residential subdivision elec power design

[gravel]

We are involved as civil engineers on site development. Usually the local electrical company does the electric service design af final plan stage. For preparing preliminary cost estimates for various site layout options, we would like some guidelines as to several issue... like spacings/locations of transformer/pads, some unit costs or wire size one would use in a single family development of say 20 half acre lots. The elec is buried service. Any thoughts would be appreciated.. just looking for some rough design parameters that electrical guys see & use. This way we can factor in the electrical as a per lot cost for that particular layout scheme.

 

[davidbeach]

Probably the best thing to do would be to hire the services of a local consulting electrical engineer who does that type of work.

 

[itsmoked]

I stand with davidbeach's suggestion.

It is very hard for us to come up with those numbers for you. Let me explain.

Local cost of living is a big one. Where I live a 700sqft fixer-upper goes for $725K. The equipment/installation here will go for several times what it would cost elsewhere. The weather has a great deal to do with the price as air conditioning jacks up energy requirements greatly, changing transformer spacing/numbers. Also the local utility may desire a specific size transformer that changes the spacing. Local authorities may have requirements that change costs. Houses may each have a well which will change things. Electric heat? The list goes on and on.

If you go to a local EE that does this for a living, he knows all these details implicitly and could probably give you a very good number or at least a ball park number. A number that accounts for the local economy.

You will also gain a relationship with someone you will ultimately need to help you. And if he/she doesn't help you you know who to avoid when you need the actual work done.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Dumbo2929]

gravel, where are you located?

 

[jghrist]

You should try contacting the local utility. They may have some written guidelines for contractors. If not, a meeting with a local division engineer or technician that does the final layout may be arranged. They may give you some guidance. Large differences in cost can occur depending on whether or not the utility will use backlot or street side distribution, particularly in terms of street crossings. Some utilities will install all cable in conduit, others direct bury except under streets. These things will have a big cost impact.

If all else fails, figure on No. 2 aluminum AWG primary URD cable with full concentric neutral, jacketed. One padmount transformer for every four lots. You didn't ask about transformer size, so I assume that the utility is installing the transformers.

 

[apowerengr]

Gravel, given 20 half-acre lots your layout probably won't materially affect the electric installation costs significantly. If anything, you could figure $10/ft as an increase from the design with the shortest curb-line to your other options (costs increase if you design cul-de-sacs). As mentioned earlier, each and every utility has differing standards regarding acceptable transformer sizes and acceptable loading levels; some provide transformers whereas some do not; front vs. back-lot construction, cable in conduit vs. direct-buried, single vs. multi-phase requirements, stand-alone vs. joint-trench, developer pays for infrastructure or utility does...........you can see where this is going.

 

[EEJaime]

In addition to what the others have said, there is the additional possible costs for street lighting, underground transformers and enclusures, (these "Byrd" enclosures are required in many new housing developments which do not have alleys and which do not want the unsightlyness of pad mounted transformers in front of the properties). There is also the possible added costs of provisions for future utility infrastructure extensions, voice/cable/digital TV distribution, etc.... An endless list of "possibles". Best bet is as stated by the other posters, a local consultant or contacting the service planning division of the power, TV and telephone service providers.

Best of luck

 

[wareagle]

2 Issues that determine transformer locations is how wide
the lots are and how far back will the houses sit from the curb. If the lots are very wide you will have transformers on every other lot line. feed back some distances.

 

[gravel]

Ok... its more involved than I imagined or really needed at a preliminary stage... here in western PA, we can lay out a generic storm sewer system, like placing inlets at every 300' and assuming a 15" storm pipe, quantify those items & multiply by a installed LF unit cost, to arrive at some base starting cost for the storm sewer system. Ditto for sewer & water. Thought elec could be ballpark priced out without any real detailing with the same concept described as storm layout.... except I don't know the transformer spacings/size (or what drives the spacing or sizes) or anything about some typical buried service. Are mainlines run on both street sides? or just 1 side & feeder lines teed off to the across street lots? Would a transformer (what's a typ size guys? some start point is what I'm looking for... I know it aint "D" battery size!) placed at every 4 lot lines on one street side with feedlines to the houses set at 75' from street... what does one used for underground wiring??? anything close says alot to me... I'm just trying to cost something out to say it will be at least this much and goes up from there. Cause we don't even know if $10K or $100 is some realistic number.

 

[fredpar]

ok you can try this:
Transformers: One Padmount transformer - 50KVA every 4 lot lines feeding 8 houses. Single phase 7620/120-240Volts. For every transformer you need a pad, it could be concrete, fiberglass, etc.

Underground wiring: To feed transformers use 1/0 Aluminum Underground 15KV on one side of the street. [Use same lenght you got for sidewalk divided by two], plus 2" grey conduit schedule 40. For each transformer add one secondary box typle flushmount and secondary wire to cross the street, let's say 500MCM Al Triplex Underground, plus 4" grey conduit schedule 40 also. For this crossing use the lenght of the right of way.
Hope this help!
[There are many things we're missing here but transformers and primary wire are the most expensive]

 

[gravel]

Fredpar... this is the guidance I was looking for... something to get some kind of logical baseline power cost, and knowing it will go up from that number... thanks