About EV Charging Load Calculator
The EV charging load calculator sizes the feeder and service for one or more electric-vehicle supply equipment (EVSE) units using the rules of National Electrical Code (NEC) Article 625. Because EV charging runs for three hours or more, each EVSE is treated as a continuous load and is sized at 125 percent of its rating (NEC 625.41 and 210.20(A)).
Enter the number of chargers, each charger rating (as power in kW or as current in amps), the system voltage and phase, the continuous-load factor (default 1.25), and an optional diversity or energy-management demand factor (NEC 625.42). The tool returns the per-charger continuous current, the total connected demand in amps and kVA, and the recommended minimum feeder or service ampacity, both with and without diversity.
How It Works
- Enter the number of EVSE units and each unit rating, either as power (kW) or rated current (A).
- Choose the system voltage and whether the supply is single-phase or three-phase.
- The calculator converts between power and current (single-phase I = P*1000/V, three-phase I = P*1000/(sqrt(3)*V)) and applies the 1.25 continuous-load factor to each charger.
- It sums the chargers for the total connected demand, then applies the diversity/EVEMS demand factor to give the design current and recommended minimum feeder/service ampacity.
Worked Example
A site has 5 Level 2 chargers, each rated 40 A at 240 V single-phase, with no energy management (demand factor 1.0). Each charger is a continuous load, so its design current is 40 * 1.25 = 50 A. The total connected continuous current is 5 * 50 = 250 A, which at 240 V is 240 * 250 / 1000 = 60 kVA. With no diversity the design current equals the total, 250 A, so the feeder/service must be sized for at least 250 A. If an EVEMS limited simultaneous demand to 60 percent (demand factor 0.6), the design current would drop to 250 * 0.6 = 150 A (36 kVA).
Formulas
- Per-charger continuous load (NEC 625.41)
I_continuous = I_rated * 1.25- Single-phase current
I = P * 1000 / V- Three-phase current
I = P * 1000 / (sqrt(3) * V)- Total connected demand and diversity (NEC 625.42)
I_total = n * I_continuous ; I_design = I_total * demandFactor- Demand in kVA
single-phase: kVA = V * I / 1000 | three-phase: kVA = sqrt(3) * V * I / 1000
Standards & References
- NEC Article 625 (Electric Vehicle Power Transfer System)
- NEC 625.41 / 625.42 (continuous load & energy management)
- NEC 210.20(A) / 215.2 (continuous load 125%)
Frequently Asked Questions
Why is EV charging sized at 125 percent of the rating?
NEC treats a load that operates for three hours or more as a continuous load. EV charging easily exceeds that, so NEC 625.41 and 210.20(A) require the circuit to be sized at 125 percent of the EVSE rating. A 40 A charger therefore needs a 50 A circuit.
What is the diversity or demand factor for multiple chargers?
NEC 625.42 lets an energy-management system (EVEMS) limit the total simultaneous demand of a group of chargers. The diversity/demand factor (a value of 1.0 or less) multiplies the summed continuous load. A factor of 1.0 means no diversity, the worst case; a lower factor reflects the controlled maximum the EVEMS allows.
How is three-phase charger current calculated?
For a balanced three-phase supply the current is I = P * 1000 / (sqrt(3) * V), where V is the line-to-line voltage and P is in kW. For single-phase the relationship is simply I = P * 1000 / V.
Does this size the conductors and overcurrent protection directly?
It gives the minimum required ampacity (the continuous design current). You still select conductors and the overcurrent device at or above that value using the NEC ampacity tables, applying temperature and conduit-fill correction/adjustment factors for the actual installation.