[mike31154]

I think you're going to cook a lower wattage lamp with that 400 in short order. From everything I've read so far, MH ballasts are matched to the lamp they are intended to fire with respect to wattage. You're asking for a possible fire.

Here's an excerpt from a 'DIY guide to MH Lighting Systems':

"Ballast

The ballast provides the proper starting voltage, operating voltage and current to the lamp to initiate and sustain the arc. HID lamps have a negative resistance characteristic, which causes them to draw an increasing amount of current, hence need a current limiting device. The ballast provides these functions.

Once the lamp is selected the next step it to select the ballast that will be used to drive the lamp. When putting together a DIY MH lighting system it is very important that the ballasts be matched to the lamp in use. An easy way to match the ballast to the lamp is through the use of the ANSI designation. For example, when using a lamp designated M-57, look for the ballast with the same designation. For lamps without ANSI designation, it is best to call the lamp manufacturer and get a recommendation on the ballast to be used.

Most ballasts (except the electronic ballasts) used for Metal Halide lighting are of the CWA (constant wattage autotransformer) type. This is lead circuit ballast, and consists of a high reactance autotransformer (core-coil) with a capacitor in series with the lamp.

Any ballast rated for the lamp will function properly, but may come in different configurations, each with its pros and cons, differences in prices and amount of DIY work required. There are several manufactures of ballasts that will operate the same lamps, and often the exact brand is not important. The ballasts can be cross-referenced with another company's ballasts if a specific brand is desired."

Proceed at your own risk, but I'd have some reservations as to the qualifications of the electrician that provided the advice in your post. I might seek a second opinion. Which you are sort of doing by posting here...