690.8(B) Conductor sizing

HoosierSparky

Senior Member
Location
Scottsdale AZ
Let me start out by saying I DO NOT LIKE REVIEWING SOLAR! Now that I got that off my chest, The issue I have is off a string the current, per the micro-inverter optimizer is 15A. There are 3 strings going from a combiner box to the inverter with 6 #10. Per 690.8(B) if I choose option 1, I only have to allow for continuous power by multiplying by 125%. I then do not have to apply any correction factors. By applying the 125% the 15A becomes 18.75A. #10 is good for 30A with no correction factors. IF I apply all the correction factors (ambient temp - 122F, conduit distance from roof adder - 40F, I end up with a derate of 0.50. Take a .0.80 derate on top of that for 6 current carrying conductors you get #10 = 16A.

Do I apply those derates? It seems 690.8(B)(1) does not take into account # of conductors or temperature.
 

Carultch

Senior Member
Location
Massachusetts
Microinverters and optimizers are two different kinds of devices. Keep track of what these terms mean, as they are not interchangeable. The example you show, uses optimizers.

How they are similar:
They are both module-level power conditioning devices that process the DC power from the modules, and also methods of satisfying 690.12 rapid shutdown.

How they are different:
1. Optimizers produce DC power, microinverters produce AC power at the grid voltage.
2. Optimizers are wired in series, microinverters are wired in parallel.
3. Optimizers rely on being connected to a string inverter for producing AC power for the grid, since they only convert DC to DC.

Moving on to the ampacity question:
690.8(A) spells out how to calculate Imax, which is the current that governs the ampacity calculations. For circuits that are uncontrolled, this is 1.25*total Isc, to account for sunlight exceeding 1 kW/m^2. For current-limited devices, such as optimizers, inverters, and microinverters, this comes directly off the datasheet.

690.8(B), then prescribes the first two conditions to be met.
690.8(B)(1): 1.25*Imax <= wire ampacity at termination/eqpt temperature rating. Usually 75C, read the fine print of 110.14(C) for more information.
690.8(B)(2): 1.00*Imax/total derate <= wire ampacity at wire temperature rating. Usually 90C.
These are not options. These are two separate conditions that need to both be satisfied.

Implied from other sections of the NEC, the wire needs to be protected at its ampacity, if there is an OCPD on the circuit in question. In your example, there isn't one needed or required, for paralleling the two optimizer strings in the inverter.
240.4: derated wire ampacity to "be protected" by the OCPD where required.
240.4/110.14(C): non-derated termination ampacity to "be protected" by the OCPD where required.
240.4(B) may allow us to consider it protected, as long as it "rounds up" to the OCPD we are using.

Depending on what edition of the NEC applies to you, the rooftop temperature adder has been modified as of NEC2017 so it only applies for conduits closer than 7/8" to the roof. If a previous version applies, you'll still have the various temperature adders for the temperature derate calculation.
 
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jaggedben

Senior Member
Let me start out by saying I DO NOT LIKE REVIEWING SOLAR! Now that I got that off my chest, The issue I have is off a string the current, per the micro-inverter optimizer is 15A. There are 3 strings going from a combiner box to the inverter with 6 #10. Per 690.8(B) if I choose option 1, I only have to allow for continuous power by multiplying by 125%. I then do not have to apply any correction factors. By applying the 125% the 15A becomes 18.75A. #10 is good for 30A with no correction factors.
So far so good.

IF I apply all the correction factors (ambient temp - 122F, conduit distance from roof adder - 40F, I end up with a derate of 0.50. Take a .0.80 derate on top of that for 6 current carrying conductors you get #10 = 16A.
So 16A>15A so the #10 satisfies both conditions. Fits with my experience that #10 is almost always good for 15A in solar.

Do I apply those derates? It seems 690.8(B)(1) does not take into account # of conductors or temperature.
No it doesn't. Think of it as an 'alternative minimum' that must be met even if required derates check out on a smaller conductor.
 

HoosierSparky

Senior Member
Location
Scottsdale AZ
So far so good.



So 16A>15A so the #10 satisfies both conditions. Fits with my experience that #10 is almost always good for 15A in solar.



No it doesn't. Think of it as an 'alternative minimum' that must be met even if required derates check out on a smaller conductor.
You missed the 125% adder for continuous power. 15A x 125% = 18.75A. So the 16A #10 is then too small for the load.
 

HoosierSparky

Senior Member
Location
Scottsdale AZ
I don't belive that's required on top of derating for optimizer outputs. For PV source circuits you'd be correct.
It is from PV source. Per the diagram, there is 15Adc from the optimizer strings. Per 690.9 you need to multiply the 15Adc by 125% due to continuous power. That takes us to the 18.75A. Per 690.8(B).
 

jaggedben

Senior Member
A
It is from PV source. Per the diagram, there is 15Adc from the optimizer strings. Per 690.9 you need to multiply the 15Adc by 125% due to continuous power. That takes us to the 18.75A. Per 690.8(B).
A PV source circuit is not the same as an optimizer output circuit, aka DC to DC converter output circuit. Check the definitions at the beginning of 690. I don't have the code in front of me, but I recall seeing an additional 125% adder only for PV source circuits. (i.e. only PV source circuits require an initial 125% before going into the either/or options of the 125% or conditions of use we already discussed).
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
What is the technical reason for the extra 125% factor for PV source circuits? Is it the possibility of the panels producing more current than under standard conditions?

Cheers, Wayne
 

Carultch

Senior Member
Location
Massachusetts
What is the technical reason for the extra 125% factor for PV source circuits? Is it the possibility of the panels producing more current than under standard conditions?

Cheers, Wayne
Yes. It has to do with the possibility of irradiance exceeding 1 kW/m^2, and the module not being current-controlled like an optimizer or inverter. In general, direct sunlight at Earth's surface is about 1 kW/m^2 on a clear day, but other factors could make it temporarily more than that. Usually due to the sunlight reflecting off of background objects, and adding to the direct sunlight. Some examples being clouds, nearby glass buildings, snowfall on the surrounding terrain.
 
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