Tenant Load, Spare Breakers, and a 1985 Service
A tenant fit-out in a 1985 office building adds 40 workstations, 22 kVA of conference AV and IT equipment with switch-mode power supplies, and two 10-ton packaged rooftop units. The building's 1200A, 208Y/120V service has a calculated existing demand of 780A, supported by 18 months of utility interval data. The tenant proposes to feed everything from a 400A panelboard in the tenant space that shows six spare 20A single-pole spaces and two spare 60A two-pole spaces. The feeder from the house panel to the tenant space is 180 ft of 4/0 copper in steel conduit. The tenant's total connected load is 95 kVA at 208V, three-phase.
1. Can the 400A panelboard and the 1200A service carry this load? Show the arithmetic and name the code basis for each step.
Read the worked answer
Convert the load first. 95 kVA at 208V three-phase is 95,000 / (1.732 × 208) = 264A of connected load. Connected load is not demand load - the calculated load must be built per NEC Article 220, applying the office receptacle and lighting demand factors and taking the largest motor at 125%.
Evaluate the service two ways.
- Calculated: existing calculated demand 780A plus the tenant's calculated demand (likely 200 to 240A after demand factors) puts you at roughly 980 to 1,020A against a 1200A service - apparently acceptable, but only if the existing 780A is a true Article 220 calculated demand.
- Measured: NEC 220.87 permits using 125% of the maximum demand over a one-year period from a recording ammeter or utility interval data in lieu of calculating the existing load. Here 780A × 1.25 = 975A before any new tenant load is added. Add even 200A and you are over the 1200A main.
This is the judgment call. The 220.87 shortcut that the tenant's engineer will reach for is exactly the method that sinks the project here. Do the Article 220 calculation for the existing building as well, take the lower defensible number, and document which basis you used. You must also verify the utility transformer capacity and the switchboard bus, not just the main breaker.
The 400A panelboard. The calculated tenant load must be compared against the 400A bus, with continuous loads at 125% and the largest motor at 125%. Six 20A single-pole spaces total 120A of single-phase capacity - that will not carry 40 workstations plus 22 kVA of equipment. A new 3-phase subpanel is required; the spare spaces are not the answer.
2. The panel schedule shows two spare 60A two-pole spaces. What is the problem with using them for the rooftop units?
Read the worked answer
A two-pole breaker is 208V single-phase, not three-phase. A 10-ton packaged rooftop unit at 208V three-phase draws on the order of 30 to 35A running and requires a three-pole breaker sized from the nameplate minimum circuit ampacity - commonly a 50 or 60A three-pole. Two-pole spaces cannot serve it, and the spaces may not even be adjacent in the panel, which you need for a three-pole device.
Even two 60A two-pole breakers at 208V single-phase only give about 12.5 kVA each - not enough for a 10-ton unit, and feeding a three-phase compressor from single-phase is not an option.
What this signals. The panel was laid out for a different tenant. The correct move is a new subpanel located near the mechanical equipment, fed three-phase from the house panel, with the rooftop units on individual three-pole breakers and disconnects at the units. Do not let the schedule's spare spaces drive the design; spare spaces are a coincidence, not a capacity analysis.
3. Evaluate the 180-ft feeder, the neutral, and the fault-current question. What would you require before signing the fit-out drawings?
Read the worked answer
Feeder ampacity. 4/0 copper in steel conduit at 75°C is 230A. The tenant's connected load is 264A, and even a demand-diversified value close to 230A leaves no margin with continuous loads at 125%. The 4/0 is undersized for the full scope; plan on 300 kcmil or larger and recheck conduit fill.
Voltage drop. For 4/0 copper at roughly 0.061 ohms per 1000 ft, VD = 1.732 × 264 × 0.061 × 0.180 / 1000 ≈ 5.0V, about 2.4 percent. That is within a 3 percent feeder guideline but leaves only about 0.6 percent for the branch circuits - too tight for a 180-ft run with motor loads. Upsizing the feeder solves ampacity and voltage drop at the same time.
Neutral. Twenty-two kVA of switch-mode power supplies means triplen harmonic currents that add in the neutral rather than cancel. Do not reduce the neutral below the phase conductors, count it as a current-carrying conductor for ampacity adjustment in this four-wire feeder, and specify a panelboard rated for harmonic loads. Consider a 200 percent neutral for the IT portion.
Fault current. A 1985 service with new breakers and new motor loads requires an available-fault-current calculation. The new panelboard's interrupting rating must equal or exceed the available fault current, and any series ratings must be listed combinations - not assumed. If the utility is upgrading the transformer, the available fault current goes up and existing equipment ratings may no longer hold.
Require before signing: the Article 220 calculation with its basis stated, a feeder and voltage-drop calculation, a fault-current study, and revised panel schedules.