PV DC WIRE Calculation and Consideration

[inforaj]

Hello All.

COMMENTS:
Design current for conductor sizing should be based on Isc x 1.56 or Isc x 1.25 corrected for conditions of use, whichever is larger. Imp x 1.56 is not a required calculation. Recommend adjusting this column to correct calculated design current values.

Typically, Imp * 1.56 (1.25 *1.25) and Isc * 1.25, whichever is greater, must be considered.

Why are they looking for Isc * 1.56 - Does anyone have any ideas? Please advise.

 

[ggunn]

1.25 X Isc is Imax. Once you have Imax, AC and DC wire sizing is the same.

 

[Carultch]

Hello All.

COMMENTS:
Design current for conductor sizing should be based on Isc x 1.56 or Isc x 1.25 corrected for conditions of use, whichever is larger. Imp x 1.56 is not a required calculation. Recommend adjusting this column to correct calculated design current values.

Typically, Imp * 1.56 (1.25 *1.25) and Isc * 1.25, whichever is greater, must be considered.

Why are they looking for Isc * 1.56 - Does anyone have any ideas? Please advise.

Imp*1.56 is not a calculation that applies anywhere. Imp without any correction factors applies to voltage drop, but not to ampacity.

Isc*1.56 is what applies for sizing terminations and for sizing OCPD's. It has to do with two 125% factors compounding with each other, the enhancement factor for irradiance exceeding a standard 1kW/m^2 "sun", and the continuous load factor.
Isc*1.25 is what applies for sizing wire with adjustment/correction factors, as well as sizing continuous-duty rated equipment. A common example is DC combiner disconnects.
On top of all that, the OCPD if applicable, must protect both the adjusted/corrected wire ampacity, and the terminations. This usually means applying 240.4(B) if you can meet its conditions, and at minimum, exceeding the previous standard size from the trip rating you are using.

For current-controlled circuits (e.g. optimizer outputs), the device's maximum current rating (Imax) takes the place of 1.25*Isc, and 1.25*Imax takes the place of 1.56*Isc.

See 690.8 and 690.9. Contextualize it also, with 110.14(C), as well as Article 310.

 

[ggunn]

On top of all that, the OCPD if applicable, must protect both the adjusted/corrected wire ampacity, and the terminations. This usually means applying 240.4(B) if you can meet its conditions, and at minimum, exceeding the previous standard size from the trip rating you are using.

I disagree; termination temperature is not a factor in determining OCPD in PV systems. If one is using 90 degree wire and 75 degree terminals the 90 degree conductor ampacity must be greater than or equal to Imax after it is derated for conditions of use, and 125% of Imax must be less than the 75 degree ampacity because of continuous use. The minimum OCPD is set at 125% of Imax and the protection of the conductors is determined by the application of 240.4, i.e., in order to be protected, the derated 90 degree ampacity of the conductors must be equal to or greater than the next size lower standard sized OCPD rating unless the rating is greater than 800A.

Continuous use considerations are to protect the terminals under normal conditions, while OCPD is to protect the conductors in a fault. With the continuous use calculation you have already ensured that the conductors will not exceed 75 degrees under normal operation because PV modules and inverters are current limited devices.

All that said, it may well be that ensuring that 125% of Imax does not exceed the 75 degree ampacity of the conductors will also mean that the next size down OCPD will always be less than the 75 degree ampacity. In that case, never mind.

 

[wwhitney]

Continuous use considerations are to protect the terminals under normal conditions

Slight quibble--not the terminals per se, just the equipment connected to them if it has different ratings for continuous and non-continuous use. Such as all non-100% rated OCPD.

[I'm still curious whether there are inverters whose DC side is rated for continuous use, allowing one to omit the 125% continuous use factor on the DC conductors when there is no OCPD on the DC side. Or if the thermal design of the DC side actually depends on upsized DC conductors as heat sinks.]

All that said, it may well be that ensuring that 125% of Imax does not exceed the 75 degree ampacity of the conductors will also mean that the next size down OCPD will always be less than the 75 degree ampacity. In that case, never mind.

If the OCPD size chosen is the smallest one that is at least 125% Imax (and no more than 800A), then the next smaller size is necessarily smaller than 125% Imax. So if the 75 degree base ampacity of the conductor is at least 125% Imax, it will be strictly greater than the next smaller size OCPD.

Cheers, Wayne

 

[Carultch]

I disagree; termination temperature is not a factor in determining OCPD in PV systems.

My point was the opposite of that. OCPD determines conductor & termination size, rather than the other way around.

One place you might see this, is a group of breakers in an inverter's recombiner, from a diverse group of combiner sizes. Most of your combiners use 400A DC breakers. But you have a select few smaller combiners, and the smallest breaker you can get for that particular configuration is a 225A breaker.

