Adding an electrical load from the main bus on, might
not be a major mod in certain installations. If it is not major, it is minor (determination of which is the intended use of the 571 Appendix A test). If it is determined to be minor, then data described in 571.06 as "acceptable" might be a suitable reference for the installation. That would include AC43.13, which includes information on determining electrical loads, and the capacity of conductors.
The design requirements (FAR Part 23, for example) are more rigorous in terms of electrical compliance of a modification/installation than AC43.13, which is the purpose of the application of Appendix A, to determine if the installer should be seeking data which is described as "approved" in 571.06 - which would include an STC (which I might issue). The demonstration of compliance for the electrical requirements required for STC issuance has more factors than need be "complied with" for a minor mod sign off.
Happily, these days, the replacement of older avionics, and some lighting, can result in very pleasing electrical load reductions, and that is good all the way around. I am regularly asked to STC approve survey and science electrical systems, some of which draw considerably more than the airframe electrical system can accommodate within its original capacity. This becomes particularly challenging when an aircraft with two alternators can carry the load when both are running, but not on only one. What happens if you're surveying along, and you loose an engine (and thus its alternator)? The other alternator tries to take up the whole load, and it too fails before the pilot can shed the load. for this, I insist on special automatic load shedding systems.
These auto load shedding systems are also required when the battery capacity of a single engine aircraft is close to the line. I've had electrical modifications where the ELA demonstrated that the capacity of the factory battery exceeded the required 30 minute [failed alternator] supply time by 25 seconds. That's not much to work with, to take up extra load, and to allow for pilot recognition of the failure, and manually load shed. These were found design compliant based upon functioning automatic load shedding systems, which relieved the pilot of that added duty.
Obviously, these systems were installed between a generating source and a primary bus, so were major mods in and of themselves, let alone the survey system they powered. STC required.
I have had many occasions when the major/minor decision was fuzzy, even when using the test. Often, the AME/AMO/Owner elected to have me STC approve the mod, rather than try to support a finding of "minor". Totally their choice. Though costly, I can STC approve minor mods, and doing so ends all arguments as to the mod possibly requiring approved data - now you have it.
Changing an entire electrical system, exactly in accordance with the parts catalog from 14 to 28 volt (or vice versa) is major, and requires an STC, because it alters the original aircraft between the bus and the alternator, even though it is also a factory system.
A much less clear trouble zone for AME's/AMO's is the wording at the bottom of every STC, and its implications (which can include electrical). It reads:
".... Prior to incorporating this modification, the installer shall establish that the interrelationship between this change and any other modification(s) incorporated will not adversely affect the airworthiness of the modified product."
That task is a tall order, and documenting that it has been thoroughly completed is more than just a brief statement. This is where the accumulation of electrical installations - even individually minor, or STC approved, can total to requiring a separate approval just to permit them to be co installed on the same aircraft. STC required, even if none in isolation was a major mod.
From the Appendix A test:
(10) affect instruments, or indicators that are installed as part of a system required by the approved type design?
EMI. If you said "no" to this, to declare the electrical change minor, and use AC43.13, you could still have an EMI affect. I've done the testing and seen that the poor positioning of a single wire, not at all associated with comm or GPS, none the less causes a GPS failure when the comm transmits.
I encourage the application of the Appendix A test as a means to legitimize a mod as minor - if it is. Our industry is not served well with the expense and delays associated with STC approval of mods which are not major. Transport Canada staff will respect more the use of a TC procedure, when it correctly employed - that helps everyone. But that places the whole evaluation task in the hands of the AME/AMO, as the use of the Appendix A test is a maintenance task, not an aircraft certification task. Correct interpretation and application of the test is important, as well as the determination of possible interrelationships of multiple STC's. On that topic, in an appropriate thread, we can discuss the unfavourable interrelationships between engine changes and STOL kits, and some combinations of Cessna wing mods STCs which AME's/AMO's should be aware of - 'cause they're signing for them!