90 C for THHN

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pvitt

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When calculating condutor ampacity for four or more current carrying THHN conductors in a conduit, do we use the 90 C column. Would we say 12 AWG is good for 30 amps and derate them to 80%? I'm just confused and now everyone at work is arguing about it. If the overcurrent device is rated for 20 amps is 80% 16 amps? I need to prove this one way or another.
 
pvitt said:
When calculating condutor ampacity for four or more current carrying THHN conductors in a conduit, do we use the 90 C column. Would we say 12 AWG is good for 30 amps and derate them to 80%? I'm just confused and now everyone at work is arguing about it. If the overcurrent device is rated for 20 amps is 80% 16 amps? I need to prove this one way or another.
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If you are using thhn then you would start with the 90C col. Since #12 is rated 30 amps taking 80% still leaves you with a 24 amp conductor. OCP still at 20 amps and you are good excluding the exceptions
 
O.K. what if the label in the panel reads, " use conductors with a 60 or 70 deg. C insulation" . Does this mean the lugs aren't rated for 90 deg. C?
 
pvitt said:
O.K. what if the label in the panel reads, " use conductors with a 60 or 70 deg. C insulation" . Does this mean the lugs aren't rated for 90 deg. C?
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If this is an old panel it could mean the lugs are only rated for 60/70C but you still could use 90C wire and derate as long as you don't use over the 60C rating . With #12 that is still 20 amps.
 
No lugs will be rated for 90 deg C. The maximum in the USA is 75 deg C. However, for derating purposes, the maximum cable temperature is used as the starting point.

Remember, your not saying that the cable is going to carry that many amps, your simply determining what the maximum amps you can carry is.

So, if I say a cable can carry 24A, but only put it on a 20A breaker, then it is not going to see anything over 20A, and in reality, your not supposed to load the circuit to more than 80%. #10, #12, and #14 are kind of anomalies anyway because they cannot be loaded to more than the NEC limit of 30, 20, and 15 amps respectively.
 
Thanks you guy's, I get it now. 310.15(B)(2) reads "the allowable ampacity shall be reduced" wich would refer you to 310.16 and on. So even with 9 conductors in the raceway, #12 THHN is still good for 21 amps. Thanks again and have a good one.
 
pvitt said:
So even with 9 conductors in the raceway, #12 THHN is still good for 21 amps. Thanks again and have a good one.
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That is correct, 9 is the magic number for 14, 12 and 10 AWG branch circuit wiring with 90 C conductors.

Once you get into larger conductors then 10 AWG you need to figure them on a case by case basis.
 
No one mentioned 110.14(C).

110.14 Electrical Connections.
Because of different characteristics of dissimilar metals, devices such as pressure terminal or pressure splicing connectors and soldering lugs shall be identified for the material of the conductor and shall be properly installed and used. Conductors of dissimilar metals shall not be intermixed in a terminal or splicing connector where physical contact occurs between dissimilar conductors (such as copper and aluminum, copper and copper-clad aluminum, or aluminum and copper-clad aluminum), unless the device is identified for the purpose and conditions of use. Materials such as solder, fluxes, inhibitors, and compounds, where employed, shall be suitable for the use and shall be of a type that will not adversely affect the conductors, installation, or equipment.

(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
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kingpb said:
So, if I say a cable can carry 24A, but only put it on a 20A breaker, then it is not going to see anything over 20A, and in reality, your not supposed to load the circuit to more than 80%.
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Got a code number for that rule?:)

steve
 
210.20(A). 80% for continuous (three hours or more) loads and 100% for loads less than 3 hours. To be conservative I always assume the loads are continuous.
 
bsh said:
210.20(A). 80% for continuous (three hours or more) loads and 100% for loads less than 3 hours. To be conservative I always assume the loads are continuous.
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Thats fine and often what I do as well, but not what the NEC requires.:smile:
 
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