How Are Electrical Feeders Sized for Commercial Buildings?

Commercial feeders are sized from the calculated load with NEC demand factors, then checked for conductor ampacity with derating, voltage drop over the run, and breaker coordination. The right size accounts for the actual run length, ambient conditions, and realistic future growth — not just the minimum the code table allows. The failure mode I see most is feeders sized from the code table alone, with no voltage drop check on long runs. A 400-foot feeder to a rooftop unit that 'meets code' can still drop enough voltage to cause compressor starting problems. The table gives you ampacity; the physics of the run gives you performance. Both have to be checked.

Start with the load: the panelboard schedule totals, with demand factors applied per the NEC article that matches the occupancy. Then select the conductor: ampacity from the tables, derated for the actual ambient temperature and the number of current-carrying conductors in the raceway. Rooftop conduits in Phoenix need real derating — this is not a theoretical exercise. Then verify voltage drop using the actual one-way run length, and finally select the feeder breaker: sized to protect the conductor, coordinated with the upstream and downstream devices. Each step can force a revision of the previous one, which is why feeder sizing is iterative on real projects.

Code compliance is the floor. Good feeder design thinks about the building's life after inspection. The details I insist on:

• Voltage drop calculated on real lengths: routing distances, not straight-line guesses
• Derating done honestly: ambient temperature and conduit fill for the actual installation conditions
• Spare conduit capacity: upsized raceways where future growth is plausible, documented on the drawings
• Breaker coordination verified: feeder protection coordinated with main and branch devices
• Pulling practicality: conduit bends, pull boxes, and wire-pulling distances that a crew can actually build