Basic high voltage switching/planning queries

[fsck]

I'm an EE helping a homeowner planning a new residence in CA. The property will be fed with 4160v/20A, and we'll have a grid-tie solar array, backup inverters & generator. The house will be fed with 208/Wye.

I'm trying to SWAG ideas & prices before he goes to the local PE for a legal design. The trouble is, while in decades past I've installed and repaired HV starters, witnessed HiPot tests, etc; I've never had to spec/cost switchgear/cable. So this is a new adventure to me; and I have some really likely elementary [to most of you] queries to help me get started.

a) We'll have a straight 1000' buried 4160v cable from the PGE meter/solar to the house. Is such usually put in Sch 40 pipe, direct buried in the trench or? What's code/practice on ditch sharing? Utilities I've observed seem to put telco/cable atop it with a foot or so of dirt standoff.

b) We want provision for a later 2nd cable; in case he builds a 2nd house. Where can I go to learn about pull lengths for such HV cable in conduit?

c) The utility will demand disconnects. What's the price differential for 4160 gear in interior vs outside cabinets? [They seem almost willing to have them within the solar building iffen they have their lock on the door...]

d) We'll need to not just disconnect from the utility but maybe also transfer to the generator. [I'm looking at several approaches.] Are double-throw center-off switches available at 4160/3ph, or will vendors think I'm crazy for even asking? (If not, I assume we can get mechanically-interlocked disconnects so only one can be closed at a time.) Suppliers to consult? How can such be run remotely: motor actuator, air cylinder....?

e) What else should I worry about...?

 

[alehman]

a. Depends on the utility. Direct burial is common.
b. Save money and put both houses' transformers on the same primary cable.
c. Not much compared to what's in the enclosures. Outdoor enclosed gear is very common.
d. Yes.
e. It seems like a bad idea for unqualified homeowner to deal with 4160 primary. Can't the utility give you secondary service? Or is the primary in so you can net meter the solar with the house? You need to hire an engineer.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[fsck]

a) That's what I thought, too.

b) PGE sorta insists; if he ever sells the 2nd house, it gets very messy. Plus, I like the idea of a Plan B if the primary cable fails.

c) Interesting; I had no clue that was true.

d) Suggestions where to start looking?

e) The 20KW of solar PV ties in at the meter. If we have 1000 ft of cable for 208/3ph 200A, with low losses, we'd need a bigger ditch....with both houses; that's 400A.

I agree with your dot.sig, and that's exactly my goal, define the problems.

I'm not worried at all about this homeowner; I've known him for years [I worked on the last house he built...], he's not an EE but has a technical degree and is NOT stupid. I used to work with operators with little education and less branes, and they routinely operated 4160 disconnects.

If we design it correctly, and it passes PGE & county inspections; we'll all have done our jobs well.

 

[racobb]

I differ.....please do not put the homeowner in the medium voltage business. Aside from being dangerous, it will cost him a mint to maintain it.

Alan

 

[fsck]

Well, I don't put him that spot; the big picture did. It was there before he called me in.

I'll sleep well if the end result is safe, dependable, and accomplishes what he wants.

He can easily afford to pay for an annual inspection by someone equipped and capable to work at that level. Since I worked for several years at a place where the MV gear was at best powered down, and dust blown out every 4-5 years; I'm not sure what beyond that you suggest is required. If PGE owned the disconnect and buried cable; what would they be doing, and how often?