[RE-wrenches] conductors and the 120% rule

Tue Mar 4 13:31:23 PST 2014

The other thing I could do is downsize the main to 190 amps or less. 190 +
60(pv) = 250 / 1.2 = 208 amps, exactly the 4/0 derate amperage.

Unfortunately I can't get away with downsizing the 60 amp pv breaker, unless
I could find a breaker rated to operate continuously at greater than 80% of
its trip rating. I guess they do exist. Then you don't have to apply the
1.25 multiplier when sizing the breaker. This is a SQD industrial panel the
inverter is feeding into. Any SQD gurus out there who know of these types of
breakers? 

Kirk Herander

VT Solar, LLC

dba Vermont Solar Engineering

NABCEPTM Certified Inaugural Certificant

NYSERDA-eligible Installer

VT RE Incentive Program Partner

802.863.1202

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jason
Szumlanski
Sent: Tuesday, March 04, 2014 4:06 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] conductors and the 120% rule

Yeah, I assumed you meant the four conductors. The exception would solve
your problem if it's physically possible.

How long of a run are we talking about? The cost difference shouldn't be bad
for a short distance.

Jason Szumlansk

i

Fafco Solar

Description: Image removed by sender.

On Tue, Mar 4, 2014 at 3:34 PM, Kirk Herander <kirk at vtsolar.com> wrote:

Whether or not a further derate has to be applied is the killer here, as I
am working with existing panels and conductors. In an old Code Corner(HP140)
J. Wiles goes through a similar scenario and calls out the allowable current
rating and conductor in 310.15, but makes no mention of applying additional
derate factors. The .8 derate for 4-6 conductors(l1,l2,l3, & n) will put the
existing 4/0 cable between feed-in and main panel at 208 amps, less than the
allowable 217. I'd hate to need to upsize the wire to 250 mcm.

Kirk Herander

VT Solar, LLC

dba Vermont Solar Engineering

NABCEPTM Certified Inaugural Certificant

NYSERDA-eligible Installer

VT RE Incentive Program Partner

802.863.1202

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jason
Szumlanski
Sent: Tuesday, March 04, 2014 2:57 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] conductors and the 120% rule

Both the bus and conductors need to be rated for 217 amps minimum. As you
mentioned, the bus is not a problem. The way I interpret it, the conductor
size required would be after derate factors are applied. The rating of the
conductor is ultimately dependent on the derate factors.

If you can locate your subpanel adjacent to the main distribution panel, you
may be able to use Exception #3 to 310.15(B)(2) by connecting the panels
with a short nipple. I assume you are just looking at a number of conductor
derate and not an ambient temperature derate.

Jason Szumlanski

Fafco Solar

Description: Description: Image removed by sender.

On Tue, Mar 4, 2014 at 12:05 PM, Kirk Herander <kirk at vtsolar.com> wrote:

Hello,

I have a 225 amp 3-phase main lug sub-panel protected by a 200 amp breaker.
My inverter breaker feeding the sub panel is 60 amps. So 225 a bus x 1.2 =
270 amps. That's less than the sum of the two breakers of 260 amps, so no
issue there. The conductors between sub and main panel have to be rated for
at least 260/1.2 = 217 amps, correct? Is this 217 amps before or after
derating the conductor?

Kirk Herander

VT Solar, LLC

dba Vermont Solar Engineering

NABCEPTM Certified Inaugural Certificant

NYSERDA-eligible Installer

VT RE Incentive Program Partner

802.863.1202

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