How to Connect a Portable Load Bank to a Residential Standby

Why this matters

A residential standby spends its life exercising at no load, which proves the engine turns and nothing else. The only way to know it will carry its rating is to make it carry its rating, and the house rarely can on a test day. A portable load bank does it on demand. The connection is where this job goes wrong: cables landed on the wrong side of the breaker, a house that was not really isolated, a sensing circuit still live in the same compartment. This card covers getting the load bank on and off the unit. What the readings must show to pass, and how they are logged, stay with the annual load test card. See related: Stationary Generator Annual Load Test Procedure.

Before anyone opens the compartment

Lock out per the shop's procedure. Controller to off, auto-start disabled, and the battery handled as the procedure says. See related: The Generator Auto-Start Lockout Before Service SOP. The unit must not crank while hands are on its output lugs.

The connection compartment is not dead just because the engine is off. Most residential standby controllers carry live circuits in or near that compartment even with the unit stopped: the utility-sensing leads from the transfer switch (240 volts, fused) and the 120-volt feed to the battery charger. Open the breakers or pull the fuses that feed them, as the maker's manual identifies, then lock and tag under 29 CFR 1910.333(b)(2), or 1926.417 where the job is construction work.

Prove it. Verify absence of voltage at the generator output breaker's load terminals, phase to phase and each phase to neutral and to ground, with a tester proven on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024). Then check the sensing and charger terminals too.

Landing cables on generator output conductors is electrical work. Where your state or locality licenses it, a licensed electrician or a qualified person under the shop's electrical safety program does it.

The connection and the isolation point

 GENERATOR                    TRANSFER SWITCH, on NORMAL
 alternator                   utility o----+
   |                                       +--> house
 [OUTPUT BREAKER 100A]        gen     o         panel
   | load-side lugs                   |
   +-- field conductors --[DISC]------+
   |                      OPEN here =
   |                      isolation point
   +-- load bank leads L1, L2, N, G
          |
   [PORTABLE LOAD BANK]  240 V single phase,
                         exhaust away from intake

The load bank leads land on the load side of the generator's output breaker, alongside the field conductors to the transfer switch. That way the output breaker still protects the alternator and the cables, and it is the switch you use to put load on and take it off.

The house is isolated at the open disconnect between the generator and the transfer switch. Many installs have one at the transfer switch or beside the unit. Where there is none, the transfer switch stays on normal with utility present, which keeps the house off the generator; but if the utility fails mid-test the switch will transfer and add the house to a fully loaded unit. Tell the household, and watch the utility indicator.

Step 1: Size the cables to the breaker, not the test

The cables have to carry whatever the output breaker can pass, because the breaker is what protects them. On a single-phase 240-volt unit with a 100-amp output breaker, every load bank lead is rated for at least 100 amps, from the load bank maker's cable chart for that cable type and length.

Use ring or compression lugs sized for the conductor and the generator's terminals, not clamps or clips. Land them to the torque on the terminal label. A clip that loosens under 80 amps of heat is a burned terminal on a customer's generator.

Why the breaker rating: a tech who sizes leads for a planned 50 percent test has cables smaller than the breaker, and one wrong dial setting on the load bank cooks them while the breaker holds.

Step 2: Set the load bank and site it

Set the load bank for the generator's configuration: 240-volt single-phase for almost every residential unit. Confirm every load step is off.

Place the load bank where its hot discharge air goes away from people, from anything that burns, and from the generator's air intake. A load bank blowing into the generator's intake heats the engine air and makes the unit look worse than it is. On a resistive load bank the discharge is hot enough to scorch siding at close range.

Step 3: Land the leads, then restore the controller

With the absence of voltage proven:

1. Land L1 and L2 on the output breaker's load-side lugs, each with its field conductor.
2. Land the neutral on the generator's neutral terminal and the ground lead on the ground terminal.
3. Tug-test every lug and close the compartment so no live part is exposed during the run.
4. Open the generator-to-transfer-switch disconnect if one exists and lock it.
5. Remove locks and tags from the sensing and charger circuits and restore them, then restore the battery and controller per the lockout procedure. Leave the controller in off, not auto.

