PV WIRE AMPACITY

[Zee]

Do I use the 90 degree chart or the 75 chart to begin derating of PV Wire? I am thinking specifically of application of the "number of current carrying conductors in a raceway" factor and the ambient temperature derate factor.

Does this change for THHN/THWN-2?

Assume the PV wire (or THWN-2) runs into inverter terminals. Are these terminals typically 75 or 90 degree rated?

Do i just stick to the good ole rule of "assume most terminals and boxes and switches etc are 75 degree rated, so just stick to the 75 degree chart for sizing the wire also, as it connects to it"?

Bonus: is cable tray a raceway, (wire mesh/snake tray) and would I apply a 50% derate for 20 PV wires in it?

 

[Carultch]

Do I use the 90 degree chart or the 75 chart to begin derating of PV Wire? I am thinking specifically of application of the "number of current carrying conductors in a raceway" factor and the ambient temperature derate factor.

Does this change for THHN/THWN-2?

Assume the PV wire (or THWN-2) runs into inverter terminals. Are these terminals typically 75 or 90 degree rated?

Do i just stick to the good ole rule of "assume most terminals and boxes and switches etc are 75 degree rated, so just stick to the 75 degree chart for sizing the wire also, as it connects to it"?

Bonus: is cable tray a raceway, (wire mesh/snake tray) and would I apply a 50% derate for 20 PV wires in it?

Terminals are generally rated for 75C, although for 100A and less, you have a burden of proof to check, as they'd be rated 60C by default.

Derating applies to wires, and not to terminals. If you use 90C wire, you use the 90C rating as the starting point for both your temperature and bundling derates, even if you have 60 or 75C terminals. This is the main advantage of 90C wire.

Other than for 1/0 & larger wires and tray rated cable that the NEC has allowed in a cable tray for several cycles, small PV Wires in a cable tray is uncharted territory. NEC2014 allows listed PV wire in all sizes, which had previously not been allowed by omission. For the most reliable way to pass inspection without it being questioned, assume it is a raceway and apply the same calculation.

 

[Zee]

Terminals are generally rated for 75C, although for 100A and less, you have a burden of proof to check, as they'd be rated 60C by default.

Derating applies to wires, and not to terminals. If you use 90C wire, you use the 90C rating as the starting point for both your temperature and bundling derates, even if you have 60 or 75C terminals. This is the main advantage of 90C wire.

Other than for 1/0 & larger wires and tray rated cable that the NEC has allowed in a cable tray for several cycles, small PV Wires in a cable tray is uncharted territory. NEC2014 allows listed PV wire in all sizes, which had previously not been allowed by omission. For the most reliable way to pass inspection without it being questioned, assume it is a raceway and apply the same calculation.

Excellent, thanks.

I want to believe i can use the 90C chart.
I have been told that sizing wire with the 90C chart will allow the wire to heat up to 90C and when connected to lesser rated (75C or 60C) terminals the HEAT itself will pass into the terminal that is not rated for such high heat. Opinion?

 

[GoldDigger]

Excellent, thanks.

I want to believe i can use the 90C chart.
I have been told that sizing wire with the 90C chart will allow the wire to heat up to 90C and when connected to lesser rated (75C or 60C) terminals the HEAT itself will pass into the terminal that is not rated for such high heat. Opinion?

That is the basic rationale for the rule, yes.

Sent from my XT1585 using Tapatalk

 

[Smart $]

...
I want to believe i can use the 90C chart.
I have been told that sizing wire with the 90C chart will allow the wire to heat up to 90C and when connected to lesser rated (75C or 60C) terminals the HEAT itself will pass into the terminal that is not rated for such high heat. Opinion?

That is the basic rationale for the rule, yes.

He can use the 90°C column for the 'field' ampacity... but he has to use the 75°C column for 'in the box', i.e. termination temperature limitation, minimum size, maximum circuit ampacity. Derating for ambient temperature and proximity applies to the former, while 125% padding for continuous loads applies to the latter. The lesser of the two is the circuit ampacity, which must be considered protected by the OCPD per Article 240.

Cable tray is not a raceway. It is a wiring method which is considered only as a support system. See 392.2.

 

[ggunn]

Excellent, thanks.

I want to believe i can use the 90C chart.
I have been told that sizing wire with the 90C chart will allow the wire to heat up to 90C and when connected to lesser rated (75C or 60C) terminals the HEAT itself will pass into the terminal that is not rated for such high heat. Opinion?

