Code - 1.56 rule or conduit/temp rating or both?

[Mhoward84]

I am trying to understand the electrical code and see that wire ampacity can be derated by several factors, including number of wires in conduit, ambient temp, the 80% conductor rule and solar 125% Isc rule (1.56 rule)

When I add all these up the ampacity requirements become a bit ridiculous and I read that you need to choose the greater of either number in conduit with ambient rule or the standard solar 1.56 rule. The article was quite old and I’m not sure if I am on the right track or there is another way of calculating the ampacity requirements. Thank you for any information in advance!

 

[Mhoward84]

Ok so apparently the answer to this is in the... code! Who knew? Here is what I found in case someone in the future needs a reference.
TL;DR - It's 125% x 125% or 125% x calculated derating, whichever is greater.

In 2020 NEC it states 690.8 A 1a = take Isc (STC, not NMOT) and multiply it by 125%.
Then 690 8 B says your wire can't carry more current than either this 125% multiplied by the greater of either

1. 690.8 B 1 - Another flat 125% OR
2. 690.8 B 2 - The derated value as calculated in 310.15. This is calculated by Table 310.15 B 1 for 86 degree ambient rating (USE-2 solar wire is 90C, use 90C table) multiplied by number of conductors in conduit rating in 310.15 C 1.

So in my case, I will have 6 conductors (not on a roof, will add temp) in 41C max temp weather, so value 2 would be 1/0.87/0.8 = 1.436, which is greater than the 125% derating, so I must use 1.436.

My panels are 10.14A STC Isc x 2 (number of strings) x 1.25 (690.8 A 1a) x 1.436 (690.8 B 2), so my total conductor rating would have to be over 36.40A. Since 90C 10AWG is rated for 40 amps, I'm good with that.

Am I correct?

EDIT: I think I may have phrased wrong. If 40A is derated by 1.436, it would now be rated for 27.855A, and I would be pushing 10.14 x 2 x 1.25 = 25.35A worst case, so it should be good, 27.85 > 25.35.

 

[Mike 134]

No. You have to check the footnotes. per sec 240.4D of 2020 NEC, 10 ga is limited to 30 amps. However, you can use the 40A rating when calculating your derating for ambient temperature and wire fill. You'll need 8 ga wire.

 

[timselectric]

Where is the conduit being ran?
How many conductors?
Length of conduit?
How many amps?

 

[Mhoward84]

No. You have to check the footnotes. per sec 240.4D of 2020 NEC, 10 ga is limited to 30 amps. However, you can use the 40A rating when calculating your derating for ambient temperature and wire fill. You'll need 8 ga wire.

The total Isc is 10.14. Multiplied by 2 would be 20.28, with the 125% (if something increases irradiance over STC) increase up to 25.35A? Wouldn't that still be ok? The 36.30A calculation was me jumping to conclusions and maybe not saying it right?

With this calculation, the 40A 90C rating would be 40/1.436 = only rated for 27.85A, where I would be under that even after adjusting my 125% amp increase (25.35A)?

 

[timselectric]

Oops
And, what voltage?

 

[Mhoward84]

Where is the conduit being ran?
How many conductors?
Length of conduit?
How many amps?

EMT from solar carport to PVC ground conduit into brick solar shed.
Two separate conduits with six conductors each
Length will be about... 67 feet
10.14 Isc x 2 on each string, 5s2p.
Voltage pretty low, around 243 before any adjustments, way under rating. (but still only 1.64% voltage drop)

 

[timselectric]

#8's in 1" conduit is what I would do.
Just speaking from experience.

 

[timselectric]

1 1/4" conduit, if the pull will be rough.
(Weird angles to pull from, or hard to get to)

 

[Mhoward84]

Ok right on I appreciate your feedback. I usually like to overbuild myself, but on the Sol Ark manual it says 10 awg max for the PV input so I really want to make sure the 10 works.

 

[timselectric]

I'm sure that 8 would fit in the terminals.
But, 10 is fine for the parameters given.

 

[Mike 134]

The total Isc is 10.14. Multiplied by 2 would be 20.28, with the 125% (if something increases irradiance over STC) increase up to 25.35A? Wouldn't that still be ok? The 36.30A calculation was me jumping to conclusions and maybe not saying it right?

With this calculation, the 40A 90C rating would be 40/1.436 = only rated for 27.85A, where I would be under that even after adjusting my 125% amp increase (25.35A)?

If your continuous current is no more than 24amps then 10 ga wire is fine. Code only allows a wire to be loaded to 80% for continuous loads(more than 3 hours) 30amps x 80%= 24A or 24 x 125%= 30 The code has multiple ways of getting the same value.

 

[Mhoward84]

If your continuous current is no more than 24amps then 10 ga wire is fine. Code only allows a wire to be loaded to 80% for continuous loads(more than 3 hours) 30amps x 80%= 24A or 24 x 125%= 30 The code has multiple ways of getting the same value.

Right on thank you. I didn’t know that about no more than 30 until your first post. Very helpful!

 

[Bluedog225]

Subscribed.

 

[CAD]

41C max temp weather

For my own system, I'm designing for much higher future temperatures wherever I can. My thinking is that with no end in sight of green house gasses we're in for a major thermal ride up in the lifetime of the equipment. In the future when you'll need your solar powered heat pump the most your wire insulation could possibly burn up, and by then it's bound to be harder to fix. my2c

 

[CAD]

No. You have to check the footnotes. per sec 240.4D of 2020 NEC, 10 ga is limited to 30 amps. However, you can use the 40A rating when calculating your derating for ambient temperature and wire fill. You'll need 8 ga wire.

240.4(D) reads, "..the overcurrent protection...", but on a DC solar string there normally isn't any "overcurrent protection", at least not a device to do this.

Instead the DC wires are protected by the upper limits on solar production capacity via Isc.

So how can 240.4(D) apply to solar DC wires when it appears that it's intended to apply to circuits with over-current protection devices?

 

[Hedges]

Often we have multiple strings in parallel, so OCP is required.
I would think without OCP we don't apply the 125% margin to breaker size, therefore should not apply to wire either. But don't know if NEC compels the margin anyway.
Do size PV array ground wires accordingly, because they will carry continuous current.

For thermal breakers, 1.25x multiple is to prevent nuisance tripping. Magnetic-hydraulic breakers aren't affected by heating so don't need that much margin, however I don't know if NEC acknowledges this.

Midnight/CBI breakers are guaranteed not to trip below 105% of rating (guaranteed to trip above 130%), so can be operated up to 100% continuously.
Carling breakers are guaranteed not to trip below 100%, guaranteed to trip above 125%, so could be operated up to 95%

The 1.25x multiple for PV would still need to be applied, because panels can deliver in excess of Isc.

 

[CAD]

Often we have multiple strings in parallel, so OCP is required.

I didn't know about this so I looked it up here. 3 or more in parallel needs OCP. 2 in parallel are exempt from OCP. For why, see their appendix B. Thanks.