Resizing Service or Feeder Loads

At one time or other, all contractors have been faced with a decision to either recalculate the size of an existing service or feeder or just guess at the amount of spare load and install new circuits while hoping for the best. Guessing will usually only get you in trouble.

Section 220.35 was inserted into the National Electrical Code (NEC) to permit actual demand loads to be used in determining existing loads so guessing is not necessary. An actual demand load is the peak load or amount of power that is consumed during a specific time period. Many utility companies provide peak demand meters at the electrical service so they can monitor peak usage of power during high demand periods, and by controlling the peak demands of the customer, the utility companies can better manage their overall demand. This power management permits the utilities to better plan their power generation and overall power supply for a region. The customer can also control their power usage by using energy management devices that will control the peak demand at their service. Utilities usually surcharge customers for high peak power usage, either during certain times of a day or during certain times of the year.

Peak demands recorded by the utility company demand meters can be a valuable resource for the contractor and most utilities will release this information directly to the contractor or to the owner of the property. The contractor then uses this peak demand as verification of the actual peak load being used at the facility. When using this peak load, the contractor must be careful to check with the owner of the property to ensure that there have been no appreciable changes during the past year regarding power consumption at the facility or unusually large equipment loads that may affect the peak demand.

Section 220.35 requires maximum demand data used for this optional calculation to be available and recorded for at least one year. However, where the maximum demand data is not available for at least one year, an exception to this requirement permits the calculated load to be based on the maximum demand of the feeder or service using a recording ammeter or power meter. This measurement must be continuously measured using an average demand read for a 15-minute period each time over a minimum of a 30-day time period. The recording meters must be connected to the highest loaded phase conductor of the service or feeder. This will ensure someone does not record the lowest load possible and use this for the calculation.

The recording must be done when the building is occupied and using normal loads; not during a holiday or some other time when work may be light and the facility is not running at peak load. There also may be seasonal conditions that affect the amount of power consumed during the recording time period, such as recording power consumption for a service or a feeder at a facility located in the desert. The facility may use air conditioning units during the summer resulting in a higher peak demand than any heating units would register in the winter, thus greatly affecting any recording done for feeders or services during the winter. If measurements can’t be accomplished during the peak usage of the equipment, then a calculation of the loads is acceptable. Remember, if the loads are seasonal, such as air conditioning or heating, these loads may be non-coincident loads, as mentioned in Section 220.21, and only the larger of the two loads is figured in the calculation. There may be other similar equipment located within the facilities that are only operated periodically or seasonally and these loads must also be taken into consideration.

Once the demand loads are measured and recorded, periodic and seasonal loads have been calculated and entered into the equation, the final part of the calculation will be done. Section 220.35(2) requires the maximum demand that was measured and calculated be multiplied by 125 percent. Any new load must be calculated using all normal requirements for load calculations in the NEC. For example, any new loads operating for three hours or more are considered to be continuous and must be figured at 125 percent, while non-continuous loads would be figured at 100 percent.

The final requirement is located in 220.35(3) and states that to use this optional calculation, a feeder must have overcurrent protection in accordance with Section 240.4 and any service must have overload protection in accordance with 230.90. This just assures standard overcurrent or overload protection.

Using this optional calculation may result in a successful bid and permit you to provide potential savings for the owner of the facility, resulting in a win/win situation. 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




About the Author

Mark C. Ode

Fire/Life Safety Columnist and Code Contributor
Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and .

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