US vs. international solar installations: a tale of two standards

The difference between solar PV on one side of the Atlantic versus the other

Most of us are probably familiar with differences between American electrical standards and those used in most of the rest of the world through personal electronics. Today’s laptop computers and phone chargers can accept either 110v or 220v so all you need to travel between regions is an adaptor for the plug. But it was not always so, and many electronic devices designed for 110v came to an early death when their owners plugged them into a 220v outlet on their European holiday.

In the world of solar power, there are similar standards that dictate the maximum voltage a photovoltaic (PV) panel can use. Like their consumer-level counterparts, these standards vary between regions.

Outside of North America, IEC standards (specifically IEC 60364-7-712) provide requirements for solar installations. In the US, the National Electric Code (NEC) governs public and private wiring installations (article 960, if you’re interested) while the National Electrical Safety Codes cover power generation, also applicable to solar PV.

It gets even more complicated because NEC standards also specifically require the use of equipment that is compliant with product standards established by Underwriters Laboratories (UL).

So, what’s the difference between solar PV on one side of the Atlantic versus the other?

In a word, it comes down to voltage. US standards cap the operating voltage of a PV panel at 1000v but conventional wiring practices typically mean the voltage is set even lower, at 600v. Meanwhile, IEC systems typically operate at 1000v or even 1500v.

This has some important implications when it comes to overall system efficiency and cost. Higher system voltage allows for:

  • A reduced of number of source circuits and a reduced numbers of conductors for a system of the same capacity
  • A reduced number of overcurrent protection devices, inverters, disconnectors and a subsequent reduction in labor cost
  • Smaller cross-section conductors (i.e., lower materials cost)
  • Reduction of wire resistive losses
  • Increased efficiency at the inverter stage
  • Increased power at the same current rate

These benefits have not gone unnoticed by the US standards bodies, and are at the center of the debate now taking place in anticipation of a revision to the NEC this year. If the codes are changed to allow higher voltage levels, we could see a drop in overall PV system costs once equipment vendors adapt to the new standard. It’s hard to say how much of a reduction the end user will see, but even a small improvement would certainly be welcome.


Comment this article(2)

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  • It is unfortunate but a fact that local standards always means extra considerations and ultimately extra cost. Many times it can also mean less reliability, as it requires specialty built rather than, proven standard product.

    Looking at the PV plant voltage level, there are many excellent points in this article. One that is not mentioned is the inverter voltage level. While a few MV inverters are available, most readily available standard products are built within the limits of LV standards. In this category many are limited to 820 V whereas a few premium manufacturers of solar inverters utilize IGBT devices that allow up to 1000 V Voc and MPPT.

  • Pablo Astorga

    Hi Marzio! Nice article.

    I believe that when you refer to 1000V as now reaching the U.S., you're really referring to the residential solar market, where NEC rules apply (as you pointed out correctly).

    The U.S. solar industry has been installing 1000V systems for years, but this has mostly been in the utility-scale sector (the so called behind-the-fence projects), where the Authority Having Jurisdiction (AHJ) can approve such a solution even if UL rating was not available back then.
    With the market growing so fast during the last 5 years, both 1000V product availability and UL certifications have increased and become very common. So we're still one step behind Europe in this regard; but getting there.