[RE-wrenches] Cable Sizing

[Bill Brooks]

Peter,

We cannot use load diversity to increase the number of conductors in a PV
conduit since there generally is little diversity among the conductors,
particularly on large arrays.

The more traditional conduit adjustment table to use is Table
310.15(B)(2)(a). The value from this table is multiplied by the temperature
adjustment factor in Table 310.16. The key is what to use as the ambient
temperature in Table 310.16. We also have the third adjustment of Table
310.15(B)(2)(c) in the 2008 NEC for conduit close to rooftops. Even if you
are excused from using the 2008 NEC by a jurisdiction, the 2005 NEC has
310.10 FPN2 that generally recommends a 17C adder on ambient temperature.

The NEC has not had any explanation as to what ambient temperature to use
until the 2008 NEC in the FPN to Table 310.15(B)(2)(c) when it referenced
ASHRAE data in an incorrect way. To be consistent with the Copper
Development Industry, we have put a proposal into the 2011 NEC to use the
ASHRAE 2% design temperatures. These values can be downloaded at
www.copper.org.

Summarizing in an example:

Assume that 8 current carrying conductors, with and Imax of 10 amps [as
defined by 690.8(a)], are in a conduit in direct sun 4" off the roof deck in
Palm Springs, California. What must be the 30C ampacity of the conductor to
meet the requirement?

Answer:

I(30C) = 10A/(conduit fill adjustment)/(Temp adjustment--direct sunlit
conduit)

Conduit fill adjustment factor = 0.7 (70%)

Direct sunlit conduit temperature = +17C above ambient
2% Design Temp for Palm Springs = 44.1C (ASHRAE 2005 Fundamentals)
Design temp = 44.1 + 17 = 61.1C --corresponds to a 0.58 factor for 90C
conductors

I(30C) = 10A/0.7/0.58 = 24.63 amps -- minimum conductor size is 14 AWG
(barely)

Most inspectors will quickly cite the fact that the ampacity cannot be
greater than the 75C column, so we check to make sure (nearly always is just
fine). The 75C column says that 14 AWG wire can handle 20 amps (Imax is
10amps) at 30C but the asterisk limits our overcurrent protection to 15 amps
(since the module has a 15 amp max fuse rating, we are already using the
required 15 amp device).

The upshot is that even a 14 AWG 90C conductor works in almost the hottest
climate in the U.S. as long as only 8 conductors or less in conduit, conduit
is at least 4" above roof, and no more than 10 amps flowing through it. Most
contractors will use 10AWG for small systems and occasionally 12AWG. 10AWG
makes it simple since it meets all wiring options in today's smaller
systems. 12AWG works in many cases, and, as our example shows, even 14 AWG
works in some circumstances (we're talking ampacity, not voltage
drop--that's a different issue). In large systems, generally we specify the
minimum wire since it adds up after a few miles of conductor.

Now wasn't that fun--I can't believe anyone could be put to sleep by that
(maybe want to commit suicide, but no sleeping here). The short answer is
that it is complicate and not well organized in the code because the
majority of wiring systems are indoor. PV and HVAC systems are the two most
common outdoor wiring systems requiring these calculations. Most electrical
engineers doing HVAC wiring are just now learning this stuff.

Most inspectors will quickly cite the fact that the ampacity cannot be
greater than the 75C column, so we check to make sure (nearly always is just
fine).


-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mark Frye
Sent: Thursday, January 08, 2009 11:08 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Cable Sizing

In any case,I believe "load diversity" refers to the duty cycle of the
various circuits with constant load levels, as opposed to the instant
current flowing in the conductors to any given load.

 
Mark Frye
Berkeley Solar Electric Systems
303 Redbud Way
Nevada City,  CA 95959
(530) 401-8024
www.berkeleysolar.com 

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Peter
Parrish
Sent: Thursday, January 08, 2009 11:01 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Cable Sizing

NEC 2005 Handbook, Annex B, page 1204, Table B.310.11

I see now that 100% diversity applies to 4-6 and 7-9 ccc's but 50% above
that. Sorry. 

- Peter

Peter T. Parrish, President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 CA Lic. 854779,
NABCEP Cert. 031806-26 peter.parrish at calsolareng.com 


-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Bill Brooks
Sent: Thursday, January 08, 2009 10:19 AM
To: 'RE-wrenches'; exeltech at yahoo.com
Cc: 'Will Logan'
Subject: Re: [RE-wrenches] Cable Sizing

Peter,

Where do you read the 50% diversity? My 2005 NEC handbook clearly says that
the raceway adjustment table is based on NO diversity--all conductors loaded
at maximum.

Bill.

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Peter
Parrish
Sent: Thursday, January 08, 2009 9:47 AM
To: exeltech at yahoo.com; 'RE-wrenches'
Cc: 'Will Logan'
Subject: [RE-wrenches] Cable Sizing

Here is an issue that has been bothering me for some time and I finally was
asked to explain it to a junior member of our Company and had to admit
ignorance...

I am referring to the issue of derating the ampacity of wire in a raceway or
conduit with other current carrying wires. I refer to Table B.310.11, Annex
B of the 2005 NEC Handbook, but the same information occurs elsewhere in the
HB as Table 310.15(B)(2)(a) and the CB.

Table B.310.11 requires a 20% derating for four to six current carrying
conductors. There is a note that this derating includes the effects of a
"load diversity" of 50%. Am I to assume that 50% load diversity means that
each current carrying wire is operating at 50% of its temperature-corrected
ampacity (or less)?

Does this table apply equally well to DC and AC circuits? What about
3-phase/3-conductor wiring?

In specific: if we have two DC PV circuits (4 wires: 2 positive, 2 negative)
in EMT and we have taken into account the "PV 125% circuit current
calculation", "cable sizing of 125% for proper operation of overcurrent
devices", and cable derating for temperature -- do we now apply the derating
from B.310.11 for 4-6 wires?

This is what we have been doing, and I'm looking for confirmation.

- Peter

Peter T. Parrish, President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 CA Lic. 854779,
NABCEP Cert. 031806-26 peter.parrish at calsolareng.com