How to choose an emergency light bar: roof fit, color, class and controller
Written by Shmaya Krinsky, K&K Emergency Systems, Opa-locka, Florida. We build and upfit ambulances, fire apparatus and police vehicles, and we sell the parts we install. Updated October 2026.
The wrong light bar shows up later as a bar that does not fit the roof, a class rating that does not cover the color you need, or a control method your controller cannot use.
The short version
- Full-size and mini bars: in these families, full-size bars typically offer room for more lightheads, while mini bars offer a shorter footprint; check the populated configuration.
- Permanent mounting is the planning default for vehicles in regular service; manufacturers publish warnings on magnetic mounting.
- Ask for the SAE class per color. "SAE compliant" alone is not a class.
- Color rules are set by each state. Check yours before you order.
- Control interface: the bar's control interface must match the controller; compare the wiring requirements for the exact serial, CAN-based or hardwired models.
- Current draw depends on the model, enabled functions and flash duty cycle; use the manufacturer's operating and maximum-load figures for the exact configuration.
Full-size or mini
What Whelen and Federal Signal publish for these families:
| Family | Maker | What the maker publishes |
|---|---|---|
| Liberty II | Whelen | Full-size, roof-mounted; the family page lists 43.22, 48 and 53.75 in lengths, 12/24 VDC and SAE Class 1/J845; specifications vary by model, and the family also includes a 60-inch Towman's variant. |
| Mini Liberty II | Whelen | 19.31 in long, 2.53 in high; 12 VDC; lists SAE Class 1 and R65 Class 1; permanent, magnetic, or magnetic/suction mounting. |
| Mini Legacy | Whelen | 19.31 in long, 1.60 in high; 12 VDC; lists SAE Class 1; same mounting options. |
| Responder LP | Whelen | Described as low-profile Super-LED mini lightbars (R1LP, R2LP). R1LP and R2LP: 17 in long; Whelen lists SAE Class 1. |
| Allegiant (legacy) | Federal Signal | Legacy Allegiant: the cited manual covers nominal 21 to 94 in models, serial or discrete versions and 12 V negative-ground operation. Federal Signal's legacy sheet lists IP65 and SAE J595/J845 Class 1 for amber, blue, red and white. Allegiant Max is a separate current generation; use its exact model's data sheet and installation manual. |
| Valor | Federal Signal | Low-profile, non-linear; nominal 44- and 51-inch models, physically 43.7 and 51.3 in long; 12 Vdc; manufacturer lists SAE J845 and J595 Class 1 for amber, blue, red and white, IP65, and serial controllers. |
| Reliant S2 | Federal Signal | Low-profile, linear; 47.6 in long, 1.3 in high; SAE J595 and J845 Class 1 (amber, blue, red, white); IP65 and IP69K. |
Not every model in a family carries the same rating. Check the exact model number.
Roof, interior, and length
Check the installed warning coverage in every required direction; the bar's optics, position and obstructions determine coverage. An interior light shines through glass, which can change output and add reflections, and the datasheet class describes the device, not your installed configuration.
Length and roof attachment. Start with the flat roof area, then leave clearance for antennas, hatches, rails and roof curvature or ribs. Clearance alone does not make a mount safe. Roof mounting follows the vehicle maker's and the mount maker's approved attachment points and load guidance. Follow the vehicle and mounting-kit instructions for attachment location, supporting structure, specified hardware and tightening torque; check both sides before drilling, and observe the vehicle maker's airbag deployment and headliner restrictions. Whelen lists standard or low-profile mounting feet and vehicle-specific fitments for Liberty II; confirm the correct kit and included hardware for your chassis and order. Zone coverage and optical power come from the whole system, not the bar alone.
Permanent or magnetic
Whelen's mini bars offer permanent, magnetic and magnetic/suction mounting. Federal Signal's light bar installation warnings say not to drive a vehicle with a magnetically mounted warning light installed, because roof construction and driving conditions vary and the light could come off.
SAE class, per color
SAE J595 covers directional warning devices; J845 covers omnidirectional and selective-coverage devices. A bar may list one or both. See our SAE classes post.
The class is claimed per color. Federal Signal's Valor data sheet lists its classes for amber, blue, red and white, and a separate California Title 13 line for amber, blue and red. Ask for the class, the standard and edition, and the color and flash pattern it applies to.
Colors and state rules
Which colors you may run, and in what combination, is set by state law and your agency. It differs by state, so we do not list it here. Confirm with your state vehicle code or fleet manager, then check that the bar offers that color at the class you need.
Serial or hardwired control
Liberty II is offered in WeCan, WeCanX, Standard Current and Low Current versions; Whelen describes individual-module control for WeCanX models. In the legacy Allegiant manual, Federal Signal describes a bar configurable for serial or discrete control: serial commands run on one communication cable to a Federal Signal controller, while discrete bars over 21 in use a nine-conductor cable. Match the bar's control type to the controller.
Power draw and wiring
Table 3 of the cited legacy Allegiant manual lists the following current at 12.8 Vdc under its "50% Flash Rate" condition; these are not maximum-current or fuse-sizing values:
| Allegiant model | Current |
|---|---|
| 21 in | 7.0 A |
| 45 in | 9.0 A |
| 53 in | 11.0 A |
Its Reliant S2 data sheet lists 9 A at a 50 percent duty cycle and 18 A maximum. The manual describes a light system as high-current, calls for separate positive and negative connections, and says a suitable fuse belongs on the positive battery connection as close to the battery as possible, with wires and fuses rated for the system amperage. It also warns that enabling a low-power function may drop output below current light-output standards. Use the exact model's installation and wiring instructions, maximum simultaneous loads and OEM-approved power and ground provisions to select wire size and circuit protection; do not size protection from average flashing current alone. See our police vehicle upfit checklist.
Frequently asked questions
Can I use a mini bar on a patrol car? Check the governing warning specification, applicable law, and the vehicle and mounting requirements for the exact bar.
Do light bars need to be SAE Class 1? Check whether your governing specification requires SAE Class 1 and confirm the applicable legal requirements.
Is a Whelen bar compatible with a Federal Signal controller? Compatibility depends on the exact bar, controller and interface. Obtain manufacturer documentation of electrical and protocol compatibility before connecting them; the manual's warning about mixed lightheads and power supplies does not establish controller compatibility.
Can I pressure wash a light bar? Federal Signal's manual says not to use a pressure washer on its light bar.
Will a 12 V bar run on a 24 V vehicle? Only if the exact model is rated for it. Whelen lists the Liberty II family at 12/24 VDC, which does not mean every model accepts both, and its mini bars above at 12 VDC; Federal Signal's Valor and Reliant S2 data sheets list 12 Vdc.
Browse our light bars, emergency lighting, sirens and speakers and auto ejects, inverters and chargers, or call (305) 317-2619 and we will help you match a bar to your roof, controller and specification.
About this article: it was drafted with AI assistance and fact-checked against the standards bodies and manufacturer publications cited in the text. AI-written content can contain errors and standards change, so facts here may be wrong. Where we describe our own practice, it is our practice, not a requirement of any standard. Confirm any requirement with the standard, the manufacturer, or with us before you buy or install.