The bottom line question: Do I need 10 or 8 THHN?
Situation:
RESIDENTIAL REMODEL.
SERVICE PANEL: Ambient temperature 105 to 113 degrees max (Albuquerque, NM) with 60 degree CB terminals.
BRANCH CIRCUITS: Three 20 amp circuits (bathroom and 2 small appliance circuits) run as follows:
Six 10 AWG THHN conductors run in 1 1/4? EMT from the service panel, up the side of the house, and bent 90 degrees into a 140 degree attic. The EMT continues 10 more feet and connects to a junction box (where there is finally a little headroom), from which the Bath and Kitchen are fed with three 12/2 NM cables.
Calculations:
FIRST TRY:
Table 310-16 says 10 AWG THHN has an ampacity of 40 amps.
Derate by .71 for 140 degree attic: 40 X .71 = 28.4 amps.
Derate by .80 for six parallel conductors in a conduit: 28.4 X .80 = 22.7 final ampacity.
Protect each circuit with 20 amp CB.
SECOND TRY:
BUT the CB terminals are only rated for 60 degrees, so I cannot use the 90 degree rating of the 10 AWG THHN and must use the 60 degree column. Luckily, a 8 AWG THHN has an ampacity of 40 amps at 60 degrees. So the calculation is the same as 10 AWG at 90 degrees: 22.7 amps final ampacity of 8 AWG at 60 degrees.
THIRD TRY:
BUT service panel (and the side ofthe house) is only 113 degrees not 140. Also the wires are no longer in a conduit.
So, can I use 10 AWG THHN at the 60 degree ampacity of 30 amps and derate the service panel portion of the wires at 105 degrees (with no parallel conductor derating): 30 * .71 = 21.3 final ampacity.
If the answer is not straightforward, maybe I should avoid inspector issues and just use 8 AWG.
Situation:
RESIDENTIAL REMODEL.
SERVICE PANEL: Ambient temperature 105 to 113 degrees max (Albuquerque, NM) with 60 degree CB terminals.
BRANCH CIRCUITS: Three 20 amp circuits (bathroom and 2 small appliance circuits) run as follows:
Six 10 AWG THHN conductors run in 1 1/4? EMT from the service panel, up the side of the house, and bent 90 degrees into a 140 degree attic. The EMT continues 10 more feet and connects to a junction box (where there is finally a little headroom), from which the Bath and Kitchen are fed with three 12/2 NM cables.
Calculations:
FIRST TRY:
Table 310-16 says 10 AWG THHN has an ampacity of 40 amps.
Derate by .71 for 140 degree attic: 40 X .71 = 28.4 amps.
Derate by .80 for six parallel conductors in a conduit: 28.4 X .80 = 22.7 final ampacity.
Protect each circuit with 20 amp CB.
SECOND TRY:
BUT the CB terminals are only rated for 60 degrees, so I cannot use the 90 degree rating of the 10 AWG THHN and must use the 60 degree column. Luckily, a 8 AWG THHN has an ampacity of 40 amps at 60 degrees. So the calculation is the same as 10 AWG at 90 degrees: 22.7 amps final ampacity of 8 AWG at 60 degrees.
THIRD TRY:
BUT service panel (and the side ofthe house) is only 113 degrees not 140. Also the wires are no longer in a conduit.
So, can I use 10 AWG THHN at the 60 degree ampacity of 30 amps and derate the service panel portion of the wires at 105 degrees (with no parallel conductor derating): 30 * .71 = 21.3 final ampacity.
If the answer is not straightforward, maybe I should avoid inspector issues and just use 8 AWG.
