Losing Control Again: Value engineering and LED dimmers


For as long as I have been in this industry, material substitutions have been a way to save money on projects. The process eventually became known as “value engineering.” Theoretically, the contractor, and maybe the owner, saves money by replacing a specified product with a less costly one. We usually look at lighting fixtures first. In the past, it was fairly easy to do. You could simply ask a vendor for a fixture package based on inexpensive fixture brands instead of the “Rolls-Royce” fixtures that were specified.


A new ‘gotcha’


This practice has become more difficult. It’s dangerous in areas that have energy usage codes. Engineers are increasingly moving to expensive light-emitting diode (LED) fixtures to meet the energy codes. When asking vendors for a substitute, you must tell them the fixture packages have to be energy equivalent in addition to the other requirements. I have heard several stories where contractors got in trouble when they accepted a fluorescent package in lieu of an LED package, since the fluorescent fixtures required more power.


Branch circuitry presents another problem when substituting fluorescent fixtures for LED fixtures. Since fluorescent fixtures draw so much more energy, be sure to include the cost of additional circuits in your calculations when you present a proposal for this type of value engineering.


Follow-up to “Out of Control”


Since my January column, “Out of Control,” I have estimated more projects with 0–10-volt (V) LED dimming controls, and, of course, the control wiring was not designed. While laying out the 0–10V wiring, I wondered if any of the metallic cabling (MC) manufacturers are making a cable with the control wiring included. Sure enough, Southwire has UL-approved cables in several configurations, including 12/2 with two No. 16 control wires as well as 12/3 with two No. 16 control wires. This is just what the doctor ordered for projects that allow MC and have 0–10V LED dimming fixtures.


I also have a correction to the article in regard to how 0–10V LED dimming works. There are actually two protocols. One protocol is known as “sourcing” and is used more often in entertainment technology. This protocol requires the controller to generate the low-voltage control signal. The protocol I see on all of my projects is known as “sinking,” where the low-voltage control signal is generated at the ballast or driver.


To estimate the cost of a sinking protocol system, first understand that the dimmer has four wires. Two are at line voltage and switch the ballast or driver on and off. The other two wires are purple and gray and handle the 0–10V signal. The driver or ballast furnishes approximately 10V and a rheostat in the dimmer alters the voltage. The return low-voltage signals the driver or ballasts electronics to “sink” the controller’s voltage, causing the fixture to dim.


These dimmers have a maximum amount of current they can handle on the low-voltage circuit. It varies typically between 30 and 75 milliamperes (mA). Calculations need to be made to ensure the dimmer’s low-voltage side is not overloaded. Tech support people at three companies have told me that it is possible to overload the low-
voltage circuit while not overloading the switching circuit.


So, do you think the engineers will make these calculations? If you do, I have a bridge for sale. Two calculations will be required to ensure you do not overload a 0–10V dimmer. The first calculation is for the rating of the on/off switch in the dimmer. You will need to make a standard amperage calculation. You can find out from the manufacturer how much current is required to switch the ballasts or drivers. The second calculation is for the rating of the dimmer’s 0–10V rheostat. Once again, contact the manufacturer to find out how much current each ballast or driver contributes to the circuit. Each ballast or driver typically contributes 0.5 to 1 mA. After obtaining the amount of current generated in the drivers, you can make the simple calculation for the control circuit.


Something else important to know about 0–10V dimmers is the dimming happens at the drivers, not the dimmer. As a result, you can gang these dimmers without derating them. These dimmers are also available without switches, for schemes where the fixture is switched from another location, such as a motion sensor.

About the Author

Stephen Carr

Estimating Columnist

Stephen Carr has been in the electrical construction business since 1971. He started Carr Consulting Services—which provides electrical estimating and educational services—in 1994. Contact him at 805.523.1575 or steve@electrical-estimating.com, and...

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