Steel Studs and N-G Bond

[EnderWiggin]

Hello all, our villa in the caribbean is going up and I am planning a three-phase 208/120Y install for our place. Either Schneider XW Pro x3 or Victron Quattro 10k x3, with 4xEG4 PowerPro for battery and a ~13kW solar array. I plan on putting in 3/4" plywood with 1/4" hardibacker over it before mounting the inverters.

There will be a critical loads panel for all inverter-powered circuits, and have a manual transfer switch to allow the critical loads panel to be powered by grid if I need to completely disable the inverters (this is a four-wire MTS that also switches neutral). All equipment will be installed in a dedicated basement (not under the main structure) concrete room with steel stud framing (wood is avoided wherever possible due to termites).

Two big questions:
1) With the steel stud-based walls, I am balking at the load that may be on them, since LF inverters are mega heavy, and there will be three of them. I plan on installing the 4xPowerPro on the floor and supplementally anchoring them to the wall with their brackets, so maybe that will ensure "bottom" stability. Any recommendations, other than 1/4" toggle bolt anchors put in/through the steel studs? Should I be additionally wood framing with 2x4's, or using long 5-6" Tapcons to additionally anchor the ply/hardi to the concrete, bypassing or through the steel studs? Or, worst case, ask the builder to remove the steel studs and then mount the ply/hardi to the concrete with Tapcons, directly? I hate this last option because it removes insulation potential and the option to thread cables behind if I want, but I don't want to hear a massive BANG in the middle of the night and see all my inverters and batteries laying on the concrete floor, face-down.
2) Since the grid does not allow sell-back, I will be operating in purely off-grid mode. If I need grid as a backup, I plan on using chargeverter(s) when SoC gets under 20%, so there will never be a tie between inverter and grid (at least directly, over AC). However, in this configuration, any upstream N-G bond in the main panel won't be "passed on" to the inverters, so I have to create a N-G bond in the inverters. I cannot create a N-G bond in the critical loads panel, because if I switch using the manual transfer back to grid to do maintenance or something, then I would have two N-G bonds. I am assuming, then, to create a manual N-G bond inside the Schneider PDP or use the Victron N-G relay in the master inverter, only. Is that correct?

Photo of the future electrical room as it stands today attached. Any input welcomed.
Ender

 

[sollap]

I would put some wood 2x4's alongside the metal studs where you plan to mount the plywood, being sure to attach the plywood to the wood studs. The studs can be attached to the wall with tapcons, L-brackets, etc. Most of the strength concern is vertical and the 2x4's will handle that. The mount to the concrete wall is just so they stay vertical. The mounts are not holding up much weight. If you want to avoid the wood, you can double up the metal studs in the area. Mount them back to back and make sure the studs are screwed to each other well. When you mount the plywood be sure to connect to both studs in each pair.

You are correct about the N-G bond in the inverter. Since it's isolated from the grid/house by the transfer switch, it doesn't conflict with anything while the inverters are not in use. When they are in use, that bond you create will provide the one needed while you are disconnected from the grid.

 

[EnderWiggin]

Greatly appreciate the feedback. This isn't a conventional install and while I know wood installation, steel stud installation is a new one for me so I didn't want the wall to buckle. I suppose going to wood is the correct approach, I'll have to look into how to termite-proof it as good as I can... I don't think I can get the builder to double the metal studs up, and the screws they used to align the vertical to horizontal members were comically small so I don't think I can trust them to keep the studs perfectly vertical.

Given your feedback, I'll look into using some 2x4 with brackets and tapcons, and maybe additionally put some toggles into the steel studs as well just for kicks. If I use tapcons, with a 6" thick reinforced concrete wall, what depth should I go? 2" or 3"? Like you said, it's not much pull weight, it's vertical pull which will be supported by the 2x4 vertically, just wondering.

Thanks also for confirming the N/G question. Originally, I'd thought to wire the inverters to the grid "in case" in the future sell back was an option, and also to allow for grid charging during prolonged solar outages, which would have provided a bond throughout, but when it was "cut" by my design in the middle, I thought I'd have to make my own bond. Sad, really, that the island frowns on sell-back, it could power the island easily with panels if they got the beaurocracy out of the way...

 

[sollap]

Pressure treated wood should help keep the termites away. 1.75-3" would be fine for the Tapcons. You said it's a reinforced wall, do you know what depth the rebar sits at? Might make sense to lean towards the shorter Tapcons to avoid hitting it.

 

[EnderWiggin]

Not sure about the rebar depth, the overall wall thickness is six inches… so probably a two or so inch Tapcon should be fine, then.

I’ll see what I can source in the island for pressure treated lumber, I would imagine it is available but honestly you never know with island supplies.

Thanks again for the input… we’re still a few months away from ordering things yet but I’m excited.

 

[Norwasian]

Not sure about the rebar depth, the overall wall thickness is six inches… so probably a two or so inch Tapcon should be fine, then.

I’ll see what I can source in the island for pressure treated lumber, I would imagine it is available but honestly you never know with island supplies.

Thanks again for the input… we’re still a few months away from ordering things yet but I’m excited.

