You're wise to be thinking about compatibility with mixed lighting types and a photocell. Here's a breakdown of potential issues and solutions when combining MH and LED fixtures on a photocell circuit

Potential Issues:
Inrush Current

LED drivers, even energy-efficient ones, can have significant inrush current when first energized. Metal Halide ballasts also have inrush current, but typically less problematic. When starting the system, the combined inrush current of four LED drivers and eight MH ballasts could potentially overload the photocell's contact or the contactor's coil. This is more of a concern with older or less robust photocells and contactors.

Photocell Compatibility with LED Loads

Some older photocells were designed primarily for incandescent or MH loads. They might not properly detect low light levels or switch on and off reliably with the significantly lower current draw of LEDs. This can lead to flickering, short cycling (rapid on/off), or the lights staying on longer than expected.

Load Rating of Photocell/Contactor

The photocell and contactor have maximum wattage or amperage ratings. You need to verify that the total load of all the fixtures (MH and LED combined) doesn't exceed these ratings. This is especially important with MH, as they draw significantly more power than comparable LEDs.

Voltage Compatibility

Ensure the voltage rating of the new LED fixtures and their drivers matches the existing electrical system (e.g., 120V, 277V, 480V). This is usually not an issue, but it's a fundamental check.

Power Factor

Metal halide lamps often have a low power factor which will increase the current draw of the devices. LED power factors are higher and will draw less current for the wattage rating.

Troubleshooting and Solutions


Calculate Total Load



Metal Halide

Find the wattage of each MH fixture (usually on the ballast or fixture label). Multiply that wattage by the number of MH fixtures.

LED

Find the wattage of each LED fixture. Multiply that wattage by the number of LED fixtures.

Total Wattage

Add the MH wattage and the LED wattage.

Total Current (Amps)

Divide the total wattage by the system voltage (e.g., 120V, 277V).2.

Verify Photocell and Contactor Ratings

Check the nameplate on the photocell and contactor. Ensure their wattage and amperage ratings significantly exceed the total calculated load. Ideally, there should be a safety margin of at least 20-30%. A larger safety margin is always better.3.

Consider a Solid-State Photocell

A modern solid-state photocell (electronic photocell, not the older electromechanical type) is generally recommended for LED compatibility. These are better at detecting low light levels and handling the inrush current characteristics of LEDs. They are also generally more reliable.4.

Use a Properly Sized Contactor

A contactor is a heavy-duty switch that is controlled by the photocell. If the photocell is not rated to switch all of the lighting, it will switch the low voltage coil on a contactor which in turn switches the higher voltage lighting circuit.5.

Install an Inrush Current Limiter (If Necessary)

If the calculated inrush current is a concern (especially with older contactors or a heavily loaded circuit), consider installing an inrush current limiter specifically designed for LED lighting. These devices limit the initial surge of current, protecting the photocell, contactor, and circuit breaker.6.

Choose Photocell with Adjustable Time Delay

Some photocells have an adjustable time delay before switching on or off. This can help prevent short cycling due to passing headlights or momentary shadows.7.

Test After Installation

After installing the new photocell and LED fixtures, carefully monitor the system for a few days. Check for: Proper on/off operation at dusk and dawn. Flickering or short cycling of the lights. Excessive heat from the photocell or contactor. Any unusual noises from the ballast, drivers, or photocell.8.

Check Power Factor and Correct if Necessary

Use a power analyzer to measure the power factor of the lighting circuit after installation. If the power factor is significantly below 0.8, consider installing power factor correction capacitors to improve efficiency and reduce the load on the electrical system.

Specific Recommendations for your situation



Replace the Photocell

Given you're already doing replacements, install a new, modern solid-state photocell that is specifically designed for compatibility with both MH and LED loads. This is the most proactive step you can take.

Check Contactor

If the contactor is old or questionable, consider replacing it with a new one that is appropriately sized for the total load and has a robust coil.

Calculate Inrush Current (if possible)

The best way to determine the inrush current is to refer to the manufacturer's specifications for the LED drivers and MH ballasts. You will likely need to contact the manufacturer for this data. If the specs aren't available, err on the side of caution and assume a significant inrush current for the LEDs.*

Consider Phasing Out MH: While not immediately necessary, advise the client that replacing the remaining MH fixtures with LEDs over time will further reduce energy consumption, maintenance costs, and potential compatibility issues.By taking these steps, you can ensure that the mixed lighting system operates reliably and efficiently with the new photocell. Good luck! Flag for review