Plan a rack and prove the power and cooling work

short · 40 min · Objective 2.4

Task

Lay out a 42U rack for the branch office, then calculate its power draw, UPS runtime and heat output. Physical installation questions are arithmetic questions, and the arithmetic is where the marks are.

Steps

  1. Lay out the rack from the bottom up: UPS at the bottom because it is heaviest, then patch panels and switches in the middle where the cable management is, then servers, then anything light at the top. Write the U position of each item.
  2. Total the power draw: two switches at 150 W each, a firewall at 100 W, three servers at 450 W each, and a wireless controller at 60 W.
  3. Size the UPS. Convert watts to volt-amps using a power factor of 0.9, then calculate runtime for a 3000 VA unit rated for a given watt-hour capacity. State how long you get at full load and at half load.
  4. Calculate the heat. Electrical power becomes heat almost entirely, so watts convert directly to BTU per hour by multiplying by 3.412. Compare against a typical 12,000 BTU/hr room air conditioner.
  5. Decide the airflow direction and state which way the equipment faces. Cold aisle at the front, hot aisle at the back, and every device pulling air the same way — one reversed device recirculates hot air into its neighbours.

Verify

python3 -c "
loads = [('switch A',150),('switch B',150),('firewall',100),('server 1',450),('server 2',450),('server 3',450),('WLC',60)]
w = sum(x[1] for x in loads)
for n, p in loads: print(f'{n:<12}{p:>5} W')
print('total       ', w, 'W')
va = w / 0.9
print('apparent power', round(va), 'VA -- a 3000 VA UPS is', 'adequate' if va < 3000 else 'too small')
wh = 1500
print('runtime at full load', round(wh / w * 60), 'minutes;  at half load roughly', round(wh / (w/2) * 60), 'minutes')
print('heat output', round(w * 3.412), 'BTU/hr -- a 12000 BTU/hr unit is', 'adequate' if w*3.412 < 12000 else 'too small')
"

The total should come to 1,810 W, about 2,011 VA, roughly 6,176 BTU/hr, and under 50 minutes of runtime on a 1,500 Wh battery. If your UPS sizing used watts against a VA rating without the power factor, you undersized it — that is the classic error.

Notes

The power-factor conversion is the examinable detail. UPS units are rated in VA, equipment is rated in watts, and dividing watts by the power factor (0.8 to 0.95 depending on the unit) is what converts between them. Buying a 2000 VA UPS for a 1,810 W load looks fine and is not.

Runtime is not linear with load, and real UPS runtime curves are worse than the simple division above at high load and better at low load. Treat the calculation as a planning estimate and the manufacturer's curve as the answer.

On the layout: heaviest at the bottom is a stability requirement, not a preference. A top-heavy rack on castors is genuinely dangerous, and rack grounding and a proper floor load calculation belong in the same conversation.