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DESIGNING TOMORROW'S POWER | EMAIL: INFO@CASTILLOPE.COM 

PV Cable Sizing, Part One: Inverter Output Conductor Sizing

By Joe Jancauskas, Senior Electrical Engineer at Castillo Engineering




Second to only PV module ratings, nothing changes faster than inverter kW ratings. In fact, inverter manufacturers revamp product ratings so often that inverter deratings are becoming commonplace in order to keep the interconnect ac rating the same and avoid reentering the cumbersome utility analysis queue. Additionally, there are widespread incorrect preconceptions about how to properly size inverter ac output conductors in light of inverter deratings, 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; now 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.


Primer on 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 kW granularity, such as: 2.0, 2.5, 3.0, 3.1, 3.2, 3.3, 3.8, 4.0, 5.0, 5.4, 6.0, 6.6, 7.0, 7.5, 7.6, 8.0, 9.0, 9.3, 9.9, 10.0, 11.4, 12.0.


2. Three-Phase String Inverters:

At the lower kW range, these inverters have the same high kW 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: 2.0, 3.0, 3.1, 3.2, 3.3, 4.0, 4.6, 5.0, 5.2, 5.4, 5.75, 6.0, 6.6, 7.0, 7.5, 7.6, 8.0, 9.0, 9.3, 9.9, 10.0, 11.4, 12.0, 14.0, 14.4, 15.0, 15.75, 17.3, 18, 20, 23, 24, 25, 27.6, 28, 30, 33, 33.3, 34.2, 36, 40, 42, 43, 48, 50, 60, 62.5, 63, 65, 66, 72, 80, 100, 125, 250, 275.


3. Central Inverters:

The definition of what rating constitutes a ‘central’ inverter is constantly changing upward, and these large devices do come with some truly odd kW ratings: 1910, 2195, 2200, 2500, 2250, 2660, 2700, 2750, 2800, 2930, 3000, 3060, 3125, 3200, 3290, 4000, 4200, 4320, 4600, 6250, 6800.


I doubt that any inverter manufacturer did a detailed market analysis and discovered that they could really corner the market with a 2930 kW rated inverter and then went out and designed it. Inverter manufacturers are just trying to reach the next higher level of output with their proven electronics platform that can get through UL testing quickly and onto the market.


Additionally, those are just the kW ratings at unity (1.0) power factor. 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 (kW and kVA), with the reactive power (kVAR) being left for the user to calculate, as shown in the image below:



Operating at other than unity power factor does lose some kWh revenue 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 Due to Manufacturers Rapidly Revamping Inverter Power Ratings


Ratings and product offerings are changing so rapidly that it is common 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 kW. The PV owner is not going to 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 models and a discontinuation of lower rated models. For example, their SE9k, SE14.4k, and SE43.2k are now discontinued and replaced by SE10k, SE17.3k, and SE50k.


Determining Whether to Use Inverter Nameplate or Continuous Output Rating to Size Output Conductors & the Impact on Project Costs


So where’s the problem?


To begin with, 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 at least 1999, and Article 690.8(A)(3) states that, for the inverter output circuit current, “the maximum current shall be the inverter continuous output current rating.” While that seems clear enough, 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 utilized, regardless of a derated value that cannot be exceeded under the terms of the Utility Interconnect Agreement, resulting in wasted material and time.


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 takes 9 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 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 blog post.


It sometimes ends up that just doing what the Owner’s Engineer wants is the path of least resistance, but it is painful to see wasted hardware and effort. If utility-scale PV installed $/kWp costs were still trending downward, it would hurt less, but they aren’t. Supply chain disruptions have also made Requests for Information (RFI) during construction asking to substitute cable sizes very common, either going up or down in size from what was originally designed. 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.


Note 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 a related 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 owners with a plethora of inverter kW sizes to choose from. The vast majority of the outgoing distribution feeder cables are not going to be sized at 1200 A because that would be total overkill. The cables are sized for the expected load and the protective relays are set accordingly. A large substation can easily have twelve, 1200 A feeder circuit breakers fed from a single 3000 A main. The ‘nameplate’ rating of the circuit breakers is ignored.




Utilities’ Acceptance & Verification of Inverter Deratings


The need for derating has become so common that utilities are widely accepting the derated values, although they are starting to reasonably request some additional confirmation that the derate was actually made. As an example, from 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 Authority Having Jurisdiction (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, as back 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 their rating standard FAC-0008. Since cables do not have nameplates, the penalty was mocked at the time on internet forums that followed NERC compliance issues, but the penalty still ended up getting paid as the path of least resistance.


Key Takeaways


On your current or upcoming solar project, think about what the true ac output ampere requirements for your inverters are, design to meet those requirements safely, and don’t waste money unnecessarily. Using the NEC requires your brain to be fully engaged, because, as it states in Article 90, it “is not an instruction manual,” and 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 blog post series, in which we will cover whether your strict voltage drop criteria is actually making you money. Do you have utility-scale solar design and engineering questions? Get in touch with one of our experts today.


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