A wire can be perfectly safe from an overheating standpoint and still starve a device of voltage over a long run — checking only the breaker rating misses the second failure mode entirely.
Ampacity (NEC Table 310.16) is a safety limit on heat; voltage drop is a performance limit on distance and resistance. A 12 AWG circuit safely rated for 20A can still show a 3%+ voltage drop at 50 feet — safe and underperforming at the same time.
Estimate with the rule, then check it against the calculator that models it properly.
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