About Electrical Load Calculator
The electrical load calculator sizes a feeder or service by applying NEC Article 220 demand factors to connected lighting, receptacle, HVAC, cooking, dryer, motor, and special loads. It is used by electrical engineers and contractors to determine demand VA, service amps, conductor size, and the main breaker for residential and commercial buildings.
Enter the occupancy, supply voltage and phases, and each load group. The tool returns the total connected VA, the demand VA after each Article 220 demand factor, the service amps, the recommended conductor and breaker, and a balanced panel schedule, all updating in real time.
How It Works
- Sum the connected VA for each load category (lighting from floor area, receptacles, HVAC, cooking, dryers, motors, and special loads).
- Apply the Article 220 demand factor to each category: lighting per Table 220.42, receptacles per 220.44, cooking per Table 220.55, dryers per 220.54, and motors per 430.24.
- Total the demand VA and convert to service amps, dividing by the voltage for single-phase or by voltage times sqrt(3) for three-phase.
- Select the conductor from NEC Table 310.16, size the main breaker, and distribute branch circuits across phases to balance the panel.
Worked Example
A dwelling general lighting load of 10000 VA is reduced by the NEC Table 220.42 dwelling demand factors: the first 3000 VA at 100% gives 3000 VA, and the next 7000 VA at 35% gives 2450 VA. The demand lighting load is 3000 + 2450 = 5450 VA, an effective demand factor of 5450 / 10000 = 0.545.
Formulas
- Dwelling lighting demand (NEC Table 220.42)
demandVA = min(VA,3000)*1.0 + min(max(VA-3000,0),117000)*0.35 + max(VA-120000,0)*0.25- Receptacle demand (NEC 220.44)
demandVA = min(VA,10000)*1.0 + max(VA-10000,0)*0.50- Motor demand (NEC 430.24)
demandVA = largestMotorVA*1.25 + sum(otherMotorsVA)- Service current
amps = demandVA / (V * phaseFactor)
Standards & References
- NEC (NFPA 70) Article 220
Frequently Asked Questions
What demand factors does the calculator apply?
It applies the NEC Article 220 demand factors: lighting per Table 220.42, receptacles per 220.44 (first 10 kVA at 100%, remainder at 50%), cooking per Table 220.55 Column C, dryers per 220.54, and motors per 430.24 (largest at 125%).
How is the service size determined?
The total demand VA is divided by the voltage for a single-phase service, or by voltage times sqrt(3) for a three-phase service, to give the service amps. The tool then selects the next standard conductor from NEC Table 310.16 and the matching main breaker.
What is the difference between connected and demand load?
The connected load is the sum of all nameplate volt-amperes, while the demand load is the connected load after Article 220 demand factors are applied. Demand factors account for the fact that not every load runs at full capacity simultaneously.
Does it handle three-phase systems?
Yes. Select a three-phase supply and the calculator computes the line current using the sqrt(3) factor and distributes branch circuits across phases A, B, and C to balance the panel schedule.