We have trouble with termites here, too. The best solution in Thailand is to use teak wood (the best) or one of two or three other hardwoods. Termites don't like gnawing on teak so much, and it tends to last a very long time. But it is expensive, and increasingly rare. The harder the wood, the less vulnerable to termites, so you may find something harder where you are (these woods are hard enough that one can barely drive a nail into them without predrilling).

But have you researched boric acid? I've theorized (have yet to put it into practice, so I cannot tell you yet if it would work) that soaking a wooden plank in boric-acid laden water for awhile would help to prevent its consumption by termites. Another possibility would be to find some penetrative and/or toxic paint, varnish, or stain and use that. I think railroad ties are pressure treated with creosote--but you probably don't have that locally available.

Just some ideas. If it were me, I'd probably weld a small frame of galvanized square tubing to put inside the wall as a backing. Termites will not eat steel, and once unnoticed inside the wall, there's little one can do about them. Persistent they are, too.

 

[EnderWiggin]

Great ideas, to be sure! Thanks!

I asked our builder about why he's using standard 2x8 framing members for our back deck, in light of termites, and he noted that he slathers all the timber with a thick black coating - didn't tell me what - and that such a coating is usually enough to keep them away for 5-10 years... and we'll be treating the bonudaries of the property also. I was considering doing that coating as well for the inverter wall.

I don't think I could bring myself to use a large sheet of teak or similar wood to mount the inverters; that wood is so beautiful I think I would cry. I think I'd also cry at the cost - teak boards near where I live back in the US are around $70 for a 1" thick by 6" wide, 8 foot board. YIKES. But I love the idea; possibly even combining some other super-hard wood with a barrier of some kind, or some other kind of material.

I'm now (I think) going to make a grid of 1 5/8" galvanized unistrut - many vertical members, each Tapcon'd into the floor. Then Tapcon some supports from behind and on the left and right (can't from the top in my setup). Finally, overlay horizontal unistrut at the correct height to mount the inverters, and run some wireway along it as well as some metallic "pegboard" stuff - basically, what some shops use for tool hanging, except in my case I can use it to mount stuff like the Lynx Distributor and T-class fusing, use zip ties for cable, mount solar disconnect boxes, etc. - that will keep all the support structure 100% wood free and immensely strong, especially since (once mounted) nearly all the inverter weight will be straight down.

Oh and if I wanted to go wild and crazy, the metallic pegboard comes in Victron Blue. But that might be a bit too much blue! SunshineEggo might do that, the smurf fanboy is strong in that one...

 

[DougfromdaUP]

Steel studs are tremendously strong as long as one face is prevented from twisting. Use thicker drywall if you’re worried. Make sure you have screws that get good bearing on the studs, or snap toggles.

 

[Sennen]

PT Wood is also insect repellent. They also make a blue coating to further help. If you go with PT, make sure it's fully dry before cutting and assembly.

 

[Bkord]

Mount unistrut from floor to ceiling. It could be inside the studs or outside the wall.

 

[EnderWiggin]

Steel studs are tremendously strong as long as one face is prevented from twisting. Use thicker drywall if you’re worried. Make sure you have screws that get good bearing on the studs, or snap toggles.

I've looked into using toggles and how well steel studs hold weight, but I'm not convinced the studs being used here are of sufficient gauge steel and/or are anchored/secured properly to their bases enough by the builder to securely hold the weight of three inverters. They're only secured with small metal screws, one each stud, in the bottom and ceiling channels. I can easily bow/flex the stud with my hand. I have a feeling that the builder counts on sheet rock adding an additional structural component to it.

Whenever the builder mounts something heavy to the wall (cabinets, etc), they always put in 2x4 and/or other materials to anchor to first... there's just too many unknowns for me to trust it and I'm no steel stud expert to do the work myself.

Inside the villa, when I have to hang something heavy it's always with snap toggles (when I can't visualize/locate/use a metal stud) or regular Togglers (if I can get the hole perfect to go through the stud), for sure...

 

[Norwasian]

I've looked into using toggles and how well steel studs hold weight, but I'm not convinced the studs being used here are of sufficient gauge steel and/or are anchored/secured properly to their bases enough by the builder to securely hold the weight of three inverters. They're only secured with small metal screws, one each stud, in the bottom and ceiling channels. I can easily bow/flex the stud with my hand. I have a feeling that the builder counts on sheet rock adding an additional structural component to it.

Whenever the builder mounts something heavy to the wall (cabinets, etc), they always put in 2x4 and/or other materials to anchor to first... there's just too many unknowns for me to trust it and I'm no steel stud expert to do the work myself.

Inside the villa, when I have to hang something heavy it's always with snap toggles (when I can't visualize/locate/use a metal stud) or regular Togglers (if I can get the hole perfect to go through the stud), for sure...

The best shape in metal to resist twisting is round, i.e. round pipe. It's amazing how much it resists twist. But round pipe would not work well in a square wall, which is why, if it were my project, I would use galvanized square tubing. Square tubing resists twist quite well, just not as well as round--but it would be plenty adequate, especially if sheet rock is added to help prevent lateral movement.

Any kind of C-channel, angle iron, etc. that has an open face/side is more vulnerable to twisting, and twisting can be a point of failure. This is why the sheet rock is structurally important with those steel studs, as it helps to prevent any movement along that side.