upgrading residential service

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difowler1

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Installing a new 320amp residential service on a detached building. The power from one of the 320amp service outdoor 200amp disconnects will be ran underground from the detached building to the customer's house panel box. If there is a ground rod at the detached building service and at the house service, does a ground wire need to be ran with the two hots and a neutral from the detached building 320amp service to the house 200 amp panel box?
 
electrofelon said:
Yeah you need an EGC run with the feeder. FYI a better way to do this is use 230.40 exception 3 and run service conductors to the house. No 4th wire and no disconnect needed
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So how would this work? Are you proposing he come off the load side of his meter with an additional set of service conductors and land them on the line side of a service entrance rated MCB or Disconnect nearest the point of entry at the outbuilding?
 
I asked this question because I am afraid I have been over-designing these in the past for folks wanting to build a detached garage or shop. If they want to just come off the load side of the meter, drop down to below grade out to their new garage and hit their new garage panel with service conductors then they can save a lot of money over installing an extra breaker, etc. Seems most folks prefer having a main breaker in their shop panel anyway. I just got schooled on the 240.21(B)(5) which allows that long tap run to be installed U/G with the OCPD located at or near the point where the feeder surfaces. That, combined with what I think I'm reading in 230.40 leads me to believe that what you are proposing is, indeed, correct and I have been overdesigning these in the past. Please correct me if I'm wrong. Also, am I correct in my understanding that this is not technically a "tap" due to being service conductors?
 
Yes he is proposing to come of the load side of the meter and install the disconnect at the house. Exception 3 allows this as stated above
 
Dennis Alwon said:
Yes he is proposing to come of the load side of the meter and install the disconnect at the house. Exception 3 allows this as stated above
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Interesting. I never knew about this exception. However, I'm a little surprised it's permitted with the parallel path issue. Seems fairly likely a metallic path could be installed between the house and barn and that path could have current on it. I guess it's no different than other installs like in a strip mall. I wonder why the code doesn't just permit a 3 wire to an out building directly from the main panel?
 
jes25 said:
Interesting. I never knew about this exception. However, I'm a little surprised it's permitted with the parallel path issue. Seems fairly likely a metallic path could be installed between the house and barn and that path could have current on it. I guess it's no different than other installs like in a strip mall. I wonder why the code doesn't just permit a 3 wire to an out building directly from the main panel?
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Well one thing to consider is the parallel path issue would not be any different if each building had its own service*. This is what we get with our (mostly) MGN distribution system.

* Unless its not a MGN distribution system and served from separate transformers.
 
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