Solar wiring question

[bluedog225]

I’m installing some medium sized solar arrays. THWN will be run through 2” buried conduit.

Would you run this close to the 10 AWG ampacity?
I thought I had lots of headroom but I do not. I’ve already unfortunately bought the 10 awg wire. But I can sell or try to exchange. Or I can drop one small array and reduce the current carrying conductors to 6 and run at 80% ampacity with plenty of room to spare.

bottom line up front

Final comparison

• Allowed = 28 A
• Required = 27.82 A

10 AWG Cu THHN/THWN-2 = 40 A

Conduit adjustment for 8 current-carrying conductors → 70 % (NEC 310.15(C)(1))

40A × 0.70 = 28 A usable ampacity

Panel Isc = 10.6 A (+/- 5%)-therefore Isc is 10.6 A × 1.05 = 11.13 A

Continuous factor (NEC 690.8)-11.13 × 1.25 = 13.91 A per string

Two parallel strings-13.91 × 2 = 27.82 A

Technically compliant, but only 0.18 A (≈0.6%) margin

No conduit fill issues (low percent filled).
No over overcurrent protection derating (only 2 strings in parallel).
Shut off terminal derating (TBD-I don’t think this is going to be limiting.)
Victron MPPT 450/200-rated at 30 amps per MC4 and ”overpanelling” ok except for loss of reverse polarity protection).

Thanks

 

[paulsomlo]

I can't speak to the ampacity of the wiring, but if you've got a long run from the inverter back to the pole, you may need to take voltage drop into account.

Some friends had a large array of panels with a long run from inverter to pole. On sunny days around noon, the inverter would shut down. The max current from the array each day at noon, flowing through the resistance of the long run back to the pole was forcing an over voltage shutdown at the inverter. The fellow who installed it had to come back and upsize the wire. He sized it originally according to NEC, but didn't seem to understand the subtleties of Ohms law.

 

[bluedog225]

I can't speak to the ampacity of the wiring, but if you've got a long run from the inverter back to the pole, you may need to take voltage drop into account.

Some friends had a large array of panels with a long run from inverter to pole. On sunny days around noon, the inverter would shut down. The max current from the array each day at noon, flowing through the resistance of the long run back to the pole was forcing an over voltage shutdown at the inverter. The fellow who installed it had to come back and upsize the wire. He sized it originally according to NEC, but didn't seem to understand the subtleties of Ohms law.

Thanks. About 320V inbound. With a few percent voltage drop. The minimum startup voltage of the inverter is around 120V. Then operation at 60V. So it should be fine.

 

[willf650]

I’m installing some medium sized solar arrays. THWN will be run through 2” buried conduit.

Would you run this close to the 10 AWG ampacity?
I thought I had lots of headroom but I do not. I’ve already unfortunately bought the 10 awg wire. But I can sell or try to exchange. Or I can drop one small array and reduce the current carrying conductors to 6 and run at 80% ampacity with plenty of room to spare.

bottom line up front

Final comparison

• Allowed = 28 A
• Required = 27.82 A

10 AWG Cu THHN/THWN-2 = 40 A

Conduit adjustment for 8 current-carrying conductors → 70 % (NEC 310.15(C)(1))

40A × 0.70 = 28 A usable ampacity

Panel Isc = 10.6 A (+/- 5%)-therefore Isc is 10.6 A × 1.05 = 11.13 A

Continuous factor (NEC 690.8)-11.13 × 1.25 = 13.91 A per string

Two parallel strings-13.91 × 2 = 27.82 A

Technically compliant, but only 0.18 A (≈0.6%) margin

No conduit fill issues (low percent filled).
No over overcurrent protection derating (only 2 strings in parallel).
Shut off terminal derating (TBD-I don’t think this is going to be limiting.)
Victron MPPT 450/200-rated at 30 amps per MC4 and ”overpanelling” ok except for loss of reverse polarity protection).

Thanks

Panel Isc is also calculated at 125% and then the continuous duty correction is applied so the calculation is: Isc * 1.25 (690.8) * (number of strings) * 1.25 (continuous duty correction) = circuit ampacity for conductor sizing * additional correction (conduit fill/ambient temp/voltage drop)

This is for the inverter DC strings conductors. I don’t do solar aside from the monitoring some large fields after install but am litterally taking some Continuing Education Courses right now for my license and picked a solar course for it.

690.8 is for calculating the ampacity of the string for calculation of the conductor size before other derating is applied. It is calculated at 1.25 for conditions that may occur above the rated environment, ie just a great day, altitude and snow glare.

The continuous duty adjustment is applied after the ampacity is calculated and is always required for solar dc string conductors and inverter ac output as well.

 

[bluedog225]

Well, I goofed it up on my original post.

I think this is accurate. I can combine at MPPT if beneficial.

Thanks all.

Per string. No combining.

• Panel Isc = 10.6 A
• +5% tolerance → 11.13 A
• x1.25 irradiance → 13.91 A
• x1.25 continuous → 17.39 A required per string

• 10 AWG THWN (90 °C) = 40 A
• 14 CCC in conduit → 50% derate
• 40 × 0.50 = 20 A usable per conductor

• Required = 17.39 A
• Available = 20 A

Margin = 2.6 A

 

[willf650]

It's up to you at this point. You have the room in the conduit and you may already have the wire from your previous post then I guess go for it.

Most applications would probably install a combiner at the array and pull a single set of larger conductors in.

You have to bring the strings together some where so where is probably based upon the material you already purchased at this point.

 

[mm08822]

I can't speak to the ampacity of the wiring, but if you've got a long run from the inverter back to the pole, you may need to take voltage drop into account.

Some friends had a large array of panels with a long run from inverter to pole. On sunny days around noon, the inverter would shut down. The max current from the array each day at noon, flowing through the resistance of the long run back to the pole was forcing an over voltage shutdown at the inverter. The fellow who installed it had to come back and upsize the wire. He sized it originally according to NEC, but didn't seem to understand the subtleties of Ohms law.

OP is on the input string side of a charge controller (dotted green in pic). You're discussing an output side issue of an inverter.

 

[paulsomlo]

OP is on the input string side of a charge controller (dotted green in pic). You're discussing an output side issue of an inverter.

That's correct.