Looking to run circuits to a new room. To limit number of conduit runs would like to run one 1/2 conduit with 2, 3-wire circuits (6 conductors), total of 4 circuits. If I use 12 AWG wire do I get 4, 15 Amp circuits? When I derate for conduit fill (0.8) and attic temp. (O.76) I get 18 Amp as max for each circuit.
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More than likely, without getting into the technical details, you will be limited to the four 15 amp circuits.
if you feel the need for 20 amp circuits, try 3/4" EMT with #10 THHN conductors.
Tom
if you feel the need for 20 amp circuits, try 3/4" EMT with #10 THHN conductors.
Tom
You have it figured correctly.
The derating factor for 4 current carrying conductors is 0.8 and for ambient temperatures between 51°C and 55°C is 0.76. So, from table §310.16 the ampacity before deration is 30A for #12 THHN. (30 x 0.8 x 0.76 = 18A)
In a single phase multiwire circuit the neutral doesn't count as a current carrying conductor for deration (see §310.15(B)(4)). But in your case having 4 or 6 conductors makes no difference.
You end up with a derated ampacity of 18A for the #12 cable.
Based on §240.4(B) you can't round up to a 20A breaker if you have any receptacles on the circuit. If any receptacles are on the circuit you can only use a 15A breaker.
As a side note, remember that regardless of the §310.16 table value, #12 wire is limited to 20A by §240.4(D) unless you have a motor or welder application.
Homer
The derating factor for 4 current carrying conductors is 0.8 and for ambient temperatures between 51°C and 55°C is 0.76. So, from table §310.16 the ampacity before deration is 30A for #12 THHN. (30 x 0.8 x 0.76 = 18A)
In a single phase multiwire circuit the neutral doesn't count as a current carrying conductor for deration (see §310.15(B)(4)). But in your case having 4 or 6 conductors makes no difference.
You end up with a derated ampacity of 18A for the #12 cable.
Based on §240.4(B) you can't round up to a 20A breaker if you have any receptacles on the circuit. If any receptacles are on the circuit you can only use a 15A breaker.
As a side note, remember that regardless of the §310.16 table value, #12 wire is limited to 20A by §240.4(D) unless you have a motor or welder application.
Homer
