Bundling pv conductors

[iwire]

I'm questioning what constitutes bundling and not maintaining spacing.

Which is always the question about that section I posted.

I can tell you that the NEC considered MC cables run through a series of bridal rings to be bundled.

I can tell you inspectors in my area consider a group of NM cables run trough the same holes in a series of studs or joists to be bundled.

But due to the sizing of PV conductors I agree it will take a lot of conductors before derating would become an issue.

 

[pv_n00b]

Definition of "Free Air'

Definition of "Free Air'

I see a lot of PV designers using the free air tables to size conductors in PV arrays. The NEC does not explicitly define free air but if you search for the use of "free air" in the NEC you will see that it says, except for uncovered trays, that to be considered in free air a conductor can't be any closer to another conductor than 2.15 X the O.D. of the largest adjacent conductor. For uncovered trays it's 1 cable diameter. Since this is rarely the case for conductor routing in an array it's better not to use the free air table.

It is much less clear about conductors being attached to support structures but my recommendation would be if those support structures get hotter than the ambient air then you need to think about derating.

 

[electrofelon]

Until you've got 50 of them in a cable tray. If the Massachusetts Electric Code applies, it is isn't a problem. But if the NEC applies, that is 14A, which is lower than the fuse......

Which brings up an interesting point. Do we get to use the ISC of the module, or do we need to have an adjusted ampacity greater or equal than the string fuse?

 

[Carultch]

Which brings up an interesting point. Do we get to use the ISC of the module, or do we need to have an adjusted ampacity greater or equal than the string fuse?

In NEC2011, the article 690.8 had a three point checklist for conductor ampacity. In summary:
A: 1.56*Isc for terminal temperature ampacity, usually 75 C
B: 1.25*Isc/total derate for wire temperature ampacity, usually 90C
C: OCPD where required shall protect the wire as sized. Thus both the terminal ampacity and the derated conductor ampacity must at least be large enough to round up (240.4(B)) to the as-built OCPD rating, and not correspond in an exact match to the previous standard OCPD.

Part A and B still exist in 690.8 for 2014, and are the same calculation, just with different wording and arrangement. Part C is mysteriously absent. I'm not sure if this removal was intended to remove this calculation, or if it was removed because it is implied elsewhere in the NEC. More likely because it is implied in the general articles for overcurrent protection.

And there are numerous examples where part C governs. Because OCPD is sized by 1.56*Isc, and then rounded up to a size that is available. Adjusting it for derate factors, it is is significantly larger than 1.25*Isc/total derate, even when you can take credit for the non-intuitive 240.4(B).

With string fuses, every integer ampere below 15A could be a standard OCPD rating for 240.4(B). So using a 15A fuse, that means you need at least 14.000000001 Amps worth of wire ampacity at conditions of use.

 

[jaggedben]

I see a lot of PV designers using the free air tables to size conductors in PV arrays. The NEC does not explicitly define free air but if you search for the use of "free air" in the NEC you will see that it says, except for uncovered trays, that to be considered in free air a conductor can't be any closer to another conductor than 2.15 X the O.D. of the largest adjacent conductor. For uncovered trays it's 1 cable diameter. Since this is rarely the case for conductor routing in an array it's better not to use the free air table.

What section is that?

 

[ggunn]

In NEC2011, the article 690.8 had a three point checklist for conductor ampacity. In summary:
A: 1.56*Isc for terminal temperature ampacity, usually 75 C
B: 1.25*Isc/total derate for wire temperature ampacity, usually 90C
C: OCPD where required shall protect the wire as sized. Thus both the terminal ampacity and the derated conductor ampacity must at least be large enough to round up (240.4(B)) to the as-built OCPD rating, and not correspond in an exact match to the previous standard OCPD.

Part A and B still exist in 690.8 for 2014, and are the same calculation, just with different wording and arrangement. Part C is mysteriously absent. I'm not sure if this removal was intended to remove this calculation, or if it was removed because it is implied elsewhere in the NEC. More likely because it is implied in the general articles for overcurrent protection.

The NEC notwithstanding, I always make sure that my THWN-2 is protected by the OCPD, i.e., that the 90 degree ampacity derated for conditions of use is at least the rating of the next OCPD down from the one used. I do not use the 75 degree ampacity derated for continuous use because that derating is for protection of the terminals, not the insulation of the wire, and that protection is already set in place by my choice of the size of the wire.

That said, I have not yet ever had to upsize the wire because of this calculation; perhaps it was dropped because it is unnecessary.

