Help me wire a sub-panel

[gearchruncher]

I am preparing to put in a HWPC. I want to set it up for 100A for future use, possibly for a second HPWC. I also have an outlet in my garage I used for welding (NEMA 6-50). So a sub-panel makes sense. I'd like to run 100A from my main panel to the garage, then to the local outlets and HWPC. I can't imagine ever charging and welding at the same time so I don't think I need overhead on the 100A.

A few questions:

1) I'm going to run #3 copper THHN to the sub-panel. However, how do you hook this to the main panel in terms of the neutral? The bus bar in my Square D Homelink won't take anything near a #3 wire. Maybe a #4. So how do you hook up that conductor?

2) At the same time, do I need a #3 neutral if my only 120V loads on the panel would be behind a 20A breaker? The welder and HWPC don't even use a neutral.

3) If I put a 100A breaker in the main, do I need another 100A in the sub panel going to the HWPC? Seems weird to have two 100A breakers in series.

 

[nwdiver]

*Standard Disclaimer: I'm not an electrician but I have wired my own HPWC... so take that for what it's worth

1/2) As you mentioned... the neutral won't carry much if any current (though I think it is used for fault detection); I ran 2/2/2 triplex to my HWPC. My panel couldn't take #2 on the neutral either so I removed a few strands of wire until it would (probably not to code). You can get a lug that you can add directly to the top of the bus bar of some panels that will accept large gauge wire... usually ~00. The neutral and gnd should be terminated in the same panel the lines are fed from. The difference between your main and sub panels is that the ground should NOT be bonded to your neutral in the sub panel. ONLY your main panel. I don't understand this enough to explain it but in the research I've done on neutral vs ground that point kept coming up... neutral and ground should only be bonded at the main panel. Most sub panels have a pull out that allows you to unbond them.

3) The breaker is to protect the line. If you have a 100A main breaker then you shouldn't need a second 100A breaker since #3 wire is rated for 100A. I added a small 100A RV panel to my HPWC. It's fed from the 100A breaker that feeds my HPWC. I ran the wire through the panel.... no additional breakers for the HPWC.

 

[182RG]

I'd like to run 100A from my main panel to the garage

@nwdiver has good advice. I used 3/3/3/6 THHN in conduit. Pay attention to his advice on removing the bonding screw (or not installing - my sub panel had it in a bag in the box). In addition to buying a larger lug for the neutral bar, you may need to buy a separate ground bar for your sub panel depending on vendor. I used a box from Home Depot. The neutral bar is isolated from the chassis of the sub panel. The screw simply connects the bar to the chassis.

Here's why no bonding

Frequently Asked Question: Why do the grounds and neutrals need to be separated in a subpanel? What happens if they aren't?

Answer: Though the neutral doesn't have significant voltage, it does carry current. Remember, it's current that kills, not voltage. In a 2-wire circuit, the neutral carries just as much current as the hot conductor. If the neutral and ground are connected in a subpanel, that current will travel on other paths, such as bare ground wires, equipment enclosures, and metal piping systems, on its way back to the service panel. One problem created by this condition is possible shock hazards, the severity of which depends on the locations of the equipment and the person touching the enclosure or piping system. Another problem is magnetic fields that do not cancel themselves out. Since the return current has multiple paths, the current remaining in the neutral will not counterbalance the current in the hot wire. The resulting imbalance creates a magnetic field that can interfere with sensitive electronic equipment. In a metal conduit system, the imbalance will induce current into the conduit, which could cause the conduit to overheat.

 

[tga]

2) At the same time, do I need a #3 neutral if my only 120V loads on the panel would be behind a 20A breaker? The welder and HWPC don't even use a neutral.

Yes. Even if you don't plan to use more than 20A @ 120V, you have no idea what the next homeowner will do. They could load up the box with multiple 120V circuits, and (if unbalanced) that could result in larger currents on the neutral. An undersized neutral won't pass inspection.

In a 2-wire circuit, the neutral carries just as much current as the hot conductor.

Yep, that's the problem.

3) If I put a 100A breaker in the main, do I need another 100A in the sub panel going to the HWPC? Seems weird to have two 100A breakers in series.

Yes, if only because you need something with terminals to attach the HPWC feeder. You'd have a 100A in the main panel, run wire from that breaker's terminal to the main lugs of the sub panel, then another wire from the sub panel's 100A breaker's terminals to the HPWC terminals.

Personally, I'd run 1-1-1-3 aluminum SER between the main and sub (assuming the breaker and subpanel are rated/sized for it) and save the #3 Cu for the HPWC circuit. At Home Depot, 1-1-1-3 SER is $2.76/foot (or even 1/0-1/0-1/0-2; it's about the same price w/ less voltage drop). #3 Copper THHN is $1.39/ft * 3 conductors = $4.17/foot, and you still need a ground wire. For a long run, that gets pricey. Properly installed, aluminum is completely safe.

 

[Craig-Tx]

Speaking of Neutrals / Grounds:
Is your garage attached or detached? If detached, then *I believe* you need to drive a ground rod into the ground and bond it to the panel. I could be off on this as I haven't looked into running power to a secondary structure in a LONG time. But I brought it up so that you can look into it.

 

[182RG]

If detached, then *I believe* you need to drive a ground rod into the ground and bond it to the panel.

Correct. Actually it is 2 ground rods, 6' apart.