Comparing UL 924 vs. UL 1008 Emergency Lighting Transfer Devices

Let’s discuss the technical aspects of the two types of emergency lighting transfer switches, UL 1008 and UL 924, and explore precisely when and how to use them.

A transfer switch is an electrical device designed to safely transfer electrical loads between two or more power sources, such as the utility grid and a backup generator. During a power outage or when the primary power source fails, the transfer switch disconnects the load from the main source and connects it to the backup source, ensuring continuous power supply to critical equipment or circuits.

Emergency lighting transfer switches enable general illumination lighting fixtures to act as emergency lighting fixtures when an emergency power source is available. These devices sense normal power and automatically connect the emergency power source to its lighting loads during a power outage.

There are two classes of emergency lighting transfer devices, UL 1008 and UL 924, with some uncertainty in the industry regarding how and when to implement each type. Recent changes to the National Electrical Code (NEC) concerning emergency lighting transfer devices have complicated matters. Perhaps the most significant confusion is that although they are both referred to interchangeably as basic “transfer devices,” their transfer capabilities serve very different functions.

Understanding the differences between the two is essential as it will affect costs, complexity, and compliance with the NEC Article 700 code. Article 700.2 of the latest NEC categorizes the UL 1008 as a Branch Circuit Emergency Lighting Transfer Switch (BCELTS) and the UL 924 transfer device as an Automatic Load Control Relay (ALCR). Both types can be used to achieve a fully compliant NEC Article 700 emergency lighting design.

However, examining the inner workings of each “transfer device” soon becomes apparent how the UL 1008 transfers between two power sources while the UL 924 transfers from input to output.

The UL 1008 transfer device has two power sources connected to its contacts, and switches between these sources (normal and emergency power) connect to the emergency lights. Engineers would classify the UL 1008 as a FORM-C type relay.

The UL 924 transfer device acts as a switch and connects the constant “on” emergency power to the emergency lights. Engineers would classify the UL 924 as a FORM-A type relay. Both transfer devices sense normal power and cause the relay to connect the emergency power to the lights upon its loss.

Because there are two different power sources for a UL 1008 device, it has to undergo a completely different test program than the UL 924. Two significant safety concerns are short circuit protection and synchronicity between the two different power sources. Nonsynchronous means that the emergency power source can be completely out of phase from the normal power source. This phase shift can lead to a 2X voltage potential difference if they are out of phase by 180 °. When this 180 ° phase shift occurs on a 277V circuit, the potential difference between the emergency and normal power can be as high as 554VAC (277VAC x 2). This situation can never happen with a UL 924 device since only ONE power source is turned on or off.

UL 1008 devices are typically more expensive than UL 924 devices due to the inclusion of overcurrent protection devices (normally fuses) as additional safety measures to satisfy the fault current requirements. The overcurrent protection may need to be reset or replaced, meaning these devices must be easily accessed. Because of these and other factors, it may be more desirable to use the UL 924 devices when possible. The easiest way to accomplish this is to use an upstream “normally on” emergency power source that feeds UL 924 devices. Two possible solutions are a generator coupled with a UL 1008 transfer switch or a central inverter that has a “normally on” power output.

Download the whitepaper to learn more about NEC updates and dive deeper into the operation of UL 1008 and UL 924 transfer devices.

Full compliance with NEC Article 700 for emergency lighting can be achieved with either UL 1008 or UL 924 transfer switches. How the emergency lighting load is controlled (line voltage dimming vs 0-10/DALI) will determine which device is used. The UL 1008 is going to be required for line voltage dimming. However, the more cost-effective

UL 924 devices can generally be used for everything else. UL 1008 transfer devices are required to transfer power from two nonsynchronous sources. UL 924 devices do not transfer two nonsynchronous power sources and simply turn lights on and off from a constant emergency power source. Both devices can be used congruently.