I have looked at 100's of facilities for harmonics and what I have seen/experienced.
Most of my customers are commercial office buildings management companies, where by per the NEC there is an over design of the electrical distribution, more capacity than will ever be utilized TYPICALLY. Normal site actual load is about 30% capacity.
From the early days: While the site distribution equipment was over designed for site loads (the facilities I saw) the utility transformers were often border line, with the advent of VFD's there were quite a few utility company transformer that were replaced due to failures. Many of these transformers were old to start with. But in many cases the new transformers failed and it was not until the utility engineers, the customer or their contractor read an article or did research was the culprit narrowed down to harmonics. Another issue we saw was with the early electronic relays. We had data centers that demanded these 1st generation electronic relays as the hot new ticket. Some of the relays had issues with the high harmonic content of the voltage and current waveforms and were "nuisance" tripping".
Another type of facility I was involved with, were call large centers (something rare in America today) . These typically were located in a warehouse office park or in a commercial office building. Neither designed for the large number of side by side row after PC's. This was one place where over sized neutrals may have been helpful. Though with the high number of switch mode power supplies and operating transformers at 90-100%, even neutrals would not have prevented the transformer failures we saw. I also so harmonics issues become a concern at casinos, as the one armed bandits were changed out from mechanical to electronics. Once again the transformers paid for this over site.
THe call centers, casinos and a few TV/radio stations were one of the few places I saw neutral currents approaching 130% of load currents.
In the typical office building common in Washington DC, I just have not seen an issue. I have checked lighting panels and never saw a concern for not using MWBC's or for over sizing neutrals.
In the data centers around Washington DC, I have seen a few issues BUT once again these were typically in converted office buildings that now had a glut of computers that they system was never designed for. IN the data centers with sensible design we seldom see issues.
The last place we have seen MAJOR issues especially in the early days of large and small data centers, was with generators. Many of the generators had mechanical governors and were sized for the load with no consideration for any possible harmonic distortion. The generators would fall on their butt when needed for emergency situations. This was also the era of little to no site testing and even if there was site testing the loads were resistive load banks.
With proper design, taking into consideration the needs of the tenant and their loads, regular maintenance that monitors site load and a customer that is diligent tracking future loads harmonics should not be an issue.
