Question on derating conductor ampacity when conductors are in an enclosure

[Anode]

I have two separate conduits with three current carrying conductors planned to come back to a wireway which is 8x8x24, both conduits would enter the wireway through the top about 6" apart.

My question is do I have to apply the .8 derate to the conductors since there are now 6 current carrying conductors in that box?

The three conductors from one conduit would be all negative conductors from three series source circuit strings (half strings essentially), and would mate with the three positive conductors from conduit #2 (the other half of the strings) forming three series source circuits. The wireway is simply a means to do that in a protected space. I won't go into why I have to do it this way, I just do.

My question is, do I have to apply a derate? If I do, I would end up having to use #8 AWG, which I am told is a problem for PV4 connectors (separate issue I have to clarify on my own).

Thanks,

 

[ggunn]

I have two separate conduits with three current carrying conductors planned to come back to a wireway which is 8x8x24, both conduits would enter the wireway through the top about 6" apart.

My question is do I have to apply the .8 derate to the conductors since there are now 6 current carrying conductors in that box?

The three conductors from one conduit would be all negative conductors from three series source circuit strings (half strings essentially), and would mate with the three positive conductors from conduit #2 (the other half of the strings) forming three series source circuits. The wireway is simply a means to do that in a protected space. I won't go into why I have to do it this way, I just do.

My question is, do I have to apply a derate? If I do, I would end up having to use #8 AWG, which I am told is a problem for PV4 connectors (separate issue I have to clarify on my own).

Thanks,

The rules for gutters are different from those for conduits. See 376.22.

 

[Anode]

The rules for gutters are different from those for conduits. See 376.22.

Second paragraph reads, " The adjustment factors specified in 310.15(B)(3)(a) shall be applicable to the current-carrying conductors up to and including the 20 percent fill specified above."

The way I read that tells me the rules do not change the way I derate conductors for ampacity. No?



In other words, hypothetically speaking, I could have a 100' long wire way, and enter three negative conductors on the end of the box, connect to three positive conductors in the center of the box, and send those three positive conductors out the other end of the 100' long box, and even though there are three circuits running straight through the whole box, I would apply a derate as if there were 6 conductors because three enter at one point and three enter at another? Which would be a .8 derate. :?

 

[Carultch]

Second paragraph reads, " The adjustment factors specified in 310.15(B)(3)(a) shall be applicable to the current-carrying conductors up to and including the 20 percent fill specified above."

The way I read that tells me the rules do not change the way I derate conductors for ampacity. No?



In other words, hypothetically speaking, I could have a 100' long wire way, and enter three negative conductors on the end of the box, connect to three positive conductors in the center of the box, and send those three positive conductors out the other end of the 100' long box, and even though there are three circuits running straight through the whole box, I would apply a derate as if there were 6 conductors because three enter at one point and three enter at another? Which would be a .8 derate. :?

30 CCC's in any cross section is the magic number. 30 CCC's and fewer means you can forget about bundling derate as long as you fill geometrically to no more than 20%.


The 31st CCC initiates the derate associated with 31 wires.

 

[xformer]

I have two separate conduits with three current carrying conductors planned to come back to a wireway which is 8x8x24, both conduits would enter the wireway through the top about 6" apart.

My question is do I have to apply the .8 derate to the conductors since there are now 6 current carrying conductors in that box?

The three conductors from one conduit would be all negative conductors from three series source circuit strings (half strings essentially), and would mate with the three positive conductors from conduit #2 (the other half of the strings) forming three series source circuits. The wireway is simply a means to do that in a protected space. I won't go into why I have to do it this way, I just do.

My question is, do I have to apply a derate? If I do, I would end up having to use #8 AWG, which I am told is a problem for PV4 connectors (separate issue I have to clarify on my own).

Thanks,

Is this raceway nonmetallic? If not, shouldn't the Negative conductors be run with the positive conductors?

 

[Anode]

30 CCC's in any cross section is the magic number. 30 CCC's and fewer means you can forget about bundling derate as long as you fill geometrically to no more than 20%.


The 31st CCC initiates the derate associated with 31 wires.

Is that an interpretation of the above code the way it reads as "up to and including the 20 percent fill specified above", I interpretted that as even if I had 1% box fill with more than 4 CCC's, I am applying 310.15(B)(3)(a). Or is there some code elsewhere stating this magic number. I am probably missing a lot.

 

[Anode]

Is this raceway nonmetallic? If not, shouldn't the Negative conductors be run with the positive conductors?

