[RE-wrenches] sub-panel supply-side tap

[Hans Frederickson]

Kirk,
You're basically doing a feeder tap, which you'll find the rules for in NEC
240.21(B). The difference from the intent of this code is that your tap  is
working in reverse. It will supply current to the feeder rather than receive
current from it. If your subpanel doesn't have a main breaker appropriately
sized to protect the feeder wiring, then technically your 80A from PV and
200A from the service could add to 280A and overload the feeder wiring
between the tap and the subpanel, as well as the subpanel bussing.  If I was
the AHJ, I would require a main breaker in the subpanel in addition to the
conditions specified in 240.21(B). This is definitely one to check on with
the AHJ ahead of time. 

Regards,
-Hans


From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kirk
Herander
Sent: Tuesday, June 05, 2012 3:44 PM
To: RE-wrenches
Subject: [RE-wrenches] sub-panel supply-side tap

Hello,

Ideally I'd like to make a supply-side tap on a feeder to a sub-panel. And I
mean tapping the feeder line-side of the sub-panels 200 amp breaker. This is
due to the limitations of the sub-panels 200 amp buss. My PV system max
output is 80 amps and therefore requires a 100 amp breaker. A no-go with the
200 amp buss. The 20% allowance won't work and I cannot downsize the 200 amp
breaker to make it work due to panel loads. I would have to upgrade the
panel to 400 amps. FYI this is 208/120 3 phase.

I have done supply-side connections before but only on a buildings main
service. Do all the same rules apply? What might be different I should take
into consideration? Thanks.

Kirk Herander
VT Solar, LLC
dba Vermont Solar Engineering
NABCEPTM Certified installer Charter Member
NYSERDA-eligible Installer
VT RE Incentive Program Partner
802.863.1202