Weekly digest #168: generator news

Why generators are back in the spotlight

Storm season prep, data center buildouts, and a wave of standby installs at small commercial sites have pushed generator work to the top of the call list this week. If you have not pulled NEC Article 445 in a while, now is the time. The 2023 cycle tightened a few requirements that catch crews off guard, especially around disconnects and overcurrent protection at the generator terminals.

Optional standby systems fall under Article 702. Legally required standby is Article 701. Emergency systems are Article 700. The wiring methods, transfer equipment, and testing rules differ for each. Misclassifying a job at the proposal stage means the inspector will catch it later, and the fix is rarely cheap.

Disconnect rules that trip people up

NEC 445.18 requires a disconnecting means for the generator that disconnects all protected conductors, including the neutral where the system is separately derived. For units over 15 kW, the disconnect must be lockable in the open position per 445.18(C). The lock provision must remain with the disconnect whether the lock is installed or not.

For one and two family dwellings, 445.18(D) allows the generator disconnect to be located outside readily accessible only if a remote shutdown is provided. Crews keep installing the disconnect on the unit itself and forgetting the remote, then failing the rough.

• Disconnect must open all ungrounded conductors and the neutral when separately derived.
• Lockable open per 110.25 with the provision permanently installed.
• Remote stop required where the disconnect is not readily accessible.
• Label the disconnect as the generator disconnect, not just "main".

Transfer switch and the neutral question

The neutral handling decision drives half the rework on residential standby jobs. If the generator is a separately derived system, you bond the neutral to ground at the generator and use a 4 pole transfer switch that switches the neutral. If it is not separately derived, the neutral stays bonded only at the service, and a 3 pole switch is fine.

Most portable and small residential standby units ship from the factory with the neutral bonded to the frame. Leave that bond in place and you need a switched neutral at the ATS. Pull the bond and the unit is no longer separately derived. Pick one and document it on the as-built.

Field tip: before you energize, put a meter between the generator neutral and ground with the unit running and the ATS in generator position. You should see near zero volts. Anything above a few volts means you have a bonding mismatch, and the GFCI receptacles on the unit will start tripping the moment a load lands.

Conductor sizing from the generator terminals

NEC 445.13 sets the conductor ampacity at not less than 115 percent of the nameplate current rating. That is the rating at the generator terminals, not the load side of the transfer switch. A common mistake is sizing the feeder for the calculated load and then finding out the generator nameplate pushes the required ampacity up a wire size.

If the design uses overcurrent protection that prevents overload, you can drop to 100 percent per the exception, but only if the OCPD coordinates with the generator output. Read the listing on the breaker, do not assume.

1. Pull the nameplate amps from the generator data tag.
2. Multiply by 1.15 for default ampacity.
3. Verify terminal temperature rating, usually 75C, sometimes 90C on larger units.
4. Check voltage drop separately if the run is over about 100 feet.

GFCI on portable and vehicle mounted units

OSHA and NEC 590.6 still require GFCI protection on 15, 20, and 30 amp 125 and 250 volt receptacles used for temporary power on construction sites. Newer portable generators ship with GFCI receptacles that work fine when the neutral is bonded at the unit. The trouble starts when crews plug into a transfer switch and the bonding moves to the service.

If you are using the generator as construction temp power, leave the bond at the unit. If you are feeding a structure through an inlet and ATS, you have to manage the bond. A switchable neutral bond kit, or a generator listed for both modes, solves it cleanly.

Maintenance, testing, and what owners forget

Article 700.3 and 701.3 require periodic testing for emergency and legally required standby systems. The frequency and load level depend on the system class, but written records are mandatory. Optional standby per 702 has no test mandate in the NEC, but the manufacturer's service interval still applies for warranty.

Battery condition kills more standby units than fuel problems. A generator that has not been exercised in six months will often crank but fail to come up to speed under load. Annual service should include load bank testing at 30 percent or higher for at least 30 minutes on diesel units to burn off wet stacking.

Field tip: log the run hours and the battery voltage at every site visit. A battery that drops below 12.4 volts at rest, or below 10 volts during crank, is on borrowed time. Replace it before the next storm, not during one.

Quick reference for this week's calls

Most generator callbacks trace to three things: bonding, disconnect labeling, and conductor sizing off the nameplate. Walk every install with that short list before you leave the site. The inspector will, and so will the next electrician who shows up after a failure.

• Article 445: generator construction, disconnects, conductor sizing.
• Article 700, 701, 702: emergency, legally required, optional standby.
• Article 250.30: separately derived system bonding.
• NEC 110.26: working space, often missed on outdoor pads.