Service Conductor Sizing for a Mixed-Demand Restaurant
A 4,800 sq ft restaurant is served by a 208Y/120 V, three-phase, four-wire service. The connected load schedule shows: 42 kVA of kitchen equipment (continuous), 18 kVA of general lighting, 12 kVA of receptacle load, and a 25 kVA HVAC load (noncontinuous). The owner wants a 400 A service with a 400 A main breaker. The utility transformer is 75 kVA. The electrical contractor proposes 500 kcmil THWN copper service-entrance conductors in rigid metal conduit. The plan reviewer notes the calculated demand load and questions both the service rating and the conductor size. Ambient temperature in the utility room reaches 40°C.
1. Using the standard load calculation method for a restaurant, determine the calculated demand load and state whether the proposed 400 A service is adequate.
Read the worked answer
Apply the general method: lighting at 100% (18 kVA), receptacles at 100% for the first portion plus the allowable demand factors (12 kVA treated per the receptacle demand table for a restaurant - commercial occupancies generally take 100% of the first 10 kVA and 50% of the remainder, so 10 + 1 = 11 kVA), kitchen equipment at 100% (42 kVA), and HVAC at 100% (25 kVA). Total ≈ 18 + 11 + 42 + 25 = 96 kVA. At 208 V three-phase, I = 96,000 / (1.732 × 208) ≈ 266 A. A 400 A service is more than adequate from a load standpoint. The real issue is that a 400 A service on a 75 kVA transformer (≈ 208 A at 208 V) is mismatched - the utility transformer cannot deliver 400 A continuously, and the service conductors must be sized to the calculated load, not the arbitrary 400 A. Sizing to 266 A allows a 300 A service with 350 kcmil copper, but the designer should coordinate with the utility on transformer capacity.
2. Evaluate the proposed 500 kcmil THWN copper in RMC at 40°C ambient. Is the ampacity adequate for a 400 A service, and what correction applies?
Read the worked answer
500 kcmil THWN copper at 75°C is 380 A (Table 310.16). At 40°C ambient, the correction factor for 75°C conductors is 0.88, giving 380 × 0.88 = 334 A - below 400 A. Even without correction, 380 A < 400 A, so 500 kcmil cannot carry a 400 A service. For a 400 A service, the conductors must have an ampacity of at least 400 A (or use the next-standard-size-up rule only where the load does not exceed the conductor ampacity, which does not apply here since 334 A < 266 A load? - it does exceed the load, so the next-size-up rule under 240.4(B) could permit a 400 A OCPD on 334 A conductors only if the load ≤ 334 A, which it is at 266 A). However, service conductors must still be sized per 230.42 to carry the load; 500 kcmil corrected to 334 A is acceptable for a 266 A calculated load, but not for a 400 A service rating. Recommend either reducing the service to 300 A or paralleling/upsizing to 600 kcmil (420 A × 0.88 = 370 A, still short) - better to use two parallel 3/0 sets or 700 kcmil. The clean answer: at 40°C, 500 kcmil is not a 400 A conductor; correct the ambient assumption or increase size.
3. The contractor argues the 75 kVA transformer limits available fault current, so smaller conductors are fine. How should the reviewer respond?
Read the worked answer
Transformer kVA limits available fault current, not continuous ampacity. Service conductors are sized for the calculated load and the service disconnecting means rating per Article 230, not for the transformer's fault contribution. A 75 kVA transformer delivers roughly 208 A continuously at 208 V, so a 400 A service is over-built relative to supply; the correct fix is to right-size the service to the load (≈ 266 A → 300 A service, 350 kcmil Cu at 75°C = 310 A, corrected at 40°C to 273 A, still above 266 A). Fault current coordination is a separate analysis (AIC ratings of equipment) and does not justify undersized conductors.