplex); VD = 2 I Ze L ÷ 1000 single-phase, √3 I Ze L ÷ 1000 three-phase, at the load current. The NEC suggests 3 % for a branch circuit or feeder and 5 % in total (210.19 and 215.2 Informational Notes).

Maximum load

The load, at the same continuous share, that reaches the first of: the device rule, the terminal-temperature minimum, or the corrected ampacity. Motors: ampacity ÷ 125 %, and the largest table motor at that current.

Maximum length

The length at which the voltage drop reaches the limit at the load current: L = VD × 1000 ÷ (k × I × Ze).

Ground fault

Informational: a bolted fault to ground at the load with an infinite source, through the circuit conductor and the EGC (Table 9 resistance, loop reactance 2X), compared with a multiple of the device rating for fast clearing (250.4(A)(5)).

Installation

Permitted uses of the wiring method in the location (Articles 330 to 340, 310.10(C), 300.5(B)); minimum cover by Table 300.5; overhead clearances by 225.18.

Cable constructions

The 3D cutaway and the cable cross-section are drawn from the manufacturer specification of each wiring method: Southwire SPEC 10000, 10025 and 10012 (THHN/THWN-2), 10005, 10006 and 10015 (XHHW-2), 10010 and 10020 (RHH/RHW-2/USE-2), 10028 (NM-B), 10036 (UF-B), 60100, 60103, 60115, 60135, 60280 and 60297 (MC), 10041 (SER), 45252 and 45103 (TC-ER), 83230 and 83248 (triplex and quadruplex). Where a construction is not published (2- and 4-conductor MC feeder and TC-ER cable), the diameter is calculated from the published 3-conductor cable of the same size and marked as calculated. Raceways are drawn at their Chapter 9 Table 4 dimensions.

Checks

Each case is solved live by this calculator and compared with a published worked example.

Limitations

• Circuits of 1000 V and less. Cable tray (392.80), Annex B underground ampacity and conductors over 90 °C are not covered.
• Single motors per branch circuit. Multimotor branch circuits, motor-operated appliances, welders and capacitors are not covered.
• Feeder loads are entered as calculated by Article 220; the neutral is the same size as the ungrounded conductors.
• Terminal temperatures, equipment ratings, the available fault current and the device time-current curves should be confirmed for the actual equipment.