Can I Really Do That?

By Mark C. Ode | Dec 15, 2007
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There has been a substantial controversy raging throughout the electrical industry for a considerable time now, and hopefully, changes made for the 2008 National Electrical Code (NEC) will finally resolve this debate. Changes to the NEC often are driven by misconceptions within the industry on the intent of the actual text within the Code, so providing some historical background on a particular section of the NEC becomes part of the solution.

Backup “emergency” power for residential, commercial and industrial facilities during temporary power outages, such as during and after hurricanes, severe thunderstorms and other natural disasters, has become a necessity for many people. The perception of emergency power is subjective. If you ask someone who has lost power to their home in the middle of an extremely hot summer or a very cold winter, they will certainly tell you air conditioning or heating is a necessity and, therefore, an emergency.

Section 702.2 defines optional standby systems as “those systems intended to supply power to public or private facilities or property where life safety does not depend on the performance of the system. Optional standby systems are intended to supply on-site generated power to selected loads either automatically or manually.” A fine print note provides, “Optional standby systems are typically installed to provide an alternate source of electric power for such facilities as industrial and commercial buildings, farms, and residences and to serve loads such as heating and refrigeration systems, data processing and communications systems, and industrial processes that, when stopped during any power outage, could cause discomfort, serious interruption of the process, damage to the product or process, or the like.”

Unlike “emergency systems” covered in Article 700 and “legally required standby systems” covered in Article 701, where the systems are legally required and the loads connected to the systems are determined by municipal, state or federal codes or other governmental agencies, the user of optional standby systems is permitted to select whatever loads can be connected to the system. Optional standby systems must comply with all load calculations in accordance with Article 220 or by another approved method, just as any load for a normal power system would be calculated. This is where some installers have deviated from the NEC.

There is a perception that any generator can be used, regardless of the size of the generator, to supply an unlimited amount of load. For example, some electrical contractors would install a 10-kW generator connected through an automatic transfer switch to a 200-ampere panelboard where the load in the panel is obviously much greater than the rating of the generator.

Nowhere in Chapter 2 of the NEC is it acceptable to automatically overload either a branch circuit or a feeder, and yet, as I previously stated, the perception is that this is perfectly acceptable for a generator since there is a branch circuit or feeder overcurrent protective device at the generator. Can a branch circuit or feeder circuit supplied by the normal service be over-loaded on a regular basis simply because it is protected by an overcurrent device? The answer to that question by any electri-cal person is absolutely not. Why is it then perceived as acceptable for the generator power circuit?

Section 702.5 covering the capacity and rating of the optional standby system has been revised in the 2008 NEC to hope-fully clear up this misconception. The new text in 702.5 appears in red as follows:

“(B) System Capacity. The calculations of load on the standby source shall be made in accordance with Article 220 or by an-other approved method.

(1) Manual Transfer Equipment. Where manual transfer equipment is used, an optional standby system shall have adequate ca-pacity and rating for the supply of all equipment intended to be operated at one time. The user of the optional standby system shall be permitted to select the load connected to the system.

(2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system shall comply with (a) or (b).

(a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.

(b) Load Management. Where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system.”

As the revised text indicates, a manual transfer permits the user to pick whatever loads are to be connected to the system. An automatic transfer switch requires the full rating of the load that will be automatically transferred onto the generator. Load man-agement devices also can monitor the load on the circuit. EC

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at [email protected].




About The Author

ODE is a retired lead engineering instructor at Underwriters Laboratories and is owner of Southwest Electrical Training and Consulting. Contact him at 919.949.2576 and [email protected]


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