If you are using 90 degree wire, derate it at 90 degrees for conditions of use. From the 75 degree column for the same gauge wire, derate it (multiply by 0.8) for continuous use. The first derate is to protect the conductor insulation and the second is to protect the terminals. Both derated ampacities must be greater than the nameplate maximum output current of your inverter(s).

 

[Carultch]

Both derated ampacities must be greater than the nameplate maximum output current of your inverter(s).

And the wire must also be protected by the OCPD, where required. Note that 240.4(B) often applies here, so in general, this means that the wire ampacity must exceed the previous OCPD and "round up" to the OCPD you actually are installing. So the 75C ampacity directly (or 60C if applicable), and the derated 90C ampacity for conditions of use, need to be between the previous OCPD size and the actual OCPD size.

One application where this might be the driving factor, is in a group of DC feeders to a central inverter, with the inverter manufacturer including a group of breakers for the DC subcombiner. Suppose you have a "runt of the litter" combiner output that is supplied by a source that is significantly smaller than the smallest breaker available in the inverter. It is possible that using a larger than default size OCPD could drive the wire size. For instance, if 1.56*Total Isc is only 149A, but the smallest breaker that fits the subcombiner is 225A. Note that most of the other combiners are connecting to 400A breakers, so the previously mentioned combiner deviates from the project norm, due to being sourced from an abnormally smaller array. A "runt of the litter" as I like to call it.

 

[Smart $]

.... So the 75C ampacity directly (or 60C if applicable), and the derated 90C ampacity for conditions of use, need to be between [greater than] the previous OCPD size and the actual OCPD size.
...

Just greater than the next lesser standard OCPD rating. The ampacity (or should I say 'ampacities'?) can equal or exceed the actual OCPD rating. 'Between' is a practice in economics, not NEC compliance.

 

[ggunn]

And the wire must also be protected by the OCPD, where required. Note that 240.4(B) often applies here, so in general, this means that the wire ampacity must exceed the previous OCPD and "round up" to the OCPD you actually are installing. So the 75C ampacity directly (or 60C if applicable), and the derated 90C ampacity for conditions of use, need to be between the previous OCPD size and the actual OCPD size.

Only the conditions of use ampacity needs to be greater than the next size down OCPD.

 

[Smart $]

Only the conditions of use ampacity needs to be greater than the next size down OCPD.

Not if you are using an oversized OCPD. The basic termination requirement is based on the load, but it also assumes one is going to use the minimum standard OCPD rating (determined using noncontinuous plus 125% continuous, same as the minimum wire size for termination). Step that OCPD rating up one size and you are looking at the potential to overheat the termination if the minimum wire size for termination is not also upsized. This is why 110.14(C) says "selected and coordinated".

 

[ggunn]

Not if you are using an oversized OCPD. The basic termination requirement is based on the load, but it also assumes one is going to use the minimum standard OCPD rating (determined using noncontinuous plus 125% continuous, same as the minimum wire size for termination). Step that OCPD rating up one size and you are looking at the potential to overheat the termination if the minimum wire size for termination is not also upsized. This is why 110.14(C) says "selected and coordinated".

For an inverter output circuit which is current limited, the conductors are already sized so that the continuous inverter current cannot overheat the terminals. In my designs it is very common for the COU@90 value to be higher than the next down OCPD but for the CU@75 value to be lower. This is per John Wiles' treatise on the subject.

 

[Smart $]

For an inverter output circuit which is current limited, the conductors are already sized so that the continuous inverter current cannot overheat the terminals. In my designs it is very common for the COU@90 value to be higher than the next down OCPD but for the CU@75 value to be lower. This is per John Wiles' treatise on the subject.

Would you please provide an example?

I was talking about general wiring, but when talking minimum wire sizing with minimum OCPD ratings of inverter output circuits, it should work out the same. The 75°C ampacity of the wire cannot be less than the inverter rated output times 125%, which is the same criteria for the minimum OCPD rating. Having a CU@75 not greater than the next lower OCPD rating is mathematically impossible... I think.

 

[ggunn]

Would you please provide an example?

I was talking about general wiring, but when talking minimum wire sizing with minimum OCPD ratings of inverter output circuits, it should work out the same. The 75°C ampacity of the wire cannot be less than the inverter rated output times 125%, which is the same criteria for the minimum OCPD rating. Having a CU@75 not greater than the next lower OCPD rating is mathematically impossible... I think.

