Right side of a Manufactured House (MH): Outside Pedestal with a 200 amps disconnect and two 20 amps breakers for the septic pump and other. Inside left side has a 200 amps service panel. New large garage/storage built at the left side of the MH, wood frame but all metal siding (mobile home metal type). Garage/storage separated about 10 from the MH and NO metal connections between.
Plan: Install a 100 amps breaker in the inside 200 amps service (plenty of room) and run a feeder to this garage/storage to a 100 amps sub-panel with a 100 breaker disconnect, this sub-panel will supply lighting, general receptacles, welding machine receptacles and others.
Situation: Because I am donating all labor and materials I would like to use some material that I have. I have a spool of AWG THHN 4; the ambient temperature is under 86 F as it is in the country in WA area (raining a lot) and I will install it in PVC 80 conduit buried 18 the 10 plus between the buildings, and both risers of this PVC 80, to the service panel and to the sub-panel will be protected. I will run 4 conductors (marking one green and another white). The neutral and ground will be un-bonded (no tied) at the sub-panel and I will install at the ground connection a bare 4 AWG (I have a spool of) to 2 ground rods (have 1 in stock).
Questions: (I am not too familiar with manufactured and metal building attachment) I think I need to bond this metal siding in any way Will bonding the four walls at each corners be effective? Or installing Flexible Metal Conduit (FMC) for the fixtures/receptacles will be enough as I can install them on those siding? Can I use this AWG 4 for this 100 amps service? Should I need two rods? Is there a 100 amps 4 conductors underground cable assembly that I can use without conduit? Does the above plan make sense?
You do two rods so you do not need to prove you have <25 ohms with one. Basically a rod is cheaper than the test.
I would go ahead and bond the building but if it is all metal and screwed together, a single bonding point should be plenty and that could be right there at the panel. Screw your lug to the framing so you have some "meat" to get a good bite in. We usually want to see a nut and bolt, not a threaded connection and definitely not a sheet metal screw.
Florida added bonding metal studs to the code after an appliance installer got lit up and they say simply mounting a grounded metal box in each isolated section will fulfill the requirement. We tend to wink at the fact that these are using Tek screws.
The bad news is your 4ga is not enough for a 100a feeder. (310.16) The 310.15(B)(6) does not apply Just use a smaller breaker. (80 or maybe a 90 with the round up rule)
I'd simply de-rate the 100A 240VAC sub-panel in the garage down to 70 Amps -- using the #4 conductors -- they being a sunk expenditure.
I'd back-feed these (2) breakers if necessary... but considering your scheme, a MLO (main lugs only) sub-panel with the feeders protected at the 200A panel ought to be the lowest cost. Walking over to kill that switch is no biggie.
You're never going to truely tap out even 70Amps at 240VAC single phase in a garage setting -- even allowing for an arc welder. (It's an intermittent load, anyway.)
A #6 would be plenty as a bonding conductor (green) between the sub-panel and the 200A panel.
With plenty of #4 to hand, you've always got the option of installing a Ufer ground -- see the NEC Handbook for the style. This beats a set of ground rods and is mandatory out here in California. It's cheaper whenever driving ground rods is a nightmare. (Which, BTW, they are here: stoney ground that destroys excavator bucket teeth.)
Puddling over the #4 with concrete provides protection for the Ufer once buried.
Schedule 80 is never used out this way: schedule 40 is plenty tough enough.
Wrap your PVC joints with painters tape (blue or green types) just above the joints -- which when later removed leaves a beautiful, clean glue line for all exposed work. Skip this step for buried glue-ups.
Though (slightly) more costly, my style preference is to use chase nipples into F/A where the PVC enters the bottom of exposed panels. Otherwise figure to use a plastic bushing, locknut, and a T/A for this connection.
The cheesy concentric knock-outs punched into most low end panels are tough going... if you don't want to screw them up. Take your time nibbling them out, ring by ring.
Considering the usage, I'd seriously consider buying GFCI circuit breakers. You should have more than enough breaker spaces to stuff them inside.
Trying to find a popped GFCI receptacle -- down the road -- in a crowded garage -- can be quite an exercise.
Also figure on having a few exterior use GFCI receptacles consequent to this project.
I'd always be sure to install receptacles towards the front -- left and right -- as one will always want to run an extension cord out of the garage to a parked vehicle project/ hydraulic wand for pavement cleaning/ de-icing.