PE Architectural Engineering · Complete public lesson
Resolve feeder capacity and equipment access together
Convert three-phase apparent power into current, then compare independent electrical and spatial constraints without trading away either one.
The situation and givens
A laboratory renovation adds a balanced three-phase load. The electrical designer and architect must decide whether the coordinated layout can proceed. Use the supplied project limits; they are exercise assumptions, not a substitute for the applicable electrical code or a manufacturer's instructions.
- Demand at the feeder: 96 kVA, balanced three phase
- Line-to-line voltage: 240 V
- Designer's confirmed permitted feeder current: 250 A
- Project-specified unobstructed front access: 42 in
- Current coordinated drawing provides 39 in
- Alternative A: retain demand and provide 42 in clear
- Alternative B: reduce permitted design demand to 88 kVA and provide 42 in clear; obtain approval for the load reduction
Complete worked method
1. Choose the correct voltage convention
For a balanced three-phase load, apparent power equals √3 times line-to-line voltage times line current. kVA is already apparent power; no extra power-factor multiplier is required.
2. Test the confirmed electrical limit
230.94 A is below the supplied 250 A limit, with 19.06 A of arithmetic headroom. That comparison does not establish compliance with every derating, protection or load-class rule; those have been supplied as the designer's confirmed limit for this exercise.
3. Test the spatial limit separately
39 in is 3 in less than the specified 42 in. The current layout therefore cannot proceed even though the electrical comparison passes. A lower electrical load does not make the missing access width disappear.
4. Evaluate the coordinated alternatives
Alternative A keeps the original load within 250 A and supplies the full access width. Alternative B also passes both given limits, but it adds an approval dependency and does not solve a current shortfall that exists under the stated 250 A limit. Recommend A for these givens, then confirm all remaining design constraints.
Common errors and why they fail
- Using 96,000 / 240 gives 400 A by treating a balanced three-phase feeder as a single-phase circuit.
- Multiplying by power factor a second time changes the given apparent power without justification.
- Approving the present layout because current passes ignores a separate spatial constraint.
- Reducing demand alone cannot resolve a 3 in access shortfall.
Apply the method
Which recommendation satisfies both supplied limits without changing the demand?
Change one condition
A revised design check lowers the permitted current to 220 A. Which alternative now passes the two supplied limits?
Reveal the worked transfer
A is no longer within the current limit: 230.94 A > 220 A. B gives 88,000 / (√3 × 240) = 211.70 A and 42 in clear, so B passes these two checks if its demand reduction is approved. Reconfirm the demand assumption, not just the arithmetic.
Put the method to work
Try different questions to check whether you can identify the governing condition and explain the next action. The free sampler includes answers and worked explanations.