Inverter Imax = 37A
Tmax = 98 degrees F, not on roof (derate = 0.91)
2 CCC's in conduit (derate = 1.0)
AWG #8 copper THWN-2 (90 degree ampacity = 55A, 75 degree ampacity = 50A)

OCPD: (1.25)(37A) = 46.0A, use 50A
COU@90: (55A)(0.91)(1.0) = 50.1A
CU@75: (50A)(0.8) = 40A (<45A)

 

[Carultch]

Inverter Imax = 37A
Tmax = 98 degrees F, not on roof (derate = 0.91)
2 CCC's in conduit (derate = 1.0)
AWG #8 copper THWN-2 (90 degree ampacity = 55A, 75 degree ampacity = 50A)

OCPD: (1.25)(37A) = 46.0A, use 50A
COU@90: (55A)(0.91)(1.0) = 50.1A
CU@75: (50A)(0.8) = 40A (<45A)

You are thinking about the calculation in reverse. The ampacity of #8 Cu wire with 75C terminals doesn't become 40A from 50A, just because there is a continuous load on it. Rather, the sizing algorithm requires you to have 125% of the continuous load amps worth of 75C ampacity, when there is a continuous load.

The ampacity of the #8 Cu wire at 75C is still 50A, and is still enough to be protected by the 50A breaker.

 

[ggunn]

You are thinking about the calculation in reverse. The ampacity of #8 Cu wire with 75C terminals doesn't become 40A from 50A, just because there is a continuous load on it. Rather, the sizing algorithm requires you to have 125% of the continuous load amps worth of 75C ampacity, when there is a continuous load.

The ampacity of the #8 Cu wire at 75C is still 50A, and is still enough to be protected by the 50A breaker.

Tomayto, tomahto. It's a semantic difference that ends up in the same place. When you are determining if a conductor is protected by OCPD, the only consideration is the conditions of use derate.

 

[Smart $]

Inverter Imax = 37A
Tmax = 98 degrees F, not on roof (derate = 0.91)
2 CCC's in conduit (derate = 1.0)
AWG #8 copper THWN-2 (90 degree ampacity = 55A, 75 degree ampacity = 50A)

OCPD: (1.25)(37A) = 46.0A, use 50A
COU@90: (55A)(0.91)(1.0) = 50.1A
CU@75: (50A)(0.8) = 40A (<45A)

CU@75: (1.25)(37A) = 46.0A... #8cu, 50A@75°C (not ≤45 OCPD)

 

[Smart $]

Tomayto, tomahto. It's a semantic difference that ends up in the same place. When you are determining if a conductor is protected by OCPD, the only consideration is the conditions of use derate.

Not really. The coordinated ampacity is the lesser of termination ampacity and the conditions of use ampacity. The correlation of that ampacity and the circuit OCPD rating must meet the requirements of 240.4.

 

[Zee]

Good discussion.

My common issue is typically can i get away with #12 or do i need #10 on my pv source circuits in conduit.

Example:
Solaredge optimizers with max amps of 15.
2 strings. ===>
4 THWN-2 conductors in conduit.


(CU) Optimizer Output: 15.00 x 1.25 = 18.75 A
(COU) Derated Conduct.: 40A (10AWG in 90C column) x 0.76 (ambient Temp on roof) x 0.80 (4 CCC in conduit) = 24.32 A
18.75 A < 24.32 A ===> Wire Sizes OK

Is it in fact OK to use the 90C chart here?

 

[Smart $]

...
Is it in fact OK to use the 90C chart here?

I believe so. A major problem with conductor ampacity requirements in e.g. 210.19, 215.2, 690.8, is that none tell you which column to use for which part of the determination. I've submitted a few proposals to get 110.14(C) included in the "125% continuous plus noncontinuous" and "before adjustment and correction" requirements but all were rejected. 110.14(C) is the only place that stipulates which column to use. How to apply these requirements properly are only passed on through the 'unwritten' Code (i.e. one professional to another).

 

[ggunn]

Not really. The coordinated ampacity is the lesser of termination ampacity and the conditions of use ampacity. The correlation of that ampacity and the circuit OCPD rating must meet the requirements of 240.4.

It makes no difference whether you multiply current by 1.25 or multiply ampacity by 0.8; you get to the same place. It's a distinction without a difference.

All my electrical plans show compliance to code this way, and AHJ plan reviewers have never questioned it