Wire voltage rating and Voc [RE-wrenches]

[Bill Brooks]

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David,

Is the system already installed? If so, the best method is to test the Voc
of the array and show how it compares with the manufacturers ratings at STC,
taking the appropriate derate for temperature (means measuring the module
temperature). You will often be 1-3% below manufacturers ratings on voltage,
enough to get you the 2Volts lower for the 600V limit. The inspector needs
to use Table 690.7, but they may have an old version of the code
(1996--that's sooo old) where 1.25 x Voc was the law (it changed in
1999--and everybody still whines about it--it could be a lot worse).

Otherwise, to use Table 690.7 straight up will require only 11 in series.
You can't comply with better wire because the modules and inverter are rated
at 600Vdc.

Bill.

-----Original Message-----
From: David Henry [mailto:david at harmonyfarm.com] 
Sent: Thursday, May 12, 2005 5:14 PM
To: RE-wrenches at topica.com
Subject: Wire voltage rating and Voc [RE-wrenches]


Wrenches,

We had an inspector on a large ground array tell us that our wire 
insulation had to be rated 25% over open circuit voltage. We are using 
THHN which is rated for 600V. We are using strings of 12 Sharp 175's 
with a Voc of 44.4 Vdc so we have 532.8 Vdc open circuit. During the 
inspection it started pouring rain and everybody had to run for cover 
and the inspector did not press the issue.

According to NEC 2002 Article 690.7 Maximum Voltage: "the maximum 
photovoltaic system voltage for that circuit shall be computed as the 
sum of the rated open-circuit voltage of the series-connected PV 
modules corrected for the lowest ambient temperature.' "the rated open 
circuit voltage shall be multiplied by the correction factor provided 
in Table 690.7". The factor in question for our climate would be 1.13 
(31 to 14 deg. F). 532.8 Vdc x 1.13 = 602 Vdc.

Is 600 v rated 90 deg THHN acceptable? If not, what do we use?

  We have more installations involving the same string sizing, (12 
modules per string), and probably the same inspector.....

Feedback is gratefully accepted

Sunny REgards,
David Henry
Harmony Farm Supply
(707) 823-9125 x14
(707) 823-1734 fax
david at harmonyfarm.com
www.harmonyfarm.com

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