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At Fresco Solar, we recently completed our first two commercial photovoltaic systems using 1,000 V DC components, both of which were also firsts for their respective authorities having jurisdiction. One is a 102 kW installation on a nine-story building in downtown San Jose, Calif. The other is a 440 kW array on a single-story fish processing plant in Hayward, Calif.

Our first concern was submitting permit drawings that would be acceptable. To ensure this, we did the following:

• Stated clearly that these would use 1,000 V DC parts;
• Submitted wet-stamped calculations showing we would not exceed 1,000 V on the coldest day; and
• Added “Danger, High Voltage, Authorized Personnel Only” labeling on our signage sheet.

In fact, the only electrical comment of note was to be sure our 480 V AC wiring was color-coded differently from the existing 208 V branch wiring throughout the building.

The fire departments of both communities were pleased when we explained that we would not be running any DC wiring into the buildings and that the ground-level AC disconnect would kill all PV power inside the building. This is a very important consideration because if the fire department does not accede to this, it may insist on a single DC disconnect on the roof, which kills the multiple inverter strategy that makes it possible to avoid having a single large inverter at ground level.

Neither planning department took an interest because the systems were not visible from ground level.

The San Jose Fire Department did take a strong position on the fact that our panels were rated FR3 - almost every PV panel is - and we were using plastic ballast pans. The department probably would have gone down a similar path even at 600 V. In the end, we needed a fire protection engineer to write a report, and we had to agree that instead of using individual ballast blocks, we would pour wet concrete into the ballast pans.

This actually worked out well as we laid out all our pans in an empty lot across the street. We brought in a concrete truck and pump and filled all the pans at ground level, leaving us with a lot less to do on the roof.

 

More with less

The availability of small 480 V inverters was key to our approach and one compelling reason to go to 1,000 V. We used ABB 27.6 kW inverters, but there are others on the market, too. The DC side of a 1,000 V inverter has much smaller components than those of a 600 V unit because the current is much less for the same AC output. This makes them a lot lighter and smaller, which, in turn, makes it possible for a crew of two to install a larger size inverter than one restricted to 600 V.

John Knueppel, owner and founder of Solar Design & Drafting, compiles our permit packages and believes 1,000 V installations should be considered for every commercial project where the electrical service is 480 V. “Even though I am not involved with purchasing to quantify cost per DC watt savings, I can safely say this is a major reduction,” he says.

The extra 400 V actually enables us to run strings almost twice as long. It is, after all, that one extra panel that puts you over the limit that matters - and one panel is a smaller fraction of a longer string.

This brings us to the single best reason to build at the higher voltage: less home run wiring. On our Hayward project, we used 15,000 feet of PV wire. We would have used twice that much - another three miles - the old way. Along with that, there is half the labor, connectors, fuses, combiners and much less current for the same power.

One other concern is sizing the strings for a design that packs every last panel onto the roof. At first, you might say it is either 18 panels or none. But this is not the case. On the San Jose installation, the developer, Silray Inc., asked us to completely fill the roof. Even then, we would be producing only a fraction of the building’s power. We found we could fit 343 panels - a number not divisible by anything. However, by using three strings of 16 on one maximum power point tracking side of an inverter and four strings of 15 on the other side, we were able to do it. The last inverter just needs to be an oddball.

Another very important reason to try to get all the inverters on the roof is that you can then reasonably say there is no public access and label the ladders and scuttles accordingly. Otherwise, you will have to fence in the inverters at ground level. This is extra cost, takes up space and, most importantly, could gain the attention of the planning department.

We eventually ran into this issue with the City of Hayward when we added a large trellis down the side of the building. We had to hire an architect and do a much more detailed site plan showing parking spaces, handicap-accessible parking spaces, signage, road widths and more. Better to try to get it all on the roof.

 

Understand the dangers

Tom McCalmont, CEO of McCalmont Engineering, which provides electrical engineering services to us, says it is important to appreciate the dangers of working on a high-voltage rooftop.

The National Electrical Code (NEC) used throughout the U.S. to ensure the safety of electrical systems is written with various voltage ranges in mind - typically under 50 V, between 50 V and 600 V, and above 600 V. Most of the NEC has been developed for voltages between 50 V and 600 V because that is the voltage range most commonly used in buildings. Per NEC article 300.2, wiring methods throughout the code are generally approved for voltages of 600 V and less. Per NEC Article 300.3(C)(2), “conductors of circuits rated over 600 V … shall not occupy the same equipment wiring enclosure, cable, or raceway with conductors of circuits rated 600 V or less.”

McCalmont points out that this language is often interpreted to mean that voltages greater than 600 V should not be used in rooftop solar systems because some of the enclosures in such systems would then contain circuits both above and below 600 V.

Systems making use of 1,000 V and higher for DC strings have gradually come into wider acceptance throughout many jurisdictions in the U.S., as long as those systems are ground-mount fields that can be isolated with a seven-foot-high security fence and that have all entrances secured by locked gates accessible only by authorized personnel. Such systems are considered “utility” fixtures that untrained and unauthorized personnel will not be able to enter. Therefore, a different set of NEC requirements applies to them.

However, this situation is not the same for rooftop PV systems, McCalmont warns. For most roofs it is not practical to prevent access to the roof by untrained and unauthorized personnel. Roofs are typically accessed by roofers for repairs and by HVAC and satellite technicians to maintain equipment, for instance.

“Such personnel typically are not trained in safety procedures to work where voltages greater than 600 V are present,” McCalmont says. “The stricture is further exacerbated by the fact that many PV arrays have their home run conductors running in free air beneath the racking of the system, which means these high-voltage strings are quite accessible to other personnel who may be on the roof.”

Because of these concerns, the ability to use 1,000 V DC strings in rooftop PV systems is still very much up to the individual jurisdiction of the local building department and their interpretation of the NEC’s voltage requirements.

 

Up on the roof

Mark Jacobi, vice president of Fresco Solar, says life on the high-voltage rooftop is safe and manageable if you take care of business:

• Use 1,000 V rated equipment and PV wire;
• Maintain four feet of additional working clearance from the equipment; and
• Establish a “behind the fence” restricted access policy for non-qualified personnel.

“I also think that it is important to have regular safety meetings and established safety protocols to heighten the awareness of the increased DC voltages of between 700 and 800 V DC,” Jacobi says.

I don’t think it is too jingoistic to say it is high time we in the U.S. caught up. This is one more step on the road to grid parity. And there is no reason the NEC cannot evolve. The technology is here, so let’s use it. R

 

Sean Kenny is CEO of Fresco Solar, a commercial solar engineering, procurement and construction contractor based in Morgan Hill, Calif. He can be reached by email at sean@frescosolar.com.