Hi Mac and All,
If you are about to do a Supply-side tap I'd encourage you to look into
the many advantages of using a Feeder Tap instead - one of which is that
since it's not on the load side of the Main the equipment need not be
Service Rated. (not that you might not want it to be anyway.)
Be aware that Supply Side Taps make total sense when you are doing only
grid-tie as you are always backfeeding and so the current produced by
the solar is always cancelling the incoming utility current. However as
soon as you add battery charging to the equation from a large or
multiple parallel multi-modal inverter, then you have the very real
possibility of overloading the Service Entrance Conductors. Basically
if you have a 200A residential service whose Service Entrance Conductors
were based on calcs that had close to maxed out the 160 Amp continuous
capacity of a 200 A service, then adding 50 Amps of battery charging to
that can push if over the edge. Once you have bi-directional flow on
your supply side tap you are basically adding loads to a service that
may not be able to handle it in the case of large batteries with large
charging capability.
Feeder Taps on the other hand force you to do 2 things 1) they limit the
bi-directional current to your Main Breaker size (200 A) and 2) they
then re-protect the main load panel downstream of the tap with another
Feeder Tap Breaker (or larger conductor) according to NEC2020 705.12(B)
- protecting it from the sum of the 200 A potential from the utililty
combined with the sell-back current potential of the grid-tie inverters.
(diagram attached). With this configuration you can safely backfeed up
to 160 Amps of solar into a 200 Amp service while still have a large
battery bank on that system and not worry at all about overloading
during heavy battery charging.
For new construction or any large install where I need to reconfigure
the Service Entrance anyway we use Feeder Taps pretty much exclusively
now - for sure so for anything with batteries. It does require having a
stand-alone Main Breaker and to do away with meter-main combo panels.
But the safety and flexibility of what you now have to work with in
terms of AC or DC coupled inverters and battery backup - including whole
home backup are far superior in my book.
And for folks already having a Main Breaker that is tappable after the
Main but before any busses, the Feeder Tap approach allows you to not
require shut-down of the service during reconfiguration.
Hope that helps!
Best,
Jeff
Mac Lewis via RE-wrenches <mailto:[email protected]>
July 19, 2023 at 10:01 PM
Hello Wrenches,
Is there any tap devices (piercing or other) that are UL Listed for
tapping on the utility side of a main breaker for a solar
interconnection? Our AHJ won't allow anything that isn't service rated.
Thanks in advance!
--
Mac Lewis
*
"Yo solo sé que no sé nada." -Sócrates
*
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