Suppose for one of your smaller combiners, the total Isc = 89.6A. Thus 1.25*Isc = 112A, and 1.56*Isc = 130A. If you could, you'd use a 150A, but your equipment limits you to a minimum of 225A. Temperature derate is 0.96. 75C terminals and 90C wire, as you can typically expect today. This means the following:
1. Wire at its 75C termination ampacity, needs to at least be 140A. Size so far: #1/0 Cu.
2. Wire at its own 90C ampacity needs at least 112A, after derate factors. Thus requiring 112A/0.96 = 117A. This rule would allow #2 Cu, but previous rule governs.
3. OCPD must protect the wire. #1/0 is not "protected" by a 225A breaker. Wire amps must be > 200A (excluding an exact match) to "be protected" by a 225A breaker. Make it 200.1A for the calculation. 200.1A/0.96 = 208.4A. Size so far = #3/0 Cu
4. OCPD must also protect wire at its termination ampacity. #3/0 Cu is an exact match to 200A, so per 240.4(B), this isn't sufficient. Concluded size: #4/0 Cu.

As you can see, steps 3 and 4 govern this example, due to using a breaker that is significantly greater than 1.56*Isc. This is likely to happen if 1.56*Isc either just slightly exceeds the previous standard size, or if manufacturer's constraints put you in a situation just like this.

 

[ggunn]

My point was the opposite of that. OCPD determines conductor & termination size, rather than the other way around.

One place you might see this, is a group of breakers in an inverter's recombiner, from a diverse group of combiner sizes. Most of your combiners use 400A DC breakers. But you have a select few smaller combiners, and the smallest breaker you can get for that particular configuration is a 225A breaker.

Suppose for one of your smaller combiners, the total Isc = 89.6A. Thus 1.25*Isc = 112A, and 1.56*Isc = 130A. If you could, you'd use a 150A, but your equipment limits you to a minimum of 225A. Temperature derate is 0.96. 75C terminals and 90C wire, as you can typically expect today. This means the following:
1. Wire at its 75C termination ampacity, needs to at least be 140A. Size so far: #1/0 Cu.
2. Wire at its own 90C ampacity needs at least 112A, after derate factors. Thus requiring 112A/0.96 = 117A. This rule would allow #2 Cu, but previous rule governs.
3. OCPD must protect the wire. #1/0 is not "protected" by a 225A breaker. Wire amps must be > 200A (excluding an exact match) to "be protected" by a 225A breaker. Make it 200.1A for the calculation. 200.1A/0.96 = 208.4A. Size so far = #3/0 Cu
4. OCPD must also protect wire at its termination ampacity. #3/0 Cu is an exact match to 200A, so per 240.4(B), this isn't sufficient. Concluded size: #4/0 Cu.

As you can see, steps 3 and 4 govern this example, due to using a breaker that is significantly greater than 1.56*Isc. This is likely to happen if 1.56*Isc either just slightly exceeds the previous standard size, or if manufacturer's constraints put you in a situation just like this.

I will run the numbers for wire sizing in your example after I have had my coffee, and I haven't used DC combiners in quite some time, but as I remember it one does not normally have OCPD at all on the output of a DC combiner. The OCPD on the combiner inputs is there to protect a fault on a string from being fed by other strings. The conductors on the output of a DC combiner are sized to withstand the maximum total current output of the combined strings and sized not to get hotter than 75 degrees (assuming 75 degree terminals), and the MPPT input on the inverter must be able to accept that current. There is no source of fault current in excess of what the conductors are sized to handle, so OCPD is not necessary.

 

[Carultch]

I will run the numbers for wire sizing in your example after I have had my coffee, and I haven't used DC combiners in quite some time, but as I remember it one does not normally have OCPD at all on the output of a DC combiner. The OCPD on the combiner inputs is there to protect a fault on a string from being fed by other strings. The conductors on the output of a DC combiner are sized to withstand the maximum total current output of the combined strings and sized not to get hotter than 75 degrees (assuming 75 degree terminals), and the MPPT input on the inverter must be able to accept that current. There is no source of fault current in excess of what the conductors are sized to handle, so OCPD is not necessary.

In my example, it's a multi-combiner system, paralleled on a central inverter. You'd be correct if an OCPD weren't applicable for the combiner output.

 

[ggunn]

In my example, it's a multi-combiner system, paralleled on a central inverter. You'd be correct if an OCPD weren't applicable for the combiner output.

I have been looking but so far I haven't found a DC combiner that has an OCPD on the output; do you know of any?

 

[ggunn]

I have been looking but so far I haven't found a DC combiner that has an OCPD on the output; do you know of any?

One more question: Are you designing PV systems of this type (or any type), or are you speaking from a theoretical perspective?

 

[Carultch]

I have been looking but so far I haven't found a DC combiner that has an OCPD on the output; do you know of any?

