The Meter Never Lies

By Michael Gambino, president of Gambino Landscape Lighting Inc.

To measure the ?EUR??,,????'??actual load?EUR??,,????'?? of a lighting circuit, set your clamp-on amp meter to the 20-amp setting and clamp it around all the cables attached to the common terminal of the secondary (low-voltage) side of the transformer. This 25-amp circuit has five lighting branch cables. The meter reads 18.50 amps of total load, which is within 80 percent of total loaded capacity (80 percent of 25 amps is 20 amps of maximum circuit load).

Prevent Nuisance Tripping?EUR??,,????'??+ Verify 12-Volt Lighting Loads

While the NEC (national electric code) states that a 120-volt circuit should not be loaded beyond 80 percent of its rated capacity, there is no such statute regarding loads on 12-volt lighting circuits. However, it?EUR??,,????'???s prudent to apply the NEC?EUR??,,????'???s 120-volt requirement when planning a 12-volt lighting system. A UL??????oe-1838 25-amp, breaker/fuse protected, 12-volt circuit is rated for 300 watts. A circuit loaded to its 80 percent capacity would carry a maximum of 240 watts or 20 amps. A 20 percent ?EUR??,,????'??buffer?EUR??,,????'?? can prevent tripping from power spikes, fluctuations in primary voltage supplied by the power company, or from surges that can occur from a sudden in-rush of current. Although not common on systems that are loaded conservatively, power surges can cause a secondary breaker/fuse inside the transformer to ?EUR??,,????'??nuisance?EUR??,,????'?? trip on a unit operating at or near full capacity. Frequent nuisance tripping will annoy your client and can damage your reputation as a lighting professional.

Measuring True Load

The only dependable way of knowing a circuit?EUR??,,????'???s true amperage draw (or load), is by using a clamp-on amp meter. If you don?EUR??,,????'???t already own one, go out and purchase a digital multi-meter (measures voltage, amps and ohms) with a clamp-on amp meter built in. This type of meter will read voltage, amperage and resistance. To find the ?EUR??,,????'??actual load?EUR??,,????'?? on a low voltage lighting circuit, set the multimeter to the 200 amp setting and clamp it around all cables (on each 25 amp circuit) attached to the common terminal of the secondary (low voltage) side of the transformer. Let?EUR??,,????'???s say you have 19.20 amps showing on your meter. According to Ohm?EUR??,,????'???s law formula, 19.2 amps is an approximate 230-watt load. Ohm?EUR??,,????'???s law is amps (19.2) x volts (12) = watts (230.4).

Load Balance Your System

Calculating total lamp wattage using Ohm?EUR??,,????'???s law is a good way to plan circuits in a lighting system, but ?EUR??,,????'??load balancing?EUR??,,????'?? the system once it?EUR??,,????'???s installed requires a field check with a clamp-on amp meter and multi-meter for voltage verification. Variables like primary voltage supply, transformer regulation (output under load), the voltage output tap you have chosen (12 to 15 volts), how and where the lamp load is distributed on each cable run, and cable size will affect the power balance of a system. These variables are not provided for in mathematical computation. Your professional obligation is to provide between 11 and 12 volts to all lamps, which optimizes light output and lamp life. Landscape professionals who install lighting should always measure actual voltage and amperage on every project they install or service.

Michael Gambino is president of Gambino Landscape Lighting Inc., an award winning low voltage landscape lighting company based in Simi Valley, Calif. Mike is responsible for the design and installation of 35 to 40 new projects per year for upscale clients?EUR??,,????'??+all personally completed by himself without the assistance of a helper. He also provides on-going maintenance for hundreds of systems he?EUR??,,????'???s installed since 1990. Find out at www.gambinolighting.com.