310.15 b 16

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Qojoe

Qojoe

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Hey guys, i must be missing something. If someone is using 310.15b 16 for sizing feeders how can you use 4/0 al and the 90c columm when 110.14(c)(1)(b)(1) says sizing anything over 100a use the 75c columm.

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Qojoe said:
Hey guys, i must be missing something. If someone is using 310.15b 16 for sizing feeders how can you use 4/0 al and the 90c columm when 110.14(c)(1)(b)(1) says sizing anything over 100a use the 75c columm.

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The 90c column is mostly just used for derating purposes. Ampacity ratings are almost always the 75c column (for reasons you stated).
 
Does that mean main lugs are on a panel are 90c rated? I ask this because for 200a services the norm out here is 4/0 al but if you use the 75c coluum you would need 250 kc mil (al) and every 200a service ive seen has had either 2/0 cu or 4/0 al for the ungrounded conductors. So im wondering how the 90c columm is used and when to use it.

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Qojoe said:
Does that mean main lugs are on a panel are 90c rated? I ask this because for 200a services the norm out here is 4/0 al but if you use the 75c coluum you would need 250 kc mil (al) and every 200a service ive seen has had either 2/0 cu or 4/0 al for the ungrounded conductors. So im wondering how the 90c columm is used and when to use it.
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The calculation for service entrance conductors allows their ampacity to be reduced to 83% so they can be smaller than the 75° C ampacity listed in 310.15(B)(16). Look at 310.15(B)(7).
 
infinity said:
The calculation for service entrance conductors allows their ampacity to be reduced to 83% so they can be smaller than the 75° C ampacity listed in 310.15(B)(16). Look at 310.15(B)(7).
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Just be be clear the reduced service conductor size is only for dwelling unit services.
 
Two factors are required in selecting minimum ampacity of a conductor.

First one is for the termination temperature rating - most of the time these days with new equipment that will allow us to use 75C, but you may occasionally need to select based on 60C

Second calculation is for conductor insulation limitations - this may be necessary when you have sections of the conductor in a high ambient temperature or have more then three current carrying conductors in a raceway.

The larger of those two selections is the minimum size conductor necessary for your application.

Then you have overcurrent protection rules to consider on top of minimum conductor ampacity.

Most of the time a conductor sized for 60 or 75 C will also be large enough for the overcurrent device of similar setting to the conductor ampacity. Also keep in mind we are allowed to go to the next size higher standard overcurrent device as long as the actual load doesn't exceed conductor ampacity for 800 amps and less overcurrent devices. This is a primary reason you see many 4/0 aluminum conductors (even when not applying 310.15(B)(7) allowances for dwelling services/feeders. 4/0 has an ampacity of 180 @ 75C, so as long as you don't have a load of 181 - 200 you still can protect it with a 200 amp overcurrent device. If your load is between 181 and 205 you must use at least 250 kcmil minimum conductor, but next standard size overcurrent device in that situation is 225 amps and could be used.
 
kwired said:
Most of the time a conductor sized for 60 or 75 C will also be large enough for the overcurrent device of similar setting to the conductor ampacity. Also keep in mind we are allowed to go to the next size higