400 Amp Service - 200 Amp Breaker Panels (Check my work please)

Question

Upgrading main service to 400 amp service. I'm having to move my meter box to the side of my house (Outer wall of the Garage) due to some building logistics. Luckily I'm in the process of redoing the walls inside the house where the new meter-base is going. Duke Energy has approved the upgrade. I've pulled the permit with the county and we are at execution phase.

The run inside the home from meter base to the Breaker Panels is 42 total feet. This run will go up in the wall and come out in the attic space (unfinished) - Single Story Ranch style home. Plan is to go across/over the Rafters at an angle in conduit. The attic space is accessed with an access hole - no permeant ladders and the access hole is about 50 foot away (from my understanding of code, this matters).

What I need to know is the following: I have 250 mcm aluminum XHHW-2 wire. The plan is to pull 6 total strands through 3 inch conduit for the entire run. 4 runs will be for the hots. 2 runs will be for the neutrals. Two 200amp Breaker Panels at the end right next to each other. Will color code the wires with different color tapes to distinguish them from each other. The meter base has a dual lug kit for the house side for the wires.

Ground will be 3 awg Copper (unsheathed inside runs and outside runs). Questioning if I need 2 separate runs inside the conduit or if I can tie the two Breaker panels together and just use one wire.

Does this sound a'ok or am I missing something drastic here. Thanks for any information. Advice is also welcome on different approaches to any of this.

Answer 1

Unfused wires and disconnects

The service conductors from meter to first disconnect are totally unfused. If you stick a nail through them, you can get up to a 10,000 amp dead short - possibly 22,000 amps depending on power company rules. That's approximately the impact energy of the Vulcan cannon on the F-15.

Most of Duke Energy's territory is on NEC 2020. NEC 2020 requires outside disconnects that will shut off the power inside the building. See NEC 230.85. While this sounds bad, it may actually be your key to success, because it moots another rule:

230.70(A)(1) sharply limits the length of those totally unfused service wires to the least possible length, with an absolute maximum around 5 feet.

So, if you insist on NOT complying with NEC 2020, you will need to place your panels (together) on an outside wall, with the meter pan on the other side of that wall.

However, if you must or choose to comply with NEC 2020, yes - you have to have disconnects at the meter, but then, you can route the fused main feeders anywhere you please, to panels that may be anywhere you please including halfway across the house from each other. You can save a ton of Romex with strategic panel placement, and avoid having 50-100' runs of eight Romex right next to each other - 70' is enough to take voltage drop over 3%!

How is this typically done?

• It's possible to install a meter pan and 2 plain old 200A breaker disconnects.
• However, in anticipation of this need, some panel manufacturers make "Meter-mains" which include the meter and 2 sites for 200A breakers in one enclosure. Neater installation. Some of these are solar ready, meaning a 3rd breaker is present for solar (which removes limits to solar size).
• The deluxe option is what I call a "Ranch Panel", which has a meter pan + two 200A breakers supplying lugs, and also one of the breakers supplies an 8-space panel. This is handy when you'll want to run feeders to outbuildings.

More than 3 active conductors in 1 conduit

The plan is to pull 6 total strands through 3 inch conduit for the entire run

The problem with that is 310.15(B)(3)(a) which requires a derate for more than 3 conductors in the same conduit. It's a thermal issue: the conductors can only carry 80% of their thermal maximums. 250 kcmil is not enough (only 184A) so you'd need to step up to 300 kcmil if you want all in the same conduit. The usual solution to that is to run 2 conduits, however price it either way.

Note that if you take advantage of the NEC 2020 ability to put the panels anywhere, you may want them going different places.

Also, attics create a complication because they can get very hot. If the wires will be running above the insulation, talk to your AHJ but you may need another thermal derate - and this is starting to become a problem, because the lugs on meter-mains and panels do have size limits.