When inspiration strikes, I like to select certain sections and articles in the National Electrical Code (NEC) that apply to a particular subject and look at their application. I recently did this with optional standby generators. Students have asked questions in my workshops on what sections in the NEC are applicable when these systems are installed temporarily or permanently. So, I decided to study these systems and write about the results.

Permanently installed
A permanently installed optional standby generator must be grounded and bonded in accordance with 250.35 of the 2011 NEC. The generator must be declared a separately derived system or nonseparately derived system. If declared a separately derived system, 250.30 requires the system to be grounded. However, if declared a nonseparately derived system, the grounding and bonding means from the generator to the first disconnecting means is sized using Section 250.122, based on the generator’s overcurrent-protection device (OCPD). If no OCPD is installed at the generator (source), a supply-side bonding jumper must be selected based on the size of the generator’s conductors that supplies the first disconnecting means per 250.102(C). When sizing the supply-side bonding jumper (a new term defined in 250.2 of the 2011 NEC), take note that 250.35(B) has been completely revised and should be carefully reviewed.

Transfer equipment
If the transfer switch is a three-pole (single-phase/3-wire) or four-pole (three-phase/4-wire) and provides switching of the grounded conductor, the generator must be grounded as a separately derived system as outlined in 250.35(A) and 250.30, Informational Note 1.

If the grounded conductor (neutral) is not switched but connected solidly to the utility’s ground and the service-equipment ground, the generator is not considered a separately derived system per 250.35(B). Therefore, the generator system is not to be individually grounded per 250.30, Informational Note 1.

Portable generator grounding
When a portable generator is installed as outlined in 702.11(A), it must be installed as a separately derived system or as a nonseparately derived system per 702.10(B). When an optional standby system is used as a separately derived system, the equipment-grounding conductor (EGC) must be connected to the generator’s grounding electrode (GE) as required by 250.30.

If the optional standby system is used as a nonseparately derived system, the EGC is required to be connected to the system’s GE. In my opinion, 250.35(A) and (B) of the 2011 NEC help clarify the grounding and bonding techniques required based on how the generator is installed.

Ampacity of conductors
Section 445.13 provides the requirements for sizing conductors from the generator to the first disconnecting means. This section permits the conductors to be sized based on the kilowatt output of the generator or from the calculated load per 215.2(A)(1).

There is an advantage where the conductors are sized at 115 percent of the generator output. These conductors appear to eliminate the means of disconnection at the generator and permit the disconnecting means to be located elsewhere. Otherwise, the disconnecting means must be located at the generator.

Generator disconnecting means
Section 445.18 requires a generator to be equipped with a disconnect that is capable of being locked in the open position by means of which the generator, all protective devices and the control apparatuses are able to be disconnected entirely from the circuits supplied by the generator. The exception is where the driving means for the generator can be readily shutdown or the generator is not arranged to operate in parallel with another generator or other source of voltage.

Portable generators
Optional standby generators can be used for supplying power for construction sites or for maintenance activities as outlined in 250.34(A)(1) and (2) of the 2011 NEC. When the generator is mounted on a vehicle, the requirements of 250.34(B)(1) through (B)(3) must be followed.

No transfer equipment
The exception to 702.5 permits an optional standby system to be connected for temporary use without providing transfer equipment. Of course, the normal power must be isolated by a lockable disconnecting means or by disconnection of the normal power supply. Proper conditions of maintenance and supervision must ensure that only qualified personnel service the installation.

Sizing generator and routing wiring
Calculations for the load on the standby source must be performed in accordance with Article 220 or by another approved method. When manual transfer equipment is used, Section 702.4(B)(1) permits the user of the generator to select the loads to be connected to the standby source. When automatic transfer equipment is used, the standby source must be sized based on the full load supplied or size for the connected load controlled by load management (system automatically manages connected load), whichever is applicable.


STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.