Electrical Service Size Calculator
Use service voltage, phase count, a user-supplied real-power demand basis, power factor, and a target utilization percentage to create a transparent U.S. English preliminary service-size estimate. This calculator does not perform an NEC service calculation, service sizing, or code-compliant service-rating selection.
- Estimated service current
- A
- Phase multiplier
- x
- Apparent service VA
- VA
- Target utilization ratio
- decimal
- Planning VA at target utilization
- VA
- Planning headroom
- VA
Calculation details
- Calculation basis
- Service-rating boundary
- Phase model boundary
Recent results
Formulas
- Phase multiplier = 1 for single-phase; sqrt(3) for three-phase
- Utilization ratio = target utilization percent / 100
- Apparent service VA = estimated real-power demand W / power factor
- Estimated service current amps = apparent service VA / (service voltage x phase multiplier)
- Planning VA = apparent service VA / utilization ratio
- Planning headroom VA = planning VA - apparent service VA
When to use this calculator
Estimate service current, apparent service VA, and a suggested nameplate VA from demand, phase, power factor, and utilization assumptions.
What the results mean
Review the calculated values together with the displayed units and assumptions. The result is a calculation aid for the stated inputs.
Formula basis
phase multiplier = 1 for single-phase; sqrt(3) for three-phase apparent service VA = estimated demand VA / power factor estimated service current amps = apparent service VA / (service voltage x phase multiplier) suggested nameplate VA = apparent service VA / (target utilization percent / 100)
Before you use the result
This is a U.S. English preliminary engineering estimate, not an NEC service calculation, code-compliance result, or final service-rating selection. It does not apply NEC load classifications, demand factors, continuous-load rules, conductor ampacity, overcurrent protection, voltage-drop limits, equipment ratings, load balancing, utility requirements, or local amendments. Verify the adopted NEC edition, AHJ direction, utility requirements, equipment data, and project conditions with a qualified electrical professional before design or installation.
FAQs
Is this an NEC service calculation?
No. This calculator creates a transparent preliminary estimate from your entered demand basis, power factor, phase count, voltage, and utilization target. It is not an NEC service calculation, not service sizing, and not code-compliance approval. It does not apply NEC service-load rules, demand factors, continuous-load requirements, or equipment-rating rules.
Why is apparent service VA higher than estimated real-power demand?
This model treats the entered demand as real power in watts before power-factor adjustment. Dividing by a power factor below 1.00 increases the apparent service VA used for current and planning VA.
Should I enter watts or VA?
Enter a real-power demand basis in watts. If your demand basis is already apparent power in VA, do not divide it by power factor again; use a separate project review to choose the correct input basis.
What does target utilization mean?
Target utilization is the share of the planning VA represented by the calculated apparent service VA. For example, an 80 percent target divides apparent service VA by 0.80 to provide planning headroom; it is not a code-required service margin.
Does this choose a 100 A, 200 A, or other service rating?
No. It estimates current and planning VA from the entered assumptions. It does not select a final service rating, conductor, breaker, meter equipment, utility service, or code-compliant installation.