The Dean of Home Renovation & Repair Advice

# COMMUNITY FORUM

jakelittel

08:57AM | 02/17/05
Member Since: 02/16/05
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.

Tom O

02:27PM | 02/17/05
Member Since: 09/17/02
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

HomerS

02:47PM | 02/17/05
Member Since: 03/20/04
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 ¬ß310.16 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 ¬ß310.15(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 ¬ß240.4(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 ¬ß310.16 table value, #12 wire is limited to 20A by ¬ß240.4(D) unless you have a motor or welder application.

Homer

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