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feeder calculations #216059
09/20/15 05:18 PM
09/20/15 05:18 PM
watersparkfalls  Offline OP
Member
Joined: Mar 2005
Posts: 222
Washington...Not DC
trying to figure out this 110.14(c) and why it lets you use the 90 degree column for derating when more than 3 current carrying conductors applies... is this for everything feeders, branch circuits? I always thought if you land in a lug (disco, panel) you had to calculate using the 75 degree column.
thanks,
H2o

2017 / 2014 NEC & Related Books and Study Guides
Re: feeder calculations [Re: watersparkfalls] #216061
09/20/15 07:57 PM
09/20/15 07:57 PM
HotLine1  Offline

Member
Joined: Apr 2002
Posts: 6,917
Brick, NJ USA
H2O:

Basically, in a short, plain English statement; you can use the 90 degree col. for derating calculations, but you must not exceed the 60 or 75 degree maximums after your calculations.


John
Re: feeder calculations [Re: watersparkfalls] #216062
09/20/15 08:04 PM
09/20/15 08:04 PM
G
gfretwell  Offline

Member
Joined: Jul 2004
Posts: 9,166
Estero,Fl,usa
The 75c is for the lug. With a simple 3 wires in a pipe, that will usually be the limiting factor.
As soon as you get more wire in the pipe and start running it through hostile areas where you have "adders" on, you are happy you can start from 90 and the wire is still going to be the limiting factor.


Greg Fretwell
Re: feeder calculations [Re: watersparkfalls] #216068
09/21/15 09:40 AM
09/21/15 09:40 AM
G
ghost307  Offline
Member
Joined: May 2005
Posts: 926
Chicago Illinois USA
Also; you can only start in the 90 degree column if the insulation is rated at 90 degrees...like THHN is.


Ghost307
Re: feeder calculations [Re: watersparkfalls] #216069
09/21/15 11:42 AM
09/21/15 11:42 AM
G
gfretwell  Offline

Member
Joined: Jul 2004
Posts: 9,166
Estero,Fl,usa
90c is becoming the norm these days since the introduction of THWN-2 and the dual rated THHN/THWN-2 conductor.
You still need to be sure what you have but what you need is usually available.


Greg Fretwell
Re: feeder calculations [Re: watersparkfalls] #216072
09/22/15 12:53 AM
09/22/15 12:53 AM
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA
To elaborate:

If the Insulation is rated for 90C, the derating may begin at the Ampacity in that column, per Table 310.16; however the actual Termination Ampacity is based on the 60C Column (for Conductors size #2 and smaller), or the 75C Column (for Conductors size #1 and larger).

Examples:

Ex. 1:
(2) 20 Amp 3 Phase 4 Wire multiwire Circuits run in single Raceway.

All Conductors are #12 THHN CU., and are Current Carrying; total of (8) Current Carrying Conductors.

Derating value for 7-9 Conductors = 0.7
Table 310.16 Ampacity for #12 THHN CU. at 90C = 30 Amps.
30 Amps X 0.7 = 21 Amps
Maximum OCPD is 20 Amps (for general circuits)

The derated value of 21 Amps is greater than the Maximum OCPD rating of 20 Amps, therefore the Circuits Terminated to 20 Amp OCPD's are compliant.

............................................................

Ex. 2:
(3) 30 Amp 3 Phase 4 Wire multiwire Circuits run in single Raceway.

All Conductors are #10 THHN CU., and are Current Carrying; total of (12) Current Carrying Conductors.

Derating value for 10 - 20 Conductors = 0.5
Table 310.16 Ampacity for #10 THHN CU. at 90C = 40 Amps.
40 Amps X 0.5 = 20 Amps
Maximum OCPD for #10 CU is 30 Amps (for general circuits).
Derated value of 20 Amps is less than the maximum OCPD value of 30 Amps; therefore, to be compliant, the Circuits will be Terminated to 20 Amp OCPD's.

That is a basic version of the derating methods.

Feel free to respond.

--Scott (EE)


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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