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Page last updated
February 28, 2005

User Advisory

Overview

When the devices connected to an electrical circuit require more current than the circuit is equipped to carry, overload occurs and a circuit breaker generally trips, resulting in interruption of power to all devices served by that circuit.

This advisory is designed to help ALS users avoid circuit overloads by ensuring that electrical devices are connected to circuits that can handle their current requirements. This document does not replace LBNL and ALS official procedures and policies concerning electrical wiring or safety. For more information on these policies, refer to Chapter 8 of the Laboratory's Health and Safety Manual, PUB-3000.

Determining Current Requirements

Before connecting any equipment to a circuit, verify that the current requirements of the equipment do not exceed what the circuit can handle. With a few exceptions, all 120-VAC circuits at the ALS are supplied through 20-A circuit breakers. The National Electrical Code allows up to 13 receptacles to be connected to one circuit breaker, and requires circuit ratings to be 125% of the continuous current load, so any devices connected to a 20-A circuit for simultaneous operation should have a total current requirement of no more than 16 A [National Electrical Code (ANSI/NFPA-70), 1973 ed., Sections 210-22, and 220-24, National Fire Protection Association, Quincy, MA]. When you have determined the current required for each device on a circuit, add the currents together to ensure the total does not exceed 16 A. (The NEC does allow a 20-A circuit to operate at the full 20 amperes if the circuit is not a continuous load i.e., a load that does not operate continuously for three or more hours. At the ALS, all circuits are considered to carry continuous loads, but that does not prevent using a circuit to its full rating for short period of time.)

Note: The ALS-installed AC power convenience receptacles in the front and rear bases of electronic equipment racks and cabinets should not be considered reliable power sources. They are designed to provide temporary power for soldering irons, handheld power tools, test instruments, etc. such that a temporary loss of power would cause only a minor inconvenience and would not result in a safety hazard or expensive equipment damage.

Common equipment

This table lists the current required by equipment commonly used at the ALS. For equipment not listed in this table, check the manufacturer's nameplate on the device; often the current required is listed in amperes (A). Contact an ALS Beamline Coordinator (ext. 7464) for assistance if the nameplate does not list current or if the listing is unclear.

These requirements are for 120 VAC at worst-case vacuum conditions.

If the Available Circuits are NOT Adequate

If the devices you must operate demand more current than the available circuits can provide, please contact a Beamline Coordinator. The Beamline Coordinator will assign appropriate ALS personnel to locate or connect additional circuits.

Electrical Cords and Extension Cords

Any electrical cords or extension cords you connect to a circuit must be rated for at least the current you expect them to carry. Check the ratings on any such devices you use to be sure they are adequate. Contact a Beamline Coordinator (ext. 7464) for assistance if current ratings are not clearly indicated on the devices. The Laboratory has a number of guidelines about the use of extension cords, including where and for how long they may be used and what types are acceptable. For more information, please refer to the ALS Safety Handbook or the Laboratory's Health and Safety Manual, PUB-3000, both of which are available from the ALS User Services Office.