7.3 Power Factor, kW/HP, Overloads, and Machinery-Room Electrical Classification

Three-Phase Power on a CIRO Screen

Industrial refrigeration motors are three-phase. Real (working) power is:

$$\mathrm{kW} = \frac{\sqrt{3} \times V \times I \times \mathrm{PF}}{1000}$$

Use line-to-line volts, line amps, and power factor as a decimal. $\sqrt{3} \approx 1.732$. Power factor (PF) is $\mathrm{kW}/\mathrm{kVA}$, or $\cos\phi$ for a simple sine wave. Induction motors are lagging (current lags voltage) because they draw magnetizing vars.

Horsepower on the shaft:

$$\mathrm{HP} = \frac{\mathrm{kW}_{\mathrm{shaft}}}{0.746} \quad \text{and} \quad \mathrm{kW}_{\mathrm{shaft}} = \mathrm{HP} \times 0.746$$

Motor efficiency sits between shaft power and electrical input:

$$\mathrm{kW}_{\mathrm{input}} = \frac{\mathrm{HP} \times 0.746}{\eta}$$

CIRO-style screens often list a 300 HP class screw, 480 V, PF 0.86, and 93% motor efficiency. Those numbers describe the machine, not a promise that the compressor is at 300 HP right now. Demand is whatever the ammeter (and PF) say.

Worked screen: amps → kW → dollars

A thermosiphon-oil-cooled screw shows 480 V, average current 312 A, PF 0.86, motor efficiency 93%.

1. Electrical input: $\mathrm{kW} = 1.732 \times 480 \times 312 \times 0.86 / 1000$. First, $1.732 \times 480 = 831.4$. Then $831.4 \times 312 \approx 259{,}400$. Then $259{,}400 \times 0.86 \approx 223{,}100$. Divide by 1000: about 223 kW.
2. Shaft load: $\mathrm{HP} = (223 \times 0.93) / 0.746 \approx 278\ \mathrm{HP}$. That is a 300 HP class motor loaded hard, but not above nameplate shaft HP.
3. Energy cost: $\mathrm{cost} = \mathrm{kW} \times \text{\textdollar}/\mathrm{kWh}$. At $0.12/\mathrm{kWh}$, $223 \times 0.12 \approx \text{\textdollar}26.80$ per hour of run time at that load. Demand charges are extra and belong with KPI/cost chapters; do not ignore them in real plants, but the exam's first skill is kW × energy rate.

Full-load check from the nameplate (if the screen asked "what current at rated HP?"):

$$\mathrm{kW}_{\mathrm{in, FL}} = \frac{300 \times 0.746}{0.93} \approx 241\ \mathrm{kW}$$ $$I_{\mathrm{FL}} = \frac{241{,}000}{1.732 \times 480 \times 0.86} \approx 337\ \mathrm{A}$$

312 A is therefore not full load. Trap: treating 300 HP × 0.746 = 224 kW as the electrical demand skips efficiency (input is higher than shaft) and skips the ammeter. Trap: dropping $\sqrt{3}$ and computing $480 \times 312 \times 0.86 / 1000 \approx 129\ \mathrm{kW}$ — that is a single-phase shape on a three-phase motor. Trap: dropping PF and reporting 259 kW (that is closer to kVA).

$$\mathrm{kVA} = \frac{\sqrt{3} \times V \times I}{1000}, \quad \mathrm{PF} = \frac{\mathrm{kW}}{\mathrm{kVA}}$$

Power-Factor Correction

Lagging motors make the plant draw more amps than the kW require. Utilities may penalize low PF or bill on kVA. Capacitors supply vars locally. Real kW of the compressor hardly changes; line current and kVA fall as PF rises toward unity. Example: 223 kW at 0.86 PF is 259 kVA. Corrected to 0.95 PF, kVA falls to $223 / 0.95 \approx 235\ \mathrm{kVA}$, and current falls in the same ratio ($0.86/0.95 \approx 0.91$).

Place and switch capacitors so you do not leave a large capacitor bank on a bus that has gone light — overvoltage and harmonic resonance with VFDs are real. Correction does not replace overloads or breakers, and it does not change the oil-pressure or high-pressure cutouts.

Overloads Versus Short-Circuit Protection

These are two different jobs:

An overload can trip without the breaker opening. A short circuit should open the breaker or fuse long before an overload heater would. Sizing overloads to FLA and breakers to a higher short-circuit rating is normal — it is not a contradiction. Do not "fix" nuisances by installing oversized overload heaters; that burns motors. Do not assume a VFD's electronic thermal overload replaces a mechanical high-pressure cutout.

Machinery-Room Electrical Classification (IIAR 2 — Do Not Invent Division 1)

Anhydrous ammonia's flammable range is about 15–28% by volume (OSHA's commonly cited band). That is a high concentration compared with toxicity (ppm) limits. 25% of LFL is the order of 40,000 ppm when LFL is taken near 16% — the same neighborhood IIAR 2 uses for the upper-detection / compressor-pump shutdown logic. Flammability is real, but it is not "gasoline vapor at the floor under normal operation."

ANSI/IIAR 2 (2014 language Ordinary Location; 2021 language Unclassified Location — treat them as the same exam idea) does not require a typical ventilated, detected machinery room to be Class I, Division 1. The design path is:

• Provide ammonia detection (IIAR 2 §6.13) and emergency ventilation (IIAR 2 §6.14.7).
• Then the machinery room is designated unclassified / ordinary as described in the electrical code.
• If emergency ventilation is not provided, the room shall be designated not less than Class I, Division 2, Group D, and electrical equipment must meet that classification.
• Electrical design documents must state ordinary/unclassified versus hazardous, and if hazardous, the Class, Division, and Group.

NEC language used with ammonia machinery rooms matches the idea: rooms with adequate mechanical ventilation that runs continuously or starts from detection at not more than 150 ppm may be unclassified. That is how IIAR 2 keeps most machinery-room electrical from being treated as a continuously hazardous classified location. It is not a claim that ammonia cannot burn.

What IIAR 2 does not say: it does not impose a blanket Class I, Division 1 requirement on a code-compliant machinery room. Division 1 would mean ignitable concentrations are expected in normal operation. The ventilation-and-detection package is specifically there so that is not the design assumption.

Ignition control still exists in an unclassified room. IIAR 2 still restricts open flames and hot surfaces in the machinery room. Emergency exhaust fans that handle ammonia are specified with spark-resistant blades in current IIAR 2 ventilation language. Emergency exhaust fan motors in the airstream or in the room are to be totally enclosed (TEFC-type enclosure) and need not be explosion-proof when the room is unclassified because of detection and emergency ventilation. Totally enclosed is not the same listing as explosion-proof Class I equipment. Do not "upgrade" every motor to XP in your head and call it IIAR 2.

E-stop versus fans: refrigerant e-stop de-energizes compressors, refrigerant pumps, and N.C. automatic refrigerant valves. Emergency ventilation is powered so that it is not killed by that e-stop. Classifying the room correctly and keeping the fans running during a leak are the same safety story: dilute and exhaust, rather than pretending every starter is Division 1.