Old Bathroom Circuit Overload and Box Fill
A 1955 dwelling has a 15 A branch circuit that feeds the hall receptacle, the bathroom vanity receptacle, the vanity light, and the exhaust fan. The bathroom receptacle is an old two-wire non-grounding type, and the box is a 3 in × 2 in × 2 1/4 in device box. The owner adds a 1,500 W hair dryer and a 1,200 W portable heater on the same circuit; the 15 A breaker trips after several minutes. The owner wants to replace the 15 A breaker with a 20 A breaker, install a GFCI receptacle, and continue using the existing 14 AWG conductors. The panel has no spare spaces, and the bathroom is on the first floor above a crawlspace. The hall receptacle is not GFCI protected and is 4 ft from the bathroom door.
1. Can the 15 A breaker be replaced with a 20 A breaker on the existing 14 AWG circuit? Explain the overcurrent protection and conductor sizing.
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No. The existing 14 AWG conductors are limited to a 15 A overcurrent device by 240.4(D). Replacing the 15 A breaker with a 20 A breaker would allow the conductors to overheat before the breaker trips. The breaker is tripping because the load is too high: 1,500 W + 1,200 W = 2,700 W, or 22.5 A at 120 V, plus the lights and fan. The correct solution is to add a 20 A bathroom receptacle circuit with 12 AWG copper and a GFCI receptacle, and keep the existing 15 A circuit for the lights and fan or replace it as needed. Do not upsize the breaker to stop nuisance tripping.
2. What circuiting and GFCI requirements apply to the bathroom receptacle, lights, fan, and hall receptacle?
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A dwelling bathroom needs at least one 20 A branch circuit for bathroom receptacles. If the circuit supplies only one bathroom, it may also supply other equipment in that same bathroom, such as the vanity light and exhaust fan. It cannot feed the hall receptacle. The bathroom receptacle must be GFCI protected. The hall receptacle is not required to be GFCI unless it is in another location covered by the GFCI rules or within the adopted distance of a sink. The existing two-wire receptacle can be replaced with a GFCI if the equipment grounding conductor requirement is handled; a GFCI can protect a grounding-type receptacle on a two-wire circuit only if marked 'GFCI Protected' and 'No Equipment Ground,' but the better fix is to run an equipment grounding conductor. Confirm AFCI requirements for the bathroom circuit under the adopted edition.
3. Can the existing 3 × 2 × 2 1/4 in box accept a GFCI receptacle with two 12/2 NM cables? Show box-fill reasoning.
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The existing 3 × 2 × 2 1/4 in box has a volume of 13.5 cu in. With one 12/2 NM cable: two circuit conductors + one ground counted as one = 3 × 2.25 = 6.75; one GFCI yoke = 4.50; total 11.25 cu in, which fits. With two 12/2 NM cables: four circuit conductors + one ground = 5 × 2.25 = 11.25; plus GFCI yoke 4.50 = 15.75 cu in, which exceeds 13.5. If cable clamps are present, add 2.25 and it is worse. Therefore the box cannot accept two 12/2 cables and a GFCI. Use a larger box or a 2-gang box, and account for the plaster ring if one is used.