One more question: Are you designing PV systems of this type (or any type), or are you speaking from a theoretical perspective?

In this discussion, it's theoretical. I did design this situation a long time ago, not necessarily with these specific numbers (I made those up), so it is worth while to understand the method.

The OCPD is in the inverter (or external subcombiner), not in the combiner.

Another similar topic for this issue, is non-standard ratings of fuses/breakers. I've worked thorough this exercise with inverters that had non-standard 330A fuses (no idea why). Since 240.4(B) is only written with standard sizes in mind, by its wording, 330A fuses would require at least 330A of wire, but 350A fuses (if they were available for that equipment) would only require 300.1A of wire.

 

[inforaj]

Imp*1.56 is not a calculation that applies anywhere. Imp without any correction factors applies to voltage drop, but not to ampacity.

Isc*1.56 is what applies for sizing terminations and for sizing OCPD's. It has to do with two 125% factors compounding with each other, the enhancement factor for irradiance exceeding a standard 1kW/m^2 "sun", and the continuous load factor.
Isc*1.25 is what applies for sizing wire with adjustment/correction factors, as well as sizing continuous-duty rated equipment. A common example is DC combiner disconnects.
On top of all that, the OCPD if applicable, must protect both the adjusted/corrected wire ampacity, and the terminations. This usually means applying 240.4(B) if you can meet its conditions, and at minimum, exceeding the previous standard size from the trip rating you are using.

For current-controlled circuits (e.g. optimizer outputs), the device's maximum current rating (Imax) takes the place of 1.25*Isc, and 1.25*Imax takes the place of 1.56*Isc.

See 690.8 and 690.9. Contextualize it also, with 110.14(C), as well as Article 310.

I agree with you, but this only applies to NEC 2020 and later. We use NEC 2017, and NEC 2017 690.8 shows Max Current, which indicates = Imp * 1.25 * 1.25. = OCPD. What do you say?

 

[Carultch]

I agree with you, but this only applies to NEC 2020 and later. We use NEC 2017, and NEC 2017 690.8 shows Max Current, which indicates = Imp * 1.25 * 1.25. = OCPD. What do you say?

The NEC has specified to use the module's short circuit current since at least as early as the 2008 cycle, when I first learned it.
Module Imp is OK to use as an operating current for voltage drop, but not as the current the wire needs to be rated to withstand.

 

[solarken]

Hello All.

COMMENTS:
Design current for conductor sizing should be based on Isc x 1.56 or Isc x 1.25 corrected for conditions of use, whichever is larger. Imp x 1.56 is not a required calculation. Recommend adjusting this column to correct calculated design current values.

Typically, Imp * 1.56 (1.25 *1.25) and Isc * 1.25, whichever is greater, must be considered.

Why are they looking for Isc * 1.56 - Does anyone have any ideas? Please advise.

There are a lot of formulas appearing in responses to this thread, but just to be clear Isc only refers to PV source and PV output circuits, not AC circuits, not DC-DC/optimizer circuits, etc. Isc is short circuit current rating of a solar module. And there really isn't a 1.56* rule for conductor sizing. Conductor sizing is based on 1.25 * the maximum circuit current, generally, for all types of circuits. For PV circuits, the NEC defines the max ckt current as 1.25*Isc, as Carultech said is meant to account for possible increased current due to increased irradiance beyond Isc which is measured at Standard Test Conditions. So Imax of a PV ckt = 1.25*Isc. And required conductor ampacity = 1.25*Imax. I would recommend just always using 1.25* Imax consistently for all circuit types and just remember that Imax=1.25*Isc ONLY for PV circuits, instead of talking about 1.56*Isc.

Imp has nothing to do with conductor sizing other than if a designer wants to use it to calculate voltage drop, as others have said.

 

[ggunn]

There are a lot of formulas appearing in responses to this thread, but just to be clear Isc only refers to PV source and PV output circuits, not AC circuits, not DC-DC/optimizer circuits, etc. Isc is short circuit current rating of a solar module. And there really isn't a 1.56* rule for conductor sizing. Conductor sizing is based on 1.25 * the maximum circuit current, generally, for all types of circuits. For PV circuits, the NEC defines the max ckt current as 1.25*Isc, as Carultech said is meant to account for possible increased current due to increased irradiance beyond Isc which is measured at Standard Test Conditions. So Imax of a PV ckt = 1.25*Isc. And required conductor ampacity = 1.25*Imax. I would recommend just always using 1.25* Imax consistently for all circuit types and just remember that Imax=1.25*Isc ONLY for PV circuits, instead of talking about 1.56*Isc.

Imp has nothing to do with conductor sizing other than if a designer wants to use it to calculate voltage drop, as others have said.

I multiply Isc by 1.25 and call it Imax. Using Imax instead of Isc there is no difference between AC and DC for wire sizing and OCPD in PV systems.