The raceway is metallic, the wireway is metallic. And due to the design of the pv system, the negative conductor home runs will be on one side, and the positives will be on another. There is also this weird circumstance in the design where we have to series 20 pv modules to another 20 pv modules, making three series source circuit strings of 40, and this question is looking at the two conduits and conductors meeting in a wireway, so they are essentially jumpers in this situation.

The actual home runs both also carry 3 pos in one conduit and 3 neg in another conduit, back to a rapid shutdown box. The conduits are metallic, and I believe the rapid shutdown box is as well.

 

[Carultch]

Is that an interpretation of the above code the way it reads as "up to and including the 20 percent fill specified above", I interpretted that as even if I had 1% box fill with more than 4 CCC's, I am applying 310.15(B)(3)(a). Or is there some code elsewhere stating this magic number. I am probably missing a lot.

376.22(B) states that 310.15(B)(3)(a) adjustment factors only need to apply in a wireway where the CCC count exceeds 30.

 

[Anode]

376.22(B) states that 310.15(B)(3)(a) adjustment factors only need to apply in a wireway where the CCC count exceeds 30.

Thank you, that was my mistake, looking at 378.22

Perfectly clear now, thank you.

 

[Smart $]

The raceway is metallic, the wireway is metallic. And due to the design of the pv system, the negative conductor home runs will be on one side, and the positives will be on another. There is also this weird circumstance in the design where we have to series 20 pv modules to another 20 pv modules, making three series source circuit strings of 40, and this question is looking at the two conduits and conductors meeting in a wireway, so they are essentially jumpers in this situation.

The actual home runs both also carry 3 pos in one conduit and 3 neg in another conduit, back to a rapid shutdown box. The conduits are metallic, and I believe the rapid shutdown box is as well.

See 300.3(B) "Conductors of the Same Circuit."

 

[Carultch]

See 300.3(B) "Conductors of the Same Circuit."

Obviously you have to separate the polarities at some point, to reach both ends of the source circuit. And you can do this within the footprint of the array. Good wire layout will group the polarities as soon as possible, especially for the wires that leave the array.

What is important is that the net current adds up to zero for conductors in a raceway. So given circuit 1A and circuit 1B, you can run a conduit between these portions, provided that both 1B+ and 1B- are in it. Note that 1B+ connects to 1A-, and we often call this a jumper.

 

[wwhitney]

What is important is that the net current adds up to zero for conductors in a raceway.

Physics-wise, why does that matter for DC?

Thanks,
Wayne

 

[GoldDigger]

Some of the rules for AC circuits in Chapter 3 are not applicable to DC circuits which are PV module outputs or combiner outputs. In particular positive and negative conductors are, I believe, allowed to run in separate raceways.

mobile

 

[Carultch]

Physics-wise, why does that matter for DC?

Thanks,
Wayne

Well unbalanced current still will magnetize a raceway in DC, but unlike its AC counterpart, the magnetization from DC is static. So the heating effects are negligible. And even with a hypothetical neodymium "superferrous" raceway, it still wouldn't be enough to be a serious problem during normal operation. Maybe for a 200A combiner worth of strings, but not just one or two strings of isolated polarity.

One particular issue is absorbing transient voltages from nearby lightning. You want to keep the area enclosed by the circuit conductors as small as practical.
http://www.tinyoffgridliving.com/pv-panels-and-lightning/

 

[Carultch]

Some of the rules for AC circuits in Chapter 3 are not applicable to DC circuits which are PV module outputs or combiner outputs. In particular positive and negative conductors are, I believe, allowed to run in separate raceways.

mobile

I wondered about that one. The SolarEdge manual specifies that you need 1000V wire for the 3-phase 480V inverters which operate at 850V, when you route both polarities in the same raceway. As I read that, I thought #1, that's what you have to do in all installations exceeding 600Vdc anyway, and #2, when would you be able to isolate the polarities and meet the NEC?

I'm not aware of 690 having any rule that overrides the general rule from Chapter 3, even though the physical issue is significantly less of a concern in DC circuits.

 

[Smart $]

Some of the rules for AC circuits in Chapter 3 are not applicable to DC circuits which are PV module outputs or combiner outputs. In particular positive and negative conductors are, I believe, allowed to run in separate raceways.

...I'm not aware of 690 having any rule that overrides the general rule from Chapter 3, even though the physical issue is significantly less of a concern in DC circuits.

That's just it. There's nothing in 690 that says you don't have to run circuit conductors together where using a ferrous wiring method, a 300.3 requirment.