What does the NEC actually require in the way of overcurrent protection for conductors supplied by a generator?
One of the more confusing areas of the Code involves the what, where, and when of overcurrent protective devices protecting conductors originating at generator terminals. We recently received a question as to whether any over-current protection was required at all for these conductors, and if so, where it had to be located. The questioner noted that some generators, especially those with series-boost systems, can supply very large ground-fault or short-circuit currents. This question is not as simple as it looks.
The EC&M Panel's response
The place to begin is Sec. 240-3, which requires conductors to be "protected against overcurrent in accordance with their ampacities as specified in Sec. 310-15, unless otherwise permitted or required in (a) through (m) below." In the common case where there is no such protection at the generator, this main rule cannot be complied with, and one of the subsections that follow must be applied. The reason for this is that overcurrent protection involves not just overload protection, which can be provided at the load end of a conductor, but short-circuit and ground-fault protection as well. This type of protection must always be ahead of a conductor.
The only possible subsection that could apply is Sec 240-3(d). This section allows tap conductors to be protected against over-current in accordance with any of several other Code rules, of which the only one that applies here is Sec. 240-21. This section comprises most of the familiar tap rules in the Code, including Sec. 240-21(i) for generator conductors.
Note that the phrase "protected against overcurrent" as used in Sec. 240-3(d) isn't really correct. Taps may or may not have ground-fault and short-circuit protection ahead of them and even if they do, the nominal rating of that protection will exceed, usually by many times, the usual rating for overcurrent devices ahead of similar conductors. For example, if the instantaneous pick-up (the relevant specification for short-circuit or ground-fault clearing) on a particular circuit breaker is (or is set at) seven times its nominal rating, and that type of circuit breaker is the protection for the feeder from which a 25-ft tap (3:1 maximum ampacity ratio) is made per Sec. 240-21(c)(2), then the instantaneous trip setting for the breaker ahead of the tap could be more than 21 (7 x 3) times its rating.
Therefore, in the true sense of the word, taps don't have short-circuit and ground-fault protection; at least, certainly not the same as for conventional conductors. They frequently, however, do have overload protection because many of the tap rules require their tap terminations to be at a single breaker or set of fuses. Overload protection without short-circuit and ground-fault protection simply isn't overcurrent protection. In the case of generator leads, there will be overload protection, but you wouldn't know it from the reference that appears in Art. 240.
Overload protection for generator conductors
This is "covered" in Sec. 240-21(i):
(i) Conductors from Generator Terminals. Conductors from generator terminals shall be permitted to be protected against over-current in accordance with Section 445-5.
As we have seen, the phrasing "protected against overcurrent" is incorrect, since ground-fault and short-circuit protection cannot be provided for these conductors. However, they could be provided with overload protection. Perhaps that is what this wording intends? Let's review Sec. 445-5:
445-5. Ampacity of Conductors. The ampacity of the phase conductors from the generator terminals to the first overcurrent device shall not be less than 115 percent of the nameplate current rating of the generator. It shall be permitted to size the neutral conductors in accordance with Section 220-22. Conductors that must carry ground-fault currents shall not be smaller than required by Section 250-23(b).
This section is completely directed at the allowable ampacity of the conductors fed from the generator. It neither specifies allowable overcurrent device ratings or settings, nor does it specify any permitted location or maximum distance from the generator. Therefore, there is no mandated overload protective device in this section.
This doesn't mean, however, that these conductors would be likely to fail due to overloading, for another reason. Sec. 445-4(a), which does address overcurrent protection, requires overload protection for constant voltage AC generators of the type referred to in the question. This protection can either be inherent to the generator or provided by separate devices. When you apply this rule along with the one in Sec. 445-5, the conductors are effectively protected against overloads. This arrangement (generator protection plus 115% rated conductors) has been in the NEC since 1940, and it seems to have stood the test of time.
It is true, however, that the location of this protective device isn't specified. One reason is that generators come in vastly differing sizes, and some of the larger ones simply can't comply with arbitrary distances, such as 25 ft. Nevertheless, although an overcurrent protective device can provide overload protection wherever it is placed in the circuit, it can only provide short-circuit and ground-fault protection to the portion of circuit downstream of itself. Each additional foot of unprotected conductor running from a generator represents some additional exposure to the building.
Therefore, even though at present there isn't any express requirement to place the overcurrent protection close to the generator source, clearly it is in the best interest of safety to have it as close as practicable. As the question noted, some generators can indeed supply significant fault current.