Can I replace Metal Halide (MH) or HPS fixtures with LEDs directly?

Short answer: rarely a straight screw-for-screw swap. Metal halide and HPS fixtures run on magnetic or electronic ballasts that push high starting voltages and a specific current waveform. LED high bays operate on constant-current drivers, usually 0-10V dimmable, and expect line voltage (120-277V or 347-480V) directly. If you just pop an LED "retrofit" lamp into an old MH socket with the ballast still in the circuit, you’ll get flicker, half the light, or a dead lamp in a week. Some so-called "direct fit" LED tubes work with pulse-start metal halide ballasts, but the compatibility list is short. HPS ballasts are even worse—they hate LEDs.

Here’s the clean way. Bypass or remove the old ballast completely, rewire the tombstones for line voltage (mains), then install a purpose-built LED high bay with a 0-10V driver. That’s what most electrical contractors do across the US and Europe. If you want to keep the existing housing, get a universal voltage (AC 120-277V) LED corn lamp or retrofit kit designed for "ballast bypass" or "direct wire" operation. But honestly? For high bays, rip the whole fixture down and put up a new LED unit. You’ll get better light distribution, longer life, and no ballast hum. Ballast bypass on an old 400W MH fixture saves you maybe forty bucks but leaves you with a beat-up reflector and questionable heat management—not worth it in a workshop or warehouse.