When I built my house 25 years ago, I put six boxes in, on various corners, under the eaves of the roof, for floodlights. Since then, I've put three quartz halogen floodlights in, two 500W in the front, and a 300W in the back. They are controlled by switches in the garage.

From time to time, I have to pull nests off the lights, for fear of ignition. This always creates disharmony in the family, as some people would rather the birds are able to choose their home, as opposed to us preventing ours from burning down. Fortunately, I am fast with an extension ladder.

The latest issue is that my youngest twins play basketball at night, and the lights get left on, and for some reason, 500W halogen bulbs don't seem to really last 2000 hours. Besides it keeps the electric meter spinning.

So I picked up a 70 watt SLA style metal halide floodlight fixture. The only problem I envision is that it is a bit heavier than the quartz halogen floods. When I put the boxes in, I used metal boxes, and mounted them on a horizontal 2x4 spanned between studs. The house is cedar tongue and groove so long screws are used to hold the three hole plate which the quartz halogen flood is attached to, into the receptical box. Structurally, those two screws are the weak point, and my concern is that the increased weight of the metal halide fixture, and the longer moment (the flood will extend further from the siding on the house) require a more substantial mounting.

What are my options? One thought I have is to mount a larger conduit style box to the siding, and make that the structural mounting point.

To review, from inside to outside... there is the metal receptical box, horizontally mounted , and recessed back to the plywood behind the T&G siding. Then there is the thickness of the T&G siding, and then the three hole cover, which the existing (light weight) 500W flood light is mounted to. The (6-32?) screws which hold the three hole cover to the receptical box are about 1.3" long, to provide for the thickness of the T&G siding and the foam gasket on the three hole cover.

Well, this has been long, but perhaps you have an idea of what my situation is... any ideas?

If the metal box is rectangle, then it should only have two 6-32 screws, if the box is round or octagonal, then it has two 8-32 screws. The 6-32 screws are not strong enough and not allowed to hold a fixture, the 8-32 screws can hold up to 35 pounds.

The 70 watt MH flood will weigh more that the quartz, probably no more than 10 lbs. And it probably has the 1/2" NPT threaded fitting that threads into a weatherproof plate, that is held by the two screws into the box.

Since you say the box is mounted horizontally, my guess is you have a rectangle box, 2" x 3" wall case, with the two 6-32 screws. If so, and it certainly not practical to change the box, then you can tap the 6-32 screw holes out with an 8-32 tap. There is a tapping tool found in the electrcial tools section of home centers that has three tap threads ,6-32,8-32, and 10-32.

But need to be careful that the box has enough metal at the tab with the hole, so if you do tap it out larger there is enough metal left.

If the box is "gangable, then it is three separate plates of metal held togther with set screws, and these may not be strong enough to support the fixture, another reason that wall cases should not be used for heavy fixtures.

If you can thread the new fixture onto a round weather proof box, use a round cover that has the 1/2" NPT hub in the center,mount the box on the surface, and use conduit nipples to go back to a rectangle weatherproof extension ring, with a 1/2" hub on the side, to accept the conduit nipple from the fixture box, and blank cover on the ext ring after splicing the wiring.

The round box has mounting tabs on the outside of the box, long wood screws, or even toggle bolts , can be driven into the plywood.

I added some pictures to help you.

Thank you for your informative answer. I will look, but I believe that the existing box is a gangable box. I have 8-32 taps. The halogen fixtures are light enough so that I'm not real worried about the stress in the 6-32 screws; the metal halide is not only heavier, but has a longer moment arm, sticking further out.

I like your solution of surface mounting a round box... I'm a little reluctant to use toggle bolts, and if there was a way I could slip some T nuts behind the siding and plywood, I would. I may be able to fab two plates to slip in.

I take it that that I need to close the flush mount box with a plate, and run a short piece of romex to the new surface mount box. Is that correct? I assume that provides some isolation of siding (wood) from the wires for fire protection, etc. I would plan on simply mounting the surface mount box right over the existing opening.

Thanks.

Using the extension ring I provided can get you out of the flush box to a new surface box off to the side of the flush box.

Or you can cover the flush box with a flat blank plate with a 1/2" conduit hole, and a chase nipple directly into the round surface box mounted directly over the blank plate.

Why a problem with toggle bolts? Thou I don't think these are necessary, only need two #10 or #12 wood screws long enough into the plywood sheathing, or a even better a framing member such as a stud or any other 2x that is in the wall.

Well the good news is that when I mounted the box horizontally, I put a 2x4 crossmember between the framing studs. I can get at least two screws into that.

Thanks for your help, and enjoy your Turkey today.

I mounted the round box over the existing receptical box. I used #10 SS deck screws (squaredrive). The top one went into a 2x4 cross member for mounting the receptical, and the bottom one went into a 2x4 I forgot I put there... but shows in the photos from building the house.

Because of the lengths of the wires, both in the receptical and on the lamp, I ran the wires from the romex through the hole on the backside of the round box. I put a piece of wirewrap around the wires heading through the threaded hole, to protect the insulation from the threads. It turns out that the round cover plate is held in with 10-24 screws, and the short screws provided were a stretch to get more than a thread or two in, I found some 10-24 stainless screws, and cut them to about 5/8 of an inch, which allowed them almost bottom out, while compressing the gasket.

The new lamp clears the soffit by 2.5 inches, which is less than the quartz halogen, but then at 70W vs. 500W, it runs cooler.

I can envision a cage around the top of the fixture, to keep birds from nesting there. I've lost track of how many nests I've removed from the 500W fixtures.

Thanks for your help, you pointed me exactly in the right direction.

I just can't figure out why HD and Lowe's don't carry metal halide floods. They carry mercury, sodium and fluorescent flood fixtures.

Thanks again.