How ChargeSpec sizes an EV charger circuit

ChargeSpec is an Android app by Sonicon Software for US electricians and EVSE installers. For one EV charger job it works out the branch-circuit load, conductor, breaker and voltage drop from NEC 2023 data. It can check the dwelling service load and produce a US Letter spec sheet. It is a calculation record, not a permit or sign-off.

The engine is a separate module that holds the NEC 2023 values. The screens show its results and never work out code values themselves.

Continuous load (125%)

ChargeSpec multiplies the charger's rated amps by 1.25 and sizes the circuit from that figure, because EV supply equipment is treated as a continuous load. A 48 A charger becomes a 60 A load. The app accepts charger ratings from 12 A to 80 A at 120 V or 240 V.

NEC 2023 data
625.41
210.19(A)(1)

Conductor

You choose copper or aluminum, the insulation rating and the termination rating (60, 75 or 90 °C). ChargeSpec uses the lower of the two as the Table 310.16 column and applies the 310.15(B)(1) ambient correction. It then picks the smallest size, 14 AWG to 500 kcmil, whose corrected ampacity meets the load.

NEC 2023 data
Table 310.16
310.15(B)(1)
110.14(C)

At 30 °C ambient there is no correction (factor 1.00). Hotter air derates the conductor and can step the size up. ChargeSpec doesn't know your cable type or how many conductors share a raceway, so it doesn't apply cable-type limits or the adjustment for more than three current-carrying conductors. Choose the temperature column your cable and terminations allow.

Breaker

ChargeSpec takes the next standard breaker rating from 240.6(A) at or above the continuous load. If that breaker is larger than the conductor's corrected ampacity, it steps the conductor up until the conductor's ampacity is at least the breaker rating. For a 60 A load, that is a 60 A breaker.

NEC 2023 data
240.6(A)
240.4

Voltage drop

ChargeSpec calculates the drop as 2 × amps × conductor resistance × run length, using the charger's rated amps and resistance per 1,000 ft from NEC Chapter 9, Table 8. It shows volts and percent. Above 3%, it flags the run and suggests the smallest larger conductor that brings the drop to 3% or less.

NEC 2023 data
210.19 Info Note

It also flags a drop above 5%. It doesn't take a feeder drop as input, so add your feeder's share yourself. Resistance values are for uncoated copper and for aluminum at 75 °C. The method is single-phase, two-wire, resistance only. Both thresholds are efficiency guidance (210.19 informational note), not code violations.

Worked: 48 A, 6 AWG copper, 250 ft, 240 V: 11.8 V, 4.91%. 3 AWG: 2.45%.

Service load check

Pro

Pro checks whether a 240 V dwelling service can take the charger, using the NEC 220.82 optional method. You enter living area, small-appliance and laundry circuits, fastened appliances, heating and cooling, and the service rating. The charger is added at the larger of 7,200 VA or its nameplate, without a demand factor. The result is pass or fail.

NEC 2023 data
220.82
220.57

A pass shows the headroom in amps, and a fail shows the shortfall.

Spec sheet

Pro

The spec sheet is a US Letter PDF with the job's inputs and results, the NEC 2023 label, the date and a professional-use disclaimer. Pro saves, prints or shares it. Anyone can preview the sheet first. It carries the service verdict when you've run the check.

The disclaimer printed on every sheet:

This sheet is an aid for a qualified professional. It does not replace the National Electrical Code, other applicable codes, or the authority having jurisdiction. Reference data: NEC 2023 data. Verify all values before installation.

Inputs and limits

NEC 2023 data only. Jurisdictions may be on another cycle or a local amendment.
Charger rating 12–80 A. Supply 120 V or 240 V; the service check is 240 V only; no 208 V.
Conductors 14 AWG to 500 kcmil, copper or aluminum. Ambient correction up to 80 °C.
Not modelled: raceway fill and the more-than-three-conductor adjustment, cable-type limits, three-phase, reactance or power factor in voltage drop, a feeder drop input.
It doesn't apply the NEC 240.4(D) small-conductor limits, so check any 14–10 AWG result against 240.4(D) yourself.
A calculation record for professional use, not the code, a permit or a sign-off.

Where the numbers come from

The values are stored in the app by NEC 2023 section: Table 310.16 ampacities, Table 310.15(B)(1) ambient factors, 240.6(A) standard breaker ratings, Chapter 9 Table 8 conductor properties, and the 220.82 and 220.57 dwelling values.

NEC is a trademark of the NFPA. ChargeSpec is not affiliated with or endorsed by the NFPA.