Mike Dolt Enterprises
Mike Dolt Enterprises
User: Charlie Bob, Location: West Tennessee
Asked on: Friday, April 8, 2011 at 11:02 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:58 am EDT
Question: I've a lot of these services around here in the past, i've always used 350kcmil for the ungrounded service conductors nd 4/0 for the grounded one. The double meter base that the POCO give us here comes with lugs where the specified torque is shown as size wire is 350 kcmil nd less. It's been a while before my last one so i went nd discuss with inspector what size of wire he wanted nd where in the code book he looks it up ( just to make sure we are on the same page!). He thinks we oughtta use 400 kcmil for ungrounded conductor based on the table for residential services. And 350 kcmil for grounded conductor. I agree with all that as well.But... in this particular residential set up, having two separated 200 amp disconnects being fed from this double meter base, would you still consider this a 400 amp service?knowing that not both houses will be using everything at all times, don't you think 400 kcmil is kinda overkill? ( i had to change the lugs in it so i could fit the 400 kcmil wire.)What size of wire you all use in your neck of the woods?Thank you all.
NEC Considerations
The National Electrical Code (NEC) requires the service conductors to be sized based on the calculated load and the maximum overcurrent protection device (OCPD) at the service entrance. For a residential service, Table 310.15(B)(6) applies if the service is over 600 volts, which is not the case here. Therefore, Table 310.15(B)(16) should be used.
NEC Table 310.15(B)(16)
This table provides the minimum allowable ungrounded conductor size for residential services based on the maximum ampere rating of the OCPD. For a 400 amp service, the table requires a minimum ungrounded conductor size of 400 kcmil copper.
Inspector's Interpretation
The inspector is correct in determining that 400 kcmil is required for the ungrounded conductors of a 400 amp residential service. This is in accordance with the NEC.
Overkill Considerations
While it is true that both houses may not be using their maximum load at the same time, the service conductors must be sized to handle the potential maximum load. The NEC does not allow for downsizing of service conductors based on assumptions about usage patterns.
Common Practices
In many areas, it is common practice to use 400 kcmil for ungrounded conductors and 350 kcmil for grounded conductors for 400 amp residential services, regardless of the actual load. This ensures compliance with the NEC and provides a reasonable margin of safety.
Conclusion:
Based on the NEC and common practices, 400 kcmil is the appropriate size for the ungrounded conductors for a 400 amp residential service, even if there are two separated 200 amp disconnects. Flag for review