Service and Feeder Load Calculations

General lighting and general-use receptacles are figured at 3 VA per sq ft of habitable area under the standard method.

Include two small-appliance branch circuits and one laundry circuit at 1,500 VA each before demand.

Apply the general-lighting demand factor: first 3,000 VA at 100%, the remainder up to 120,000 VA at 35%.

Household ranges and dryers use their own demand tables; a single 12-kW range is calculated at 8,000 VA and a dryer at 5,000 VA minimum.

Base ampacity comes from Table 310.16, then apply ambient temperature correction and conductor-fill adjustment factors.

More than three current-carrying conductors in a raceway require adjustment: 80% for 4-6, 70% for 7-9.

Termination temperature limits per 110.14(C) cap the final usable ampacity at the 60 C or 75 C column.

Dwelling service and feeder conductors may use the 83% allowance of Section 310.12.

Use the table full-load current (Table 430.250 for three-phase) rather than nameplate for sizing conductors and protection.

Single-motor branch conductors are sized at 125% of FLC; feeders use 125% of the largest motor plus the sum of the others.

Transformer current is VA divided by voltage (times 1.732 for three-phase); primary-only protection is limited to 125%.

Motor overload is set at 125% for service factor 1.15 or greater, otherwise 115% of nameplate.

Single-phase voltage drop = (2 x K x I x L) / circular mils; use 1.732 instead of 2 for three-phase.

Recommended limits are 3% on a branch circuit and 5% for feeder plus branch combined.

Available fault current at a transformer secondary equals full-load amps divided by the per-unit impedance (infinite primary assumption).

Larger conductors (more circular mils) reduce both voltage drop and conductor heating.

General-use receptacles are figured at 180 VA each; receptacle loads over 10 kVA may be demanded at 50%.

Show-window lighting is calculated at 200 VA per linear foot and each sign circuit at a minimum 1,200 VA.

Continuous loads are increased to 125% when sizing conductors and overcurrent devices.