Wiring Up Emergency Ballasts

Today, we will be doing a follow-up of another article, “Driver V Ballast”, in which we talked about the differences between LED Drivers and both Magnetic and Electronic Ballasts. We touched on the basic principles and theory on how each works, the history of light fixtures, and briefly dove into the dimming capabilities of both ballasts and drivers. Now, we will look into the emergency ballasts: what their purpose is, wiring diagrams, and how they function. Grab a cup of coffee or a cold beer, sit back, and let’s dive into today’s topic: Emergency Ballasts.

Power Sentry emergency backup battery pack – Source: acuitybrands.com


*Disclaimer – For all intents and purposes of today’s article, we will be using the most recently published version of the NEC(2020). Some references may differ depending on the version of code you are using.

Emergency ballasts and emergency LED drivers act the same way their non-emergency counterparts do with one addition: a back-up battery. Why do we need emergency ballasts or backup lighting devices? For this answer, let’s refer to Article 700 Part IV. 

700.16 Emergency Illuminations

  1. General – Emergency Illumination shall include means of egress lighting, illuminated exit signs, and all other luminaires specified as necessary to provide required illumination

Basically, people need to be able to see down hallways and in rooms to get to exits when necessary. This is what egress lighting, or lighting that helps guide one’s way out (along with illuminated exit signs and other required luminaires), is for. For this lighting, it is required that there be one lumen per square foot (one foot candle). 

Wiring Diagram

Image 1 below shows an example of a wiring diagram for an emergency LED driver.

Image 1: Wiring Diagram

Example emergency light wiring schematic with ballast and battery pack – Source: rablighting.com

*Disclaimer – For our discussion purposes today, we will be referring to the image provided. The text will be typical of said image and may not apply to all manufactured products. Please consult all manuals and diagrams with specifications to that product.

The emergency driver pictured above is a typical wiring diagram for both drivers and ballasts. The emergency driver or ballast will work in conjunction with its respective counterpart to provide illumination in the occurrence of power loss. Let’s take a look at the image above and try to understand the way it’s set up. On the left side of image 1 we have our feed in (switched ungrounded or non-switched ungrounded), grounding conductor and our grounded conductor (neutral or common). In a typical setup, unless pertaining to a “night light” application where the emergency luminaire will stay on 24/7, we want the emergency light to act as a regular light in which we can turn off or on by use of a manual switch or occupancy/vacancy sensor; therefore, we will have two ungrounded conductors, one switched and one constant hot, the grounded conductor, and the grounding conductor. Following the path from the junction box, we make our way to our emergency driver. Notice, both ungrounded conductors are wired to the emergency driver. 

  • The black conductor will wire together with the corresponding black wire of the emergency driver, feeding the back-up battery. 
  • The red conductor, that would act as a switchleg if applicable, will either be ran as a switched conductor or fed constant power. This is the conductor that will power our light in normal operation. It will tie together with the red/white conductor of the emergency driver.
  • The white conductor, referred to in the image as “common”, wires into both the emergency driver and regular driver’s corresponding neutral conductor.

This leaves us with a yellow/black, a yellow, a black/white and a blue conductor. 

  • The black/white conductor carries the 120-277V through to the regular driver, as an extension of the switchleg that controls the on/off function of the light.
  • The yellow/black conductor ties all the negative terminals of the luminaire trim, the driver, and the emergency driver. Notice, the negative conductor from the driver could be either black or blue, instead of yellow/black. This is a component of the low voltage DC output. 
  • The yellow conductor will connect to the positive terminal of the luminaire trim. This is also a component of the low voltage DC output. 
  • The blue conductor from the emergency driver then ties to the positive terminal of the driver, which is red. This is also a component of the low voltage DC output.

What does all this mean?!

What’s actually going on here? What do all these connections mean and how does it all work?

We mentioned that the switched conductor from our line side feeds through the emergency driver at 120-227V to the regular driver which converts it to DC, and the other ungrounded conductor, or constant hot, is converted into DC and feeds the battery inside the emergency driver. As stated before, the switched conductor can be tied to an uninterrupted source, as well. In this instance, we would typically see that in a “night light” configuration. To complete the AC circuit, the grounded conductor, or neutral, ties to the neutral conductor of the emergency ballast. 

From there, the switched conductor feeds through to power the regular driver via white/black wire, which then converts the AC voltage into DC. Then the driver outputs a DC voltage to the lamp from the positive terminal (red colored) and flows through the blue conductor of the emergency driver. Power is then sent through the yellow conductor to the lamp, and returns to the negative terminal (blue or black colored) and negative conductor of the emergency driver (yellow/black). This is normal operation. 

In the event of a loss of power to this circuit, the emergency driver senses this loss and switches to emergency mode in which the lamp is then powered by the back-up battery. When no power flows through the unswitched conductor, the system automatically knows to transfer its output from feeding the driver to directly feeding the lamp. DC voltage will travel through the yellow conductor to the lamp then back through the yellow/black conductor to complete the circuit.

What about the inverter wires and integral test button? I’m glad you have been paying attention! The two ends of the inverter leads need to be connected for power to charge the battery. Lastly, the integral test button needs to be wired in for maintenance personnel and technicians to test if the battery still has a good core charge. Over time, batteries can go bad. This is why it is imperative these emergency lights be tested once a month or per manufacturer’s direction to ensure optimal operation in time of power loss. This is also a good time to remind you that we are supposed to check GFCI devices and breakers once a month, also. Yeah, I’m looking at you.

Hopefully, by now, we have a better understanding of how these devices work and their importance to the well-being and safety of people in the event of a loss of power. Not only is the knowledge of how to properly install these devices a crucial aspect, but it is also imperative to perform regular maintenance on these luminaires by testing the battery. 

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