 

[pv_n00b]

What section is that?

Searching for "free air" turns up several, 330.80, 332.80 are two. If I search for the name of the free air table, 310.15(B)(17), it's mostly referenced in the cable tray section 392. Basically the NEC is not really clear when to use it. Many people seem to be applying it anytime the conductor is outside of conduit in a PV array but I have not found anything in the NEC that would justify that. I'd be interested in seeing it if it is there.

 

[pv_n00b]

The NEC notwithstanding, I always make sure that my THWN-2 is protected by the OCPD, i.e., that the 90 degree ampacity derated for conditions of use is at least the rating of the next OCPD down from the one used. I do not use the 75 degree ampacity derated for continuous use because that derating is for protection of the terminals, not the insulation of the wire, and that protection is already set in place by my choice of the size of the wire.

That said, I have not yet ever had to upsize the wire because of this calculation; perhaps it was dropped because it is unnecessary.

The only time I have had the terminal temperature drive the conductor size calculation is when conditions of use adjustments are 1 or higher, so ambient temp 30°C or less and 3 or fewer conductors in conduit. Even then it's not that often, using 90°C conductor and 65°C terminals makes it more likely. It should still be checked every time but I can see why people who don't do it don't often run into problems. Since the most likely problem will be intermittent tripping of a CB it's a pain to troubleshoot if it is a problem.

 

[jaggedben]

Searching for "free air" turns up several, 330.80, 332.80 are two. If I search for the name of the free air table, 310.15(B)(17), it's mostly referenced in the cable tray section 392. Basically the NEC is not really clear when to use it. Many people seem to be applying it anytime the conductor is outside of conduit in a PV array but I have not found anything in the NEC that would justify that. I'd be interested in seeing it if it is there.

Okay... I don't see how any of that defines free in air or applies to PV circuits on arrays. The first two sections you mention both refer to conductors on a messenger as well as exposed with the spacing you mentioned. 330.80 refers to a different table. Cable tray rules are all over the place with respect to this discussion but none of them get at defining 'free in air'.

 

[iwire]

In my opinion any conductor run bundled with / along with other conductors or too a structure is not 'single free air conductor'.

Lets take a look at the names of the tables

Table 310.20 Ampacities of Not More Than Three Single Insulated Conductors, Rated 0 Through 2000 Volts, Supported on a
Messenger, Based on Ambient Air Temperature of 40°C (104°F)

Table 310.17 Allowable Ampacities of Single-Insulated Conductors Rated 0 Through 2000 Volts in Free Air, Based on Ambient
Air Temperature of 30°C (86°F)

Notice they treat conductors bundled on a messenger differently than they treat single free air conductors.

Here is an example of single free air conductors

 

[Carultch]

That said, I have not yet ever had to upsize the wire because of this calculation; perhaps it was dropped because it is unnecessary.

You've probably never encountered a situation where you have a "runt of the litter" combiner. Consider a group of 8 combiners, 7 of which have 24 source circuits. But the 8th one only has 8 source circuits.

You have to use the 8 breaker configuration subcombiner, with breakers in the "bin" from 225A to 400A. So even though the "runt of the litter" feeder doesn't need the full 225A, and probably could do with closer to 150A, it has the 225A breaker due to the equipment available.

 

[ggunn]

You've probably never encountered a situation where you have a "runt of the litter" combiner. Consider a group of 8 combiners, 7 of which have 24 source circuits. But the 8th one only has 8 source circuits.

You have to use the 8 breaker configuration subcombiner, with breakers in the "bin" from 225A to 400A. So even though the "runt of the litter" feeder doesn't need the full 225A, and probably could do with closer to 150A, it has the 225A breaker due to the equipment available.

Oh, right; thanks for the reminder. Actually, I have indeed run into that situation; it's been a few years and I had forgotten, and yes, I had to use the smallest breaker in the bin and upsize the conductors so that they were protected by the breaker.

 

[Andrew445]

Good discussion. My 2 cents -

DC conductors open/bundled within the array should be sized per table 310.15 (Underground), and should include the raceway fill derate as well as whatever applicable temp. derate you are comfortable with. If you actually run into a problem with this, then split up the string bundles or something. The code is not explicit in this matter, but luckily the conservative take usually pans out.

Many PV sites I have designed do indeed use a messenger wire with cables bundled and supported in hangers. For this application, use table 310.20 (messenger wire), and also derate for raceway fill/temp.

Indeed, you need to ensure the derated ampacity is always protected. No clue why this was removed in NEC 2014.