I'm going to build one of these: https://ibuildit.ca/projects/simple-loa ... ic-switch/
About 60% of the way down the page, after the picture of the 2-gang box with the #10 screws pointed up, he says: "...the box MUST be grounded for safe operation."
I don't understand why it is necessary to ground the box. Aside from his use of the box as a heat sink, I don't understand why the box has to be metal. If I bring in 3 pig tails, each with 3 wires, what is wrong with simply putting all the ground wires under one wire nut and calling it a day?
As I see it, this is just a fancy extension cord, with a + branch to the vac controlled by a relay. What am I missing here? I'd like to use a 3-gang plastic box and I'll use a heat sink on the relay or possibly a metal cover if the heat sink doesn't want to fit.
Of course, the schematic is deficient in not showing the mains ground wires. Also, there won't be any loose hanging female pigtails. I'll use a standard outlet mounted in the project box for the vac and the saw, breaking the parallel connection between the 2 receptacles.
Other reference materials:
Transformer https://catalog.triadmagnetics.com/Asset/CSE187L.pdf
Relay http://www.fotek.com.hk/solid/SSR-1.htm
I'd appreciate hearing your thoughts on the need for a metal box.
Load sensing switch
Moderators: pompeiisneaks, Colossal
Re: Load sensing switch
If you're using a metal box, and one of the mains wires comes loose and contacts the box ... and then you touch the ungrounded box ... you'll receive a shock. The ground is a safety measure in that case. Who knows if the box has enough thermal mass to account for the load to be dissipated. Sounds like he's done it before, but you'd want to know the recommended heat sinking configuration of the relay for the load you plan to use. Think of the heat of a 1500W hair dryer - much like a heat gun.
While not as elegant, you could wire an outlet box in line with the switch of the primary device and plug the secondary device into the outlet box. For something like dust collection or vacuum, leave the power swtch "On" for the secondary load and when you turn the primary on the secondary will also come on. That's the way my router table is set up. Challenge is having a large enough breaker to supply both loads. Something like a table saw can put you close to the capacity of a 15 Amp breaker, especially if you're loading the blade with thick cuts. Having a secondary load can trip the breaker in that case.
I've watched a few of this guy's woodworking videos.
While not as elegant, you could wire an outlet box in line with the switch of the primary device and plug the secondary device into the outlet box. For something like dust collection or vacuum, leave the power swtch "On" for the secondary load and when you turn the primary on the secondary will also come on. That's the way my router table is set up. Challenge is having a large enough breaker to supply both loads. Something like a table saw can put you close to the capacity of a 15 Amp breaker, especially if you're loading the blade with thick cuts. Having a secondary load can trip the breaker in that case.
I've watched a few of this guy's woodworking videos.
Re: Load sensing switch
A reply! Thank you.
Yes, the saw needs 15A and the shop vac is rated at 6A. I've got a 20A circuit. I'm thinking maybe I'll get by. The vac will start after the saw starts and the saw always starts without a load.
I agree about the need to ground a metal box. That's clear as day. What I don't understand is why I can't use a plastic box. If the hot wire gets loose inside the plastic box, it's still insulated. If it touches the neutral or ground, it trips the breaker. What is the problem? I can't see it.
I've got a heat sink. I don't think it will be an issue. The transformer will generate enough current to close the relay. I think it will be barely enough volts to close the relay switch. Just how hot can that get? I can attach it to a heat sink!
Maybe we have an electrician here?
Yes, the saw needs 15A and the shop vac is rated at 6A. I've got a 20A circuit. I'm thinking maybe I'll get by. The vac will start after the saw starts and the saw always starts without a load.
I agree about the need to ground a metal box. That's clear as day. What I don't understand is why I can't use a plastic box. If the hot wire gets loose inside the plastic box, it's still insulated. If it touches the neutral or ground, it trips the breaker. What is the problem? I can't see it.
I've got a heat sink. I don't think it will be an issue. The transformer will generate enough current to close the relay. I think it will be barely enough volts to close the relay switch. Just how hot can that get? I can attach it to a heat sink!
Maybe we have an electrician here?
Re: Load sensing switch
No one ever said you can't use a plastic box. But if you use a metal box, for safety sake, connect the green ground wire to it. If for no other reason, because NEC says so.
Re: Load sensing switch
You forced me to read it again, Steve. I did not read carefully. You are right. He says if you use a metal box it has to be grounded. No disagreement there on that point. I think I'm good to go on this project as soon as the relay arrives. The transformer arrived today. It's itty-bitty. maybe .75 sq. in. and quite literally 1 turn on the primary (exposed, I can see it) of really fat wire.
Re: Load sensing switch
Only thing I can think of is how hot does it get? Will it damage a plastic box? If you have proper heat sinking and air venting, maybe it's not an issues or maybe you can mount it such that it wouldn't be an issue. Otherwise, there are plenty of plastic outlet boxes used these days. It's typically how you add an additional outlet in a existing wall.