Practice project

Office Lighting Load: Connected Power and Current

A short project: total the connected lighting power for three spaces, convert to apparent power and estimate the current, then write the calculation up so another engineer can check it.

Electrical & Power · Last reviewed

Brief

A hypothetical office has three spaces. Calculate the connected lighting input power and estimate the current on a simplified single-phase supply. This exercise covers the arithmetic only. It leaves out code-based demand factors, circuit and phase allocation, conductor sizing, protection, emergency lighting and installation design.

Given

SpaceLuminairesInput power each
Open office2432 W
Meeting room825 W
Corridor1018 W

Assume all luminaires operate at once, the supply is 120 V single-phase and the power factor is 0.95. Treat these as fixed exercise inputs.

Worksheet

ItemYour result
Open-office connected power____ W
Meeting-room connected power____ W
Corridor connected power____ W
Total connected real power____ W
Apparent power (real power ÷ power factor)____ VA
Estimated current (apparent power ÷ voltage)____ A
Show the worked answer
  1. Open office: 24 × 32 = 768 W.
  2. Meeting room: 8 × 25 = 200 W.
  3. Corridor: 10 × 18 = 180 W.
  4. Total real power: 768 + 200 + 180 = 1,148 W.
  5. Apparent power: 1,148 ÷ 0.95 = 1,208.4 VA.
  6. Current: 1,208.4 ÷ 120 = 10.07 A.
1,148 W | 1,208 VA | 10.07 A

Review questions

  • Can you pick a breaker from 10.07 A? No. A breaker choice also needs the circuit layout, the continuous-load rules and the installation conditions, none of which this exercise gives.
  • What changes if the power factor falls? For the same real power and voltage, apparent power and current rise.
  • Why label W and VA separately? They are different quantities. Treating them as the same hides the power-factor assumption.

Your submission

Write a one-page calculation showing the inputs, assumptions, formulas, units, results and exclusions. Add a revision date and a short list of what a real design would still need.

Try a variation: an LED retrofit

Suppose new LED fixtures use 40% less input power than the originals, with the same quantities and the same 0.95 power factor.

  1. Open office: 768 × 0.6 = 460.8 W. Meeting room: 200 × 0.6 = 120 W. Corridor: 180 × 0.6 = 108 W.
  2. Total real power = 688.8 W. Apparent power = 688.8 ÷ 0.95 = 725.1 VA.
  3. Current at 120 V = 6.04 A, down from 10.07 A. At 277 V the current is 2.62 A, down from 4.36 A.

What a real design adds

  • Demand factors from the code for the occupancy, in place of assuming all luminaires are on at once.
  • The continuous-load rule, which applies a 125% factor to loads that run for three hours or more.
  • Emergency and exit lighting on their own circuits.
  • Controls, dimming and the power factor of the actual drivers.
  • Circuit and phase allocation, conductor size and protection.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

More worked problems

Questions

Why 120 V?

It keeps the arithmetic simple. Many commercial lighting circuits run at 277 V instead, and the same method applies with the voltage changed.

Is 0.95 a realistic power factor?

It is a fixed exercise input. Real fixtures vary, so use the manufacturer data for a real design.

What is the difference between connected load and demand load?

Connected load is the sum of everything that could run. Demand load is the load the code or the designer expects to run at the same time. Demand is usually the smaller figure.

Why convert watts to volt-amperes?

Wires, breakers and transformers carry current, and current depends on apparent power (volt-amperes), not only real power. Dividing watts by power factor gives the apparent power.

Do LED drivers change the power factor?

Yes. Driver power factor varies by product, from below 0.9 to above 0.95. Use the manufacturer data for a real design.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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