PV cable sizing pt 1: Inverter output conductors

Nov 07, 2023

By Joe Jancauskas, Senior Electrical Engineer at Castillo Engineering | April 4, 2023

Credit: Castillo Engineering

Second to only PV module ratings, nothing changes faster than inverter kilowatt ratings. In fact, inverter manufacturers revamp product ratings so often that inverter derating are becoming commonplace in order to keep the interconnect alternating current (AC) rating the same and avoid reentering the cumbersome utility analysis queue.Additionally, there are widespread misconceptions about how to properly size inverter AC output conductors in light of inverter derating, particularly for central inverters, that result in unnecessary material costs and lost time.

Below I provide a primer on inverter ratings for the three main categories of inverters; the prevalent inverter deratings that are largely being accepted and verified by utilities; and how to save time and money by properly sizing inverter output conductors.

Ratings for different categories of inverters

There are three main categories of inverters, and it is worth looking at a selection of recently available ratings for each group as a background to the topic of cable sizing for both string and central inverters:

1. Single-phase string inverters:

These inverters come in a bewildering array of sizes, with a lot of kilowatt granularity, from 2 to 12 kW with many iterations in between.

2. Three-phase string inverters:

At the lower kilowatt range, these inverters have the same high-kilowatt granularity of single-phase string inverters, however, at the higher end of the range, there are bigger gaps in kW ratings, as the manufacturers keep jumping up in available sizes. These inverters can range from 2 to 275 kW.

3. Central Inverters:

The definition of what rating constitutes a "central" inverter is constantly moving upward, and these large devices do come with some truly odd kilowatt ratings. They start at 1,910 kW and leap to 6,800 kW.

I doubt that any inverter manufacturer did a detailed market analysis and discovered that they could really corner the market with a 2,930-kW rated inverter and then went out and designed it. Inverter manufacturers are trying to reach higher levels of output with their proven electronics platform that can get through UL testing quickly and onto the market.

Additionally, those are just the kilowatt ratings at unity power factor 1.0, an ideal power setting. With the higher penetration of inverter-based distributed energy systems on the grid, many inverters are now offering extended ratings for non-unity power factor operation. Ten years ago, string inverters were only rated in watts, but now they are rated in both real and total apparent reactive power capabilities (kilowatts and kilovolt-amperes [kVA]), with the reactive power (kVAR) being left for the user to calculate, as shown in the image below:

Credit: Castillo Engineering

Operating at power factors other than unity power factor 1.0 does cause some kWh revenue loss in order to generate the kVAR, but operating at a leading power factor can sometimes make for a less expensive utility interconnection.

Derating inverters for interconnection because manufacturers rapidly revamp inverter power ratings

Ratings and product offerings are changing so rapidly that by the time inverter orders are placed, the inverter that the utility approved in the Coordinated Electric System Interconnect Review (CESIR) is no longer available, but one with a slightly higher rating has taken its place as the ratings keep inching up in kilowatt. The PV system owner will not be allowed to connect a higher AC power system to the grid without another torturous trip back through the utility analysis queue, so the result is that the new inverters need to be derated to keep the interconnect AC rating the same.

As an example of inverter rating changes, SolarEdge just had a major product revamp on May 1, 2022, with new, higher output inverter models and a discontinuation of lower-rated models. For example, its SE9k, SE14.4k and SE43.2k are now discontinued and replaced by SE10k, SE17.3k, and SE50k.

Whether to use inverter nameplate or continuous output rating to size conductors & the impact on project costs

Central inverters are particularly an issue, even though their output conductors are typically short runs to get the inverter's high amperage output of 480 V, 600 V, 630 V or 800 VAC into a step-up transformer to convert it to medium voltage. Due to lead time considerations, the inverter and transformer are often ordered separately, rather than as a factory-integrated skid, and need to be connected in the field with custom bus (tricky) or cables (more common).

The National Electric Code (NEC, NFPA 70) rules for sizing the inverter AC output conductors has been the same since 1999. Article 690.8(A)(3) states that, for the inverter output circuit current, "the maximum current shall be the inverter continuous output current rating." So, does that mean the value stamped on the nameplate that went through UL testing, or the value of the electronically restricted output?

We have seen several recent instances where an owner's engineer has been adamant that the full output current on the nameplate must be used, regardless of a derated value that cannot be exceeded under the terms of the Utility Interconnect Agreement, resulting in wasted material and time.

Credit: Castillo Engineering

The high amperage outputs dictate a lot of parallel runs of conductors and a lot of bolted connections. Connecting an inverter derated to 2500 kVA requires nine sets of 750 kcmil AL at 75 0C, but do you really want to run a 10th set for its full nameplate rating of 2800 kVA? You will start running up against space and lug limitations quickly. Whether to use the NEC 75 0C, 90 0C, or another rating method will be covered in a subsequent story.

Sometimes doing what the Owner's Engineer wants is the path of least resistance, but it can result in wasted hardware and effort. If utility-scale PV installed dollar-per-kilowatt costs were still trending downward, it would hurt less — but they aren't.

Supply chain disruptions have also made requests to substitute cable sizes during construction very common. This can result in termination issues from either too many smaller conductors that don't have enough places to land or conductors that are too large for the factory-provided terminals.

The NEC does not say "per the inverter nameplate," but "the continuous output rating." There is no religious sanctity to the inverter nameplate and its implied association with NEC article 690.8.

As an example, when an electric utility buys indoor, metal-clad switchgear for a substation, the 15-kV circuit breakers only come in three basic sizes: 1200 A, 2000 A and 3000 A. They don't have the luxury of PV system owners with a plethora of inverter kilowatt sizes to choose from. Most of the outgoing distribution feeder cables are not going to be sized at 1200 A because that would be overkill. The cables are sized for the expected load and the protective relays are set accordingly. A large substation can easily have 12 1200 A feeder circuit breakers fed from a single 3000-A main. The nameplate rating of the circuit breakers can be ignored.

Utilities’ acceptance & inverter deratings verification

The need for derating has become so common that utilities are widely accepting the derated values, although they are starting to request some additional confirmation that the derate was actually made. As stated in Orange & Rockland's, DER Interconnection Design Package Requirements:

"Inverter Nameplate: if the inverter nameplate exceeds application threshold, then a manufacturer letter stating the inverter will be curtailed at the factory is required. The letter can be generic (not project specific at this point). If the project proceeds to construction, then a project specific letter stating the project number and inverter serial numbers will be required from the inverter manufacturer."

If the electric utility that approved the project accepts the derate, there should be no reason why an AHJ or independent engineer should not accept it as well. Other options to avoid a nameplate rating fixation is to simply order a modified nameplate with the inverter or attach a placard in the field with the derated value.

Nameplate fixations are not new. In 2009, the electric utility group that I managed was fined $7,000 by the National Electric Reliability Council for not including "cable nameplate" ratings in conformance with its rating standard. Cables do not have nameplates, but the penalty was still paid.

Key takeaways

On your current or upcoming solar project, think about what the true AC output ampere requirements are for your inverters; design to meet those requirements safely; and don't unnecessarily waste money. The NEC "is not an instruction manual," as stated in Article 90. When you do follow it, the result may be safe, but it may not be "efficient, convenient, or adequate for good service."

Stay tuned for part two of our PV cable sizing series, in which we will cover whether your strict voltage drop criteria is actually making you money. If you have utility-scale solar design and engineering questions, get in touch with one of our experts today.