Should I isolate the ground and neutral in the 30A safety switch box or keep them bonded until the sub panel it is feeding (as service disconnect)?

Question

I am in Michigan if that makes a difference to you.

I am wiring a subpanel (QO 100-Amp Rainproof Main Lug Load Center) to my detached barn. I am feeding the subpanel from a 30A double breaker in my house's main panel. The feeder is 10/3 with ground, buried in conduit, 2 conductors, 1 neutral, 1 ground, supplying 240V at the subpanel.

In order to make working in the subpanel more safe, I have elected to install a 30A fused safety switch (GE TG3221) inline before the subpanel inside my barn, feeding the subpanel from the safety switch. All the wires will be the same 10/3 w ground.

Inside the safety switch box, there is a 3-lug isolated neutral bar, with an optional green screw to bond that neutral bar to the box. There is no equipment grounding bar in the safety switch box.

Now, I am well read on the fact that the neutral and ground in the subpanel MUST be ISOLATED. but in the safety switch box, I'm not sure. Since I'm feeding from my main panel (where the neutral and ground are bonded) I'm thinking that they should stay bonded inside the safety switch and only get isolated in the subpanel.

I have driven an 8' grounding rod, which I plan to connect (with #8 bare Cu) to the ground inside the subpanel, which will, once again, be isolated from neutral inside the subpanel.

Can anyone advise me on the question of whether or not the ground and neutral should stay bonded inside the safety switch? I am thinking that the options are:

1. Bond ground and neutral simply by using the neutral bar lugs to connect all the neutrals and grounds (not sure if using the bonding screw would be needed here)

2. Bond ground and neutral by connecting neutrals to neutral bar, installing bonding screw. Next installing an equipment grounding bar (to the box) and connecting the grounds to that (bonded) ground bar.

3. Isolate by connecting the neutrals to the neutral bar (NO BONDING SCREW). Installing an equipment grounding bar (to the box) and connecting the grounds to that bar (which will be isolated from the neutral bar w/o the bonding screw).

I've tried to include all the relevant details, but let me know if I missed anything.

Answer 1

My panel is full. How do I add circuits? is one of our most popular questions. The underlying reason is somebody tried to save a couple bucks when selecting a panel. In other words, what you're doing right now. As such, we Very Much Implore You to radically oversize the panel. It's cheap now.

Having a shutoff switch at the outbuilding is mandatory. Most people select a main-breaker panel and use the main breaker as the shutoff switch. Your way is fine too, and will save you a walk back to the house. I'm less thrilled with the equipment being outside though. I like equipment that's inside.

However, that switch is driving me crazy. From extensive Web search, the switch looks like a 3-pole switch, but the text all says it's a 2-pole switch. If it's got 3 poles, it's better to switch neutral. However neutral must always be switched with the hots, because if only neutral is severed, that creates even worse problems! That means a gang switch is fine, a 3-pole common-trip breaker is fine, but fuses are not. If using fuses, don't fuse neutral. Really there's no reason to fuse anything at this switch, the breaker in the house protects.

You do need a ground rod also. However you need two ground rods unless you do a really weird test to affirm one will suffice. Folk advice is just set another ground rod. Really, if lightning is around, you can't have enough ground rods.

Neutral is never ground

A lot of people struggle with the neutral-ground relationship and when/why they should be bonded. It's not random/whatever. It has very specific purposes and I'll hit two of them.

• To keep all conductors near earth potential, i.e. so your hots are 120V from earth instead of 2400V from earth. It would be fine to use a 2-volt 5KVA transformer as your neutral-ground bond -- at which neutral would be intentionally biased 2 volts AC from ground. You could use a car battery for the N-G bond -- in which case neutral would be biased 12 volts DC from ground. We only use a strip of copper for a 0.0 volt bias, because it's cheaper and lower maintenance. There's nothing magical about 0.0 volts, in fact, neutral will not be 0.0 volts from ground anywhere but at the N-G bond.

• To return ground fault current to source, which is the neutral line off the transformer. If something breaks and shorts 500A from hot to ground, that 500A won't flow (it'll just electrify things) unless we are able to carry it efficiently back to supply neutral -- in which case it will flow and will trip the breaker, as intended. Now imagine your main panel N-G bond fails, and you bonded neutral and ground at your outbuilding. The fault current will seek out your #10 ground wire to the barn, hop the illicit N-G bond, ride the #10 neutral back to source. That's a mighty ton of current going through those long wires, is either one breakered? No. The long wire run will impede current somewhat, which will make the supply breaker to the failing device slow to trip. It'll spin the electric meter really fast until something fries/breaks/burns.

So the answer is that anywhere neutral and ground arrive in separate wires, "whether to bond them" has only one answer: absolutely never. Except, of course, in the one official location where the equipotential bond must be located.

That shutoff switch

The shutoff switch you bought is probably intended for an air conditioner or motor (hence my suspicion it may have 3 terminals; for 3-phase, to satisfy a rule in factories that every hardwired machine must have a maintenance shutoff switch in direct line of sight to the machine.)

• If it has 3 poles, use one for neutral and use the bus for ground. Leave the chassis bonding screw in.
• If it only has 2 poles, then use them for the hots, use the bar for neutral (remove chassis bonding screw) and then either buy an accessory bar for ground (I see mounting holes) or just wirenut all the grounds together since all the wires are copper. To ground the switch chassis you'll need to find a hole tapped 10-32 and pull a pigtail off it.

Answer 2

Ground and neutral only meet at one place, and that place is the main panel

You must keep ground and neutral separate everywhere but the main panel -- this is the cardinal rule of how grounding works. As a result:

• Land the feeder grounds on the terminal block/bar in the safety switch, bonding it to the case with the green screw
• And wirenut the neutrals to each other, but do not connect them to anything else in the switch

That way, the safety switch case is grounded, and the grounds and neutrals run through to the subpanel, but ground and neutral do not meet here.

You don't really need the safety switch -- there are cheaper ways to make this as safe as you desire

While the safety switch does provide a disconnecting means for the structure, it may be less costly to put a QOM2100 in the subpanel (all but the smallest QOs are convertible from main lugs to main breaker or vice versa) and use that as a disconnect instead.

In order to keep some dunderhead from turning the subpanel back on while you're working on it, what you can do is get a lockout device for your main panel and then use a thin-shackled padlock to lock out the breaker for the subpanel while you're working on it, keeping the key in your pocket while the work is taking place. That way, any miscreant who wants to zap you will have to bust the lock first, which should be enough of a clue to someone who's not truly malign in intent that they shouldn't be doing that!