Weekly digest #198: generator news

Generator interconnection: the 705 vs 702 question

Most generator callbacks this week trace back to the same mistake: treating an optional standby system like an interactive one. NEC 702 covers optional standby (the typical home or shop generator with a transfer switch). NEC 705 covers interconnected sources that run in parallel with utility power. They are not interchangeable, and the inspector will catch it.

If the generator never closes onto the utility, you are in 702 territory. Sizing follows 702.4, and the transfer equipment must be listed and suitable for the application per 702.5. The moment a contractor mentions "soft loading" or "no break transfer," you have crossed into 705 and need full parallel protection.

Watch for portable generators wired through interlocked breakers in residential panels. That setup is legal under 702 only when the interlock is listed for that exact panel and breaker combination. A field-fabricated bracket is a violation, full stop.

Standby grounding: the bonded vs unbonded neutral trap

This is the most common write-up on transfer switch jobs. A separately derived system requires a neutral-to-ground bond at the generator per 250.30. A non-separately derived system (where the neutral is switched at the transfer switch, or more commonly, where it is not) keeps the bond at the service only.

If you install a 3-pole transfer switch (neutral solid through), the generator neutral must be unbonded. If you use a 4-pole switch that breaks the neutral, the generator becomes a separately derived system and needs its own bond and grounding electrode per 250.30(A) and 250.30(B).

Field tip: pop the generator panel before energizing and verify the factory bonding strap. Many residential units ship bonded. If you wired a 3-pole switch, that strap has to come out, and you log it with a photo.

GFCI requirements on portable units

OSHA and NEC overlap here, and the 2023 cycle tightened things further. Portable generators 15 kW or less manufactured after January 1, 2015 must have GFCI protection on all 125V and 125/250V receptacles per UL 2201 and referenced in 590.6 for temporary installations.

The trap: older rental fleet generators may not be compliant, and contractors keep using them with cord-and-plug tools on jobsites. If you are the EC of record on a temp power install, you own that compliance check. Verify the receptacle has the GFCI test/reset, not just a label.

• Confirm GFCI marking on each 125V receptacle
• Test trip on every receptacle before turnover, not just the first one
• For 240V tools, verify the twist-lock outlet has GFCI per the 2020 NEC update to 590.6(A)(3)
• Document serial and manufacture date for any pre-2015 unit you flag

Conductor sizing from generator to first means of disconnect

NEC 445.13 governs the minimum ampacity of conductors from the generator terminals to the first overcurrent device. The conductors must have an ampacity not less than 115% of the nameplate current rating of the generator.

The exception in 445.13(A) allows 100% sizing if the generator overcurrent protection is integral and the conductors are protected accordingly. In practice, run the 115% number unless you have documentation showing integral protection. Inspectors rarely accept verbal claims on this.

For paralleled gensets feeding a common bus, each set of conductors gets sized independently to its own source. Do not aggregate, then divide. That math fails fast when one unit drops offline.

Disconnect location and the 445.18 rewrite

The 2023 NEC reorganized 445.18 and added explicit language on disconnect location and remote shutdown. The disconnect must be located within sight of the generator or be capable of being locked open per 445.18(A).

For one and two family dwellings with permanently installed generators over a certain capacity, an emergency shutdown device is now required outside the structure per 445.18(D). The intent is firefighter safety. Position it where it is reachable but not vulnerable to landscaping damage or accidental contact.

Field tip: mount the emergency shutdown on the same side of the building as the gas shutoff and main service disconnect. First responders look in one place, you pass inspection, and the homeowner remembers where it is during a real event.

Battery backup interplay and the rise of hybrid systems

More residential jobs now combine a generator with a battery system, often through an automatic transfer switch with multiple inputs. This pulls 706 (energy storage), 705 (interconnected sources), and 702 (optional standby) into the same enclosure.

The article that controls depends on the operating mode. If the battery and generator can run in parallel with utility, 705 applies and you need anti-islanding on the inverter side. If they only operate during utility loss with a true open transition, 702 still governs but you need clear listings on the transfer equipment for multiple input sources.

1. Confirm the inverter listing covers the generator input you are using
2. Verify the transfer scheme (open transition vs closed transition) on the submittal
3. Check that overcurrent protection on the battery output meets 706.30
4. Label every disconnect with source identification per 705.10

The inspector wants a clear single-line. Bring it to rough-in, not final. Catching a labeling or listing issue before drywall saves the job.