Advanced Load Calculations Flashcards

A 3-phase, 4-wire, 208Y/120V system supplies a balanced load of 60 A per phase at 208V line-to-line. What is the total three-phase kVA?

Answer: 21.63 kVA

3-phase kVA = V_LL × I_L × √3 / 1000 = 208 × 60 × 1.732 / 1000 ≈ 21.63 kVA.

Per NEC 220.61, the neutral load for a feeder or service shall be the maximum unbalance of the load. For 3-wire, 2-phase circuits, the neutral carries current only when:

Answer: Phases are unbalanced

The neutral conductor in a multi-wire branch circuit or feeder carries only the unbalanced portion of the load between phases.

A 1,500 sq ft dwelling's small appliance branch circuits require a minimum load of:

Answer: 3,000 VA

NEC 220.52(A) requires a minimum of two small appliance branch circuits at 1,500 VA each, totaling 3,000 VA.

When calculating the feeder neutral for a 3-phase, 4-wire system with a large nonlinear load, NEC 220.61(C)(1) limits the neutral reduction to what percentage of the phase conductor ampacity?

Answer: No reduction permitted

NEC 220.61(C)(1) prohibits reducing the neutral for that portion of the load that consists of nonlinear loads supplied from 3-phase, 4-wire systems.

A dwelling unit has a 5 kW, 240V dishwasher and a 1.5 kW, 120V garbage disposal. What is the combined demand load using the NEC standard calculation method?

Answer: Both must be included at full nameplate rating

Per NEC 220.53, household appliances are subject to 75% demand when four or more are present; with only two appliances, both are counted at 100% of nameplate rating.

Per NEC 430.24, when calculating feeder ampacity for two or more motors, the feeder must be sized at the largest motor FLC at 125% PLUS:

Answer: The sum of all other motor FLCs at 100%

NEC 430.24 requires feeder ampacity = 125% of largest motor FLC + 100% sum of all other motors' FLC.

A 480V, 3-phase generator is rated 500 kW at 0.8 power factor. What is the generator's kVA rating?

Answer: 625 kVA

kVA = kW ÷ power factor = 500 kW ÷ 0.8 = 625 kVA.