Rtu's

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raider1

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Logan, Utah
Welcome to the forum.:)

If the minimum circuit ampacity is 139 amps you must use a conductor with an ampacity of at least 139 amps.

#1 copper THHN has an ampacity of 130 amps at the 75 degree column of T310.16. So you would need to use a 1/0 copper THHN which has an ampacity of 150 amps according to T310.16 at 75 degrees C.

Take a look at Part IV of Article 440.

Chris
 

yired29

Senior Member
I have a rtu with minimum ampacity of 139 and maximum of 150 amps. Can i use #1 thhn cu. conductors for this application?
If the RTU has 90 degree terminantons then yes, I don't believe it does so you would need at least 1/0 under the conditions of use stated in table 310.16. Also one should be familiar with 110.14. For the most part the 90 degree column is only for corrections and adjustments. There are few applications where 90 degree is the maximum tempeerature at the point of termination
 
I was reading 240.4b,123 and it says i could use the next higher standard overcurrent device rating above the conductors being protected, if the three conditions are met, which i think they are
 

jeffhornsby

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Location
Destin, FL
If the RTU has 90 degree terminantons then yes, I don't believe it does so you would need at least 1/0 under the conditions of use stated in table 310.16. Also one should be familiar with 110.14. For the most part the 90 degree column is only for corrections and adjustments. There are few applications where 90 degree is the maximum tempeerature at the point of termination

Don?t forget to pluralize points to my knowledge there are no circuit breakers rated for 90C
 

augie47

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Tennessee
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State Electrical Inspector (Retired) DINOSAUR
I was reading 240.4b,123 and it says i could use the next higher standard overcurrent device rating above the conductors being protected, if the three conditions are met, which i think they are

The first "condition" of the conductor would be covered by 210.19(A):
210.19 Conductors ? Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served.

A pretty good rule of thumb is a three step process: (1) determine the load (in this case Art 440 applies--139 amps); (2) select the conductor
(in this case 210.19) and then select the overcurrent device (generally Art 240, but in this case, 240.4(G) allows you to apply Art 440.)
But in every instance the conductor ampacity must be equal or greater than the load.


and Wecome to the Forum.
 

don_resqcapt19

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Location
Illinois
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retired electrician
I was reading 240.4b,123 and it says i could use the next higher standard overcurrent device rating above the conductors being protected, if the three conditions are met, which i think they are
Typically these nameplates will have a box for "minimum circuit ampacity". The conductor cannot be smaller than this number. It is not an OCPD value and there is no rounding up.
 
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