Fw: Fw: Interpretation of NEC code / can of worms [RE-wrenches]

Allan Sindelar allan at positiveenergysolar.com
Fri Sep 5 14:35:31 PDT 2003


Re: Fw: Interpretation of NEC code / can of worms [RE-Wrenches,
I forwarded Vince's post to John Wiles. His response follows. I have copied the attachment to which he refers at the bottom of this post.
Allan at Positive E

----- Original Message ----- 
From: John Wiles 
To: Allan Sindelar 
Sent: Friday, September 05, 2003 12:25 PM
Subject: Re: Fw: Interpretation of NEC code / can of worms [RE-wrenches]


Allan:
We may not have the entire story.  Here is some general information:


1.  THHN is a 90°C conductor rated for dry and damp conditions at this temperature.  All exterior conduits are considered wet locations.  If it were dual marked THHN/THWN-2, it would have a wet 90°C rating and be suitable for use in outdoor conduits at elevated temperatures.  Probably THWN with a wet rating at 75°C would be suitable for the run into the building.
2.  USE-2 with no other markings, is not suitable for use in conduit inside buildings since it has no flame retardant properties.  Dual marked USE-2/RHW-2 is ok for both runs.
3.  Conductors may be spliced in conduit bodies only when the conduit body is marked with the volume in cubic inches and the the number of conductors in the body meet code limitations on conduit body fill.
4.  Supplementary grounding electrodes, in addition to the minimum code requirements, are useful in controlling lightning damage.  The faster and more directly lightning can get to earth, the better.  A supplementary grounding electrode near the PV array may reduce damage from nearby lightning strikes.


You may post the above if desired.


Allan:  See attached.  No need to apply both 80% conductor ampacity derate and "conditions of use" corrections (conduit fill and temperature) at the same time. Apply separately and then select largest size.  The first two steps of a somewhat complex procedure to do it right, but may be able to use a smaller conductor in PV circuits


  Care to comment?
  Allan

  ----- Original Message -----
  From: "Vince McClellan" mail at solarenergydesign.com
  >
  > I just had an inspection today by an inspector that has obviously been
  studying
  > NEC article 690,which was a refreshing difference from what I usually
  encounter.
  > However, he had  some very different interpretations of the code than any
  I've
  > heard so far, so I thought it would be a good thing to get some second
  opinions.
  > On this installation I have #10 USE-2 wire coming down off of a roof
  mounted
  > array from 18 Sharp modules, through a weather head, and then junctioning
  to #10
  > thhn in a conduit body, then to a Sunny Boy inverter. The inspector wants
  me to
  > replace the thhn with a 90 deg. rated conductor all the way to the
  inverter, even
  > though the conductors are not in a high temperature location. He says
  because it
  > is part of the original circuit it has to be the same insulation rating
  through
  > out the whole circuit. Anybody know where I can find THAT in the NEC code?
  > The other issue that I have is most of the inspectors I'm working with
  want me to
  > install an additional grounding electrode with a #6 grounding electrode
  conductor
  > coming straight down from the array. It was my understanding that there
  should be
  > only one grounding electrode in a system. It states in the notes in the
  NEC
  > handbook in article 690.47 that "The ac and dc grounding electrode
  conductors
  > where both are present should connect to a common grounding electrode or
  > grounding electrode system.
  > I remember back to some of the first Trace inverter manuals when the
  engineers
  > at Trace encouraged installers to ground to only one grounding electrode
  because
  > of the  of a potential voltage difference between two grounding
  > electrodes in the event that lightning strikes closer to one grounding
  electrode.
  > Their observation was that the voltage potential between two electrodes
  would of
  > course cause current flow through the system damaging sensitive
  electronics such
  > as inverters.
  > Anyone care to comment on any of this.
  >
  > Vince McClellan
  > Energy Design

Selecting Overcurrent Devices and Conductors in PV Systems
1. Define Continuous Currents

The unique nature of PV power generators dictate that all ac and dc calculated currents are continuous and are based on the worst-case conditions. There are no non-continuous currents.

A. DC currents in PV source and output circuits are 125% of the short-circuit current (Isc) (690.8(A)(1)).

B. AC inverter (stand-alone or utility interactive) output currents are at the rated output of the inverter (690.8(A)(3)).

C. DC Inverter input currents from batteries are at the rated output power of the inverter at the lowest battery voltage that can maintain that output (690-8(A)(4)).

2. Select Overcurrent Device

A. Rated at 125% of continuous current (690.8(B)(1)). 

a. If listed in enclosure for 100% duty, then use 100% continuous current (690.8(B)(1) EX)). 

b. May round up to next standard rating (<=800A)(240.4(B). PV circuits standard values are 1-15 amps in 1 amp increments (690.9(C)).

In PV source circuits, the value should be at less than or equal to the value of the maximum series protective fuse marked on the back of the module. If desired (for unknown reasons), this selected value could be increased to the size of the maximum protective fuse found on the back of the module.

B. If overcurrent device is exposed to temperatures (operating conditions) greater than 40°C and/or less than 25°C, must use temperature correction factors on the device rating (110.3(B)).

3. Select Conductor

A. Select conductor with 30°C ampacity >= 125% of continuous current (215.2(A)(1)).

B. Conductor selected must have 30°C ampacity after corrections for conditions of use (ambient temperature and conduit fill) >= continuous currents (no 125%).

(a) Apply at all points of different temperatures and or conduit fill.

(b) Use 10%/10-foot rule where appropriate (310.15(A)(2) EX)

C. Select largest conductor from 3.A. or 3.B (310.15(A)(2))

4. Evaluate conductor temperature at each termination

A. Use ampacities for conductor size selected in 3.C from 310-16 60°C or 75°C table depending on conductor temperature rating of device terminals (110.14(C)).

B. Ampacity must >= 125% of continuous current

C. Increase conductor size, if necessary, to meet 4.B. at all terminations.

5. Verify Overcurrent Device Protects Conductors

A. The rating (after any corrections for conditions of use-2.B.) of the overcurrent device selected in 2. <= ampacity of the conductor selected in 4.C. The ampacity used for the conductor is that found under the conditions of use (3). Rating round-up is allowed (240.4(B))

B. Select a larger conductor size if not protected by the overcurrent device.

Wiles 5/9/03 505-646-6105

[Non-text portions of this message have been removed]

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