Load Bank Test Formulas

Load bank test formulas for a generator: the rated power, the test steps, the bank current and the kVA loading of a resistive bank.

The formulas

S is the rating of the set in kVA, pf its rated power factor, x the share of the step as a fraction, V the line voltage and pfbank the power factor of the bank (1 for a resistive bank). A resistive bank loads the engine to its kW rating but loads the alternator only to P / S in kVA, so a set rated at 0.8 shows 80% of its kVA at full kW on a resistive bank. A reactive bank is needed to test the alternator at its rated kVA. The steps of 25, 50, 75 and 100% are common, and the test plan, the duration of each step and the pass limits come from the standard of the test or the customer. The values here are examples.

Worked example: a 500 kVA set rated at 0.8 on 415 V

1. Prated = 500 × 0.8 = 400 kW
2. At 75%, P = 0.75 × 400 = 300 kW
3. A resistive bank at 400 kW draws I = 400 × 1000 / (√3 × 415) = 556.5 A
4. The kVA loading of the alternator is 400 / 500 = 80%

Test steps for the 500 kVA set on a resistive bank at 415 V

Frequently asked questions

Why test with a load bank?

A site load may be too small or too variable to prove the rating. A bank gives a known load, so the set can show that it carries the rated power and that its cooling and fuel system hold up.

Why does the resistive bank not load the alternator fully?

The alternator is rated in kVA, and a resistive load has a unity power factor, so the current is lower than at rated kVA. A reactive bank adds the missing current.

What does the test show besides power?

The temperatures, the pressures, the frequency and voltage regulation and the fuel use at each step. Record them against the maker limits.

Why run the bank at a low load first?

A set that runs long on a light load can build up unburned fuel. Test steps and the time at each step follow the maker advice.

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