400A Underground residential service

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dgrey

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Plymouth, MA
I am being asked to quote a 400 A/320 constant underground service that will be about 240' in length. My plan is to pipe to a 2 position meter main with two 200A breakers. Each breaker will serve a MLO panel, one in the main house and one in the in-law section.(This is a BIG place, kind of two houses attached together with electric heat to boot!!) My calculations lead me to 250MCM's AL in parallel in two separate conduits from the utility pole to the meter socket. Am i close here or am I going to run into voltage drop issues? Are 250's even the right call?

GEC- #2 copper Burndyweld to two 3/4" copper rods in trench
 
I have worked mostly in VA, NC and TN throughout the years. In those areas, the POCO would be responsible for sizing and installing the conductors. They just need a raceway. Is that not the case where you are located?
 
250.66(A) allows for #6 copper GEC to rod electrodes...#2 would not be in violation though.

Everything else looks good...but concern for voltage drop can only be calculated with the demand.

What is your load calculation?
 
I am in Southeastern Mass and here the power co only connects at the pole or handhole and install meter. Homeowner owns and is responsible for the service lateral on the underground. As far as load goes I did not calculate the existing house just replacing existing 200 A MLO. Owner want second meter because he doesnt want to pay for his mother in-laws new addition. 17282 KVA after all percentages taken. Need the 200 amp for the 42 circuits
 
dgrey said:
I am in Southeastern Mass and here the power co only connects at the pole or handhole and install meter. Homeowner owns and is responsible for the service lateral on the underground. As far as load goes I did not calculate the existing house just replacing existing 200 A MLO. Owner want second meter because he doesnt want to pay for his mother in-laws new addition. 17282 KVA after all percentages taken. Need the 200 amp for the 42 circuits
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That runs up against the restrictions on multiple services to one building, yes?
 
I actually come up with parallel 500 mcm (Al) if the system was loaded to 320 amps, but we know that would never happen. I do think that the 250 mcm parallel would be sufficient but I would like to see how you calculated the load and make a determination based on those values. I am also curious about your grounding, specifically why you aren't installing one to the water pipe where it enters the premises. If you are then you could reduce the size of your supplemental ground to #6 (Cu) as has been previously stated.

Bob
 
dgrey said:
I am being asked to quote a 400 A/320 constant underground service that will be about 240' in length. My plan is to pipe to a 2 position meter main with two 200A breakers. Each breaker will serve a MLO panel, one in the main house and one in the in-law section.(This is a BIG place, kind of two houses attached together with electric heat to boot!!) My calculations lead me to 250MCM's AL in parallel in two separate conduits from the utility pole to the meter socket. Am i close here or am I going to run into voltage drop issues? Are 250's even the right call?

GEC- #2 copper Burndyweld to two 3/4" copper rods in trench
Click to expand...
What is your calculated load and what is your acceptable voltage drop level? If you have more then one service disconnecting means like you do, you can have less conductor capacity then you have overcurent protection. What I mean here is if you only have 275 amps of calculated load you only need a minimum of 275 ampacity conductor even though you have 400 amps worth of overcurrent protection.

Beyond that voltage drop is a good reason to increase conductor size, but it would only need to be based off 275 amps (if that is the total load) of load not 320 or 400.

If in your situation I would likely have run parallel 4/0 alum, unless I knew I was loaded pretty heavy, then I would look harder at a realistic max load and voltage drop and adjust accordingly.
 
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Municipalities in this area use plastic water pipe. Existing metal water pipes cannot be trusted as the municipality may at any time replace it with plastic from the street shut off to the meter. Wells use plastic the entire length from well head to inside expansion tank thus the driven or "made" rods
 
dgrey said:
Municipalities in this area use plastic water pipe. Existing metal water pipes cannot be trusted as the municipality may at any time replace it with plastic from the street shut off to the meter. Wells use plastic the entire length from well head to inside expansion tank thus the driven or "made" rods
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In PG&E land here in Cali, the gas installation is moving from metallic to plastic feeders from the main to the meter too, FWIW.

When the metal gas pipe to my house was damaged (and that is another interesting story) the repair guys dug down the the 6" main and ran plastic pipe under the street inside the old metal pipe which they had cut off at ground level.
 
dgrey said:
Municipalities in this area use plastic water pipe. Existing metal water pipes cannot be trusted as the municipality may at any time replace it with plastic from the street shut off to the meter. Wells use plastic the entire length from well head to inside expansion tank thus the driven or "made" rods
Click to expand...
And is a reason NEC requires a water pipe electrode to be supplemented with at least one other type of elecrode. Does not change the fact that a qualifying water pipe is almost always a better electrode then any ground rod that may supplement it.
 
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