Step 4: Start, then step the load

Start the unit from the controller in manual run. Let it reach speed and settle, and read no-load volts and hertz at the load bank's meter or a meter on the output.

Close the output breaker if it is open, then add load in steps. Use the generator's rating for the fuel it is running on. Natural gas ratings are usually lower than propane ratings on the same unit. See related: Reading a Generator Spec Sheet and Derating.

At each step, hold until the readings settle, at least a few minutes, and record:

• Voltage L1 to L2, and each leg to neutral.
• Frequency.
• Amps on each leg.
• Anything the engine does: hunting, smoke, a change in sound.

Take the full-load gas pressure reading at the same time while you have full load on. See related: How to Measure Gas Pressure at the Generator, Static and Under Load.

Worked example: a 22 kW unit on natural gas

A residential air-cooled standby sold as "22 kW." The spec sheet (illustrative for this example) rates it 22 kW on propane and 19.5 kW on natural gas, which is what this house runs. The test is to 19.5 kW, not 22. Output breaker 100 amps; leads rated 100 amps.

Amps are kW times 1,000 divided by 240: 19,500 / 240 = 81.25, written 81.3. Cross-check one step in reverse: 40.6 amps times 240 volts is 9,744 watts, which is the 9.75 kW step.

The readings track the dial within an amp on each leg, which says the load bank is balanced and the meter agrees with the setpoint. Voltage drops 4 volts and frequency drops 0.4 hertz from no load to full; whether that is inside this unit's regulation spec is a pass-fail call that belongs to the annual load test card, read against the maker's numbers.

What a wrong setup would have shown. Test to 22 kW on this natural gas unit and you are asking it for 113 percent of its natural gas rating; it will droop, possibly shut down on underfrequency, and the shop will write up a healthy generator as failing. Read amps on one leg only and a load bank with a dead element on L2 looks like a full-load pass on paper.

When to shed load mid-step. If frequency keeps sliding instead of settling, the engine is short of fuel or power: drop back one step, note where it happened, and take the gas reading there. If frequency holds but voltage sags on both legs, the problem sits in the alternator or its regulator, not the fuel. If one leg's amps read well under the other's at the same setting, stop and check the load bank before blaming the generator. Hunting, black smoke or a rising exhaust note under load means shed it to zero and stop; a unit that cannot hold a step is a diagnosis, and pushing it to the next step only adds heat.

Step 5: Cool down and restore

1. Shed load in steps back to zero.
2. Let the unit run unloaded for the cooldown the maker specifies, so the engine sheds heat with air moving.
3. Stop the unit from the controller.
4. Lock out again exactly as before, including the sensing and charger circuits, and prove absence of voltage at the output lugs with a proven tester before and after.
5. Remove the load bank leads; check the field conductors' lugs are still tight and re-torque if any was disturbed.
6. Close the compartment, close the generator-to-transfer-switch disconnect, restore the sensing and charger circuits, battery and controller.
7. Put the controller in automatic and confirm the display shows ready in auto, with utility sensed. A unit left in off or manual after a load test will not start in the next outage.

What changes the setup

Three-phase units on light commercial work land three phase leads plus neutral, and the load bank is set for that voltage and configuration. Read every phase.

No neutral load on the load bank. A 240-volt load bank with no neutral connection loads L1 to L2 only and never tests 120-volt regulation on either leg. That is acceptable for an engine and alternator capacity test if your load test card says so; it does not tell you how a lightly loaded leg behaves with a heavy 120-volt load on the other.

A liquid-cooled unit can sit at full load much longer than an air-cooled one, and the load test card sets the duration by unit type.

References

• NFPA 70E, Standard for Electrical Safety in the Workplace, current edition, Article 120; 29 CFR 1910.333 and 1926.417
• The generator maker's manual for output terminals, sensing and charger circuits, ratings by fuel and cooldown; the load bank maker's cable chart and setup instructions
• See related: Stationary Generator Annual Load Test Procedure; The Generator Auto-Start Lockout Before Service SOP; What a Load Bank Test Proves and What It Does Not