Wire size and fusing for 3P array

[zanydroid]

Pretty sure it is more than 3 (not including ground), not 2 (I’m using two 3-conductor + ground metal clad conduit for my 3 6 AWG home runs for precisely that reason).

It is 3. But 3 very rarely happens in split-phase residential, it only occurs in 3-phase systems. L1/L2/N does not count as 3 CCCs, it counts as 2 b/c I_L1+I_N = I_L2 is always true.

3 may even be impossible in code compliant split phase systems since all conductors from a circuit need to be in the same conduit / cable, which forces a multiple of 2.

(Terminal temperature) This is the area I’m still confused about.

You start derating from the max temperature of the wire type as allowed by code (sometimes it's lower than the certification on the jacket, sometimes it's derated when wet. I haven't designed with PV wire so I don't know the ins/outs of that wire type. 690.31 is where the details of PV wiring method are in NEC). And then the resulting ampacity must be lower than the ampacity allowed for that AWG / metal type / terminal temperature combination, factoring in any special case restrictions in code for the wire type.

The conduit fill derating is certainly irrelevant by the time the terminals are involved, since the terminals are not sitting in the conduit.

I believe this is to give sufficient margin for the device to operate correctly. I'm not sure how the device termination temperature need to be derated for ambient temperature though.

 

[FilterGuy]

I suppose the argument is that the wire heating up to it’s maximum temperature can cause the fuse to exceed it’s maximum rated temperature.

That is exactly the reason.

 

[fafrd]

That is exactly the reason.

Most MC4 fuses are rated to 194F / 90C: https://www.bougerv.com/products/5-...MInJz9_-qS_AIVFQKtBh3wPQTHEAQYBCABEgJzVvD_BwE

So I must have been confused when I thought I saw a reference to 75C somewhere.

But I’ll be sure to make sure my MC4 fuses are rated for at least the temperature of my lowest-temperature elements within the circuit.

 

[fafrd]

It is 3. But 3 very rarely happens in split-phase residential, it only occurs in 3-phase systems. L1/L2/N does not count as 3 CCCs, it counts as 2 b/c I_L1+I_N = I_L2 is always true.

3 may even be impossible in code compliant split phase systems since all conductors from a circuit need to be in the same conduit / cable, which forces a multiple of 2.

I’m using 3 conductors per conduit for my DC-coupled home runs. 3 + conductors in one conduit and 3 -conductors in the other.

You start derating from the max temperature of the wire type as allowed by code (sometimes it's lower than the certification on the jacket, sometimes it's derated when wet.

Are you saying that when running conductors rated for 90C / 194F through an attic that may reach 140F, I don’t need to plan for any derating at all?

I haven't designed with PV wire so I don't know the ins/outs of that wire type. 690.31 is where the details of PV wiring method are in NEC). And then the resulting ampacity must be lower than the ampacity allowed for that AWG / metal type / terminal temperature combination, factoring in any special case restrictions in code for the wire type.

FilterGuy and I went over the appropriate limits for PV wire in air early in this thread. in case you skipped that part.

I’m sure the limits are very different when running PV wire in conduit, but my plan is to run individual conductors in air at top-of-rail height above roof surface…

The conduit fill derating is certainly irrelevant by the time the terminals are involved, since the terminals are not sitting in the conduit.

Again, I’m not using conduit on the roof.

It’s the 10AWG MC4 connectors that limit my current rating on the roof (50A @ 85C).

I need to stick to 10AWG to get into my roof-mounted junction box (Soladeck).

Within the Soladeck, I’m using a Dinkle Terminal Block rated for 60A to 120C to combine 10AWG strings to 6AWG home run wires running from roof to basement (through attic) in conduit (3 conductors per conduit).

Those 6AWG conductors terminate into another Dinkle terminal block in a junction box in the basement.

I believe this is to give sufficient margin for the device to operate correctly. I'm not sure how the device termination temperature need to be derated for ambient temperature though.

On the roof, I’ll never reach temps of even 110F / 43C.

Through the attic, I should plan for worst-case temps of 140F / 60C.

The basement never gets over 75F / 24C.

 

[zanydroid]

I’m using 3 conductors per conduit for my DC-coupled home runs. 3 + conductors in one conduit and 3 -conductors in the other.

Unless there is a DC or PV exemption, 300.3 says all conductors of the same circuit needs to be in the same conduit, so DC current carrying conductors also have to be a multiple of 2.

Going from 3-4 will mostly affect temperature derating margin since #10 copper cannot anyway be used for more than 30A because of small conductor rule 240.4(D)

Ah never mind your attic runs will be #6. In which case going from 3-4 will probably constrain you bc you likely want to hit 75C ampacity.

Are you saying that when running conductors rated for 90C / 194F through an attic that may reach 140F, I don’t need to plan for any derating at all?

No, there is a derating table you use. It takes as input the conductor temp and ambient temp and outputs the correction factor. So for 90C wire (labeled AND allowed by NEC chapter for that wire type) you can use the 90C column.

Table 310.16

https://media.distributordatasolutions.com/ThomasAndBetts/v2/part2/files/File_7437_emAlbumalbumsOcal20(USA)oc_1_g_nec31016pdfClickHerea.pdf

 

[zanydroid]

Instead of conduit, have you considered using 3x 6/2 MC and physically protecting it as appropriate? If derating becomes a problem. MC is allowed for PV DC circuits if physically protected. There is a prescriptive thing, EG for certain AWG it needs guard strips that are taller than the cable.

 

[fafrd]

Instead of conduit, have you considered using 3x 6/2 MC and physically protecting it as appropriate? If derating becomes a problem. MC is allowed for PV DC circuits if physically protected. There is a prescriptive thing, EG for certain AWG it needs guard strips that are taller than the cable.

The only thing I’ve commuted that is my two home conduits runs of 6AWG wires (3 conductors each).

It was almost impossible to get those run and I never want to have go up there again.

I read the reasoning for requiring both conductors to run in the same city suit to get comfortable with parallel conduit runs carrying all 3 + conductors in 1 and all 3 - conductors in the other.

More than 2 runs of conduit wound not have been an option (had to perform unnatural acts to get even 2 run..,).

At this point, those two MC conduits of 6AWG are the primary fixed point of my DC-coupled array (racking system and Soladeck being the other).