The NEC requires illumination for all working spaces about electrical equipment [110.34(D)], but does not define what's adequate. Theoretically, a single 40W incandescent lamp or a flickering fluorescent fixture 10 ft away is sufficient because it meets the "letter of the law." However, it would be pointless for the NEC to include this requirement if the intention were not to ensure that those working around electrical equipment have sufficient light to work safely.
NEC: Light ‘em Up
How much light is sufficient, and how do you provide it?