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#96407 - 11/23/05 10:45 PM 320 amp service
watersparkfalls Offline
Registered: 03/08/05
Posts: 212
Loc: Washington...Not DC
i am designing a 320 amp service for an existing shop and future house(under construction now).
the shop is currently fed with a 200 amp meter at the begining of a drive way about 200 feet away underground(meter on 4X4 pole and no disco between it and shop panel)
the power company will change the x-former to a 320 for new house.
here is my plan i will build a stand(using maraine plywood and two 4X4's)to mount a 320 amp meter can from which i will feed two 200 amp 3R disco's one for new house other for existing shop. i will drive two ground rods here for my new service split bolt off grounding electrode for 4th wire to feed new house 200 amp sub-panel and drive two ground rods there for seperate building and all neccessary bonding(h20,gas,ect...)for new sub- panel.
I will seperate grounds and neuts from existing shop panel since this will be a sub-panel(shouldn't it already be since its 200 feet from meter?)here is where it gets tricky there isnt a 4th wire back to the meter since the previous electrician treated this as a main panel and now it will be a sub panel and require the 4th wire(ground) can i split bolt the 4th wire at the house and trench over to the shop(about 75 feet instead of the 200 back to the new 320 service)to meet the 4th wire requirement?

any thoughts will be much appreciated,

thanks h20
p.s. is a 320 considerd a parallel service?
there are two sets of line conductors single phase(which seems self explanitory) but i am somewhat confused if each disco fed from load side of meter can should follow 310.4 rules.
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#96408 - 11/24/05 06:37 AM Re: 320 amp service
Tom Offline
Registered: 01/01/01
Posts: 1044
Loc: Shinnston, WV USA
Provided that there is no other conductive path between the house & the shop that would require bonding (thereby creating a parallel path), then you do not need a 4 wire feed to the shop & you do not need to separate the neutrals & equipment grounds. Take a look at 250.32(B)(2) and see if you think this might apply to this installation.

Your thought of running an equipment ground over from the house to the shop might be OK if you were dealing with DC circuits, but it would be useless for AC and a violation of 300.3(B) Soares Book on Grounding covers this subject really well & is worth the $45 or $50 investment.

Parallel conductors would have both ends joined together, since you're running to two separate structures, then 310.4 would not apply to the feeders unless you are actually running parallel conductors to each building.

Few things are harder to put up with than the annoyance of a good example.
#96409 - 11/24/05 06:44 AM Re: 320 amp service
earlydean Offline
Registered: 12/22/03
Posts: 751
Loc: Griswold, CT, USA
You could also wire 2 services, one to each building. The metering location could remain, as the location of the metering is not the determination for where the service ends. It ends in the enclosure(s) where the main disconnect and OC protection is located. You could even leave the 2-200 amp CBs at the meter (just don't call them the main panel, call them building isolation switches). You have to install main disconnects and OC devices at each building anyway.
This way, each building needs only hots and neutral, no ground. Each building's main panel will be service rated (required anyway). Bond ground rods and metal water pipe (if present) to the GEC and neutral at each building and at the metering structure as always for a service.
#96410 - 11/24/05 06:34 PM Re: 320 amp service
tdhorne Offline
Registered: 03/22/01
Posts: 345
Loc: Maryland, USA
Take advantage of exception number three to 230.40. It allows you to run two sets of service entrance conductors supplied from one service drop or lateral to supply a single family home and one detached structure.

230.40 Number of Service-Entrance Conductor Sets.
Each service drop or lateral shall supply only one set of service-entrance conductors.
Exception No. 3: A single-family dwelling unit and a separate structure shall be permitted to have one set of service-entrance conductors run to each from a single service drop or lateral.
You would have the new 320 amp service installed to a meter can that has provision for two sets of conductors on the load side. You then run two separate underground service entries with one serving each building. Since the service disconnecting means is located at each building you need make no change to the shop service equipment. You just run the new service entry to the house and wire it as a separate service.

A second approach would be to replace the service conductors to the shop or to parallel them with identical conductors of the same length. If those conductors were properly sized when installed then a second set of identical conductors run in parallel will raise the ampacity of the service entry conductors to 400 amps. You would then be able to mount a second 200 ampere service disconnecting means beside your shop panel to supply the feeder from there to the house. The feeder to the house would be four wire and the building disconnecting means and lighting and appliance panel boards at the house would be wired as feeder supplied panels that many in the craft call sub panels. Since the feeder to the house would be the "main feeder to a dwelling unit" it could be sized in accordance with

310.15 Ampacities for Conductors Rated 0–2000 Volts.
(B) Tables. Ampacities for conductors rated 0 to 2000 volts shall be as specified in the Allowable Ampacity Table 310.16 through Table 310.19 and Ampacity Table 310.20 through 310.23 as modified by (1) through (6).
(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders.

That would allow you to run 2/0 copper or 4/0 aluminum as the feeder from the shop to the house.

If the supply from the POCO to the meter is overhead you may want to install a grounding electrode system at the meter and connect the grounding electrode conductor to the neutral at the service head. This will tend to shunt lightning to earth rather than burning up your underground service entry conductors.
Tom Horne

[This message has been edited by tdhorne (edited 11-24-2005).]
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison

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