Simple Voltage drop calc

[ecohouse]

Sorry if this is obvious, but I am seeking some clarification:

Solar Job: AC combiner after inverter (obviously, but just helping to paint the picture)

I have a 300' run. The maximum amps coming from my AC combiner box is 75. Do I need to multiply this value by 1.25 when sizing wire considering Vd?

I used the quick and dirty southwire phone app and determined that I need #1 thhn. Hope this is correct, already purchased the wire but haven't yet pulled it.

Thanks

 

[charlie b]

A voltage drop calculation only needs to consider 100% of the current that will flow. If you are saying that the 75 amps is what the photovoltaic power source can supply, then you use that value. The 125% factor would come into play when you adding up the things that will use electricity, and selecting a wire size that is big enough to handle all those things. You calculate that load by taking 125% of the continuous load plus 100% of the non-continuous load, and selecting a wire that has at least that high a value of ampacity.

Welcome to the forum.

 

[ecohouse]

Thanks for the reply Charlie. Good to be here!

 

[Carultch]

Sorry if this is obvious, but I am seeking some clarification:/ion

Solar Job: AC combiner after inverter (obviously, but just helping to paint the picture)

I have a 300' run. The maximum amps coming from my AC combiner box is 75. Do I need to multiply this value by 1.25 when sizing wire considering Vd?

I used the quick and dirty southwire phone app and determined that I need #1 thhn. Hope this is correct, already purchased the wire but haven't yet pulled it.

Thanks

For the DC side, use Imp * number of strings in parallel to calculate voltage drop and compare with Vmp * number of modules in series.
For the AC side, take the sum total datasheet current values of all applicable inverters without any safety factors for calculating voltage drop. Compare with nominal voltage.

Keep in mind that voltage drop is a "soft rule", and not a rigid rule where one can say it is definitely a violation. It is from an informational note with a recommendation. The consequences of too much voltage drop results in a performance hit and nuisance tripping of the inverter, rather than a serious failure.

 

[pv_n00b]

There is no code requirement for voltage drop requirements in PV systems. These requirements are generally driven by two things. On the AC side you have to watch out for too much voltage rise on the line due to AC line resistance since that can cause the inverters to drop out if the utility voltage is on the high side, the industry standard is to limit this to 2%. On the DC side it's all pretty much up to you and sometimes the EPC or financier will have values. The industry standard seem to be about 3%.

The voltage drop is not based on maximum current and voltage but Imp and Vmp. Using these values is the industry standard but you can find several alternative interpretations that say much higher voltage drop than the basic 3% can be allowed considering the cost of conductor verses the marginal energy saved. It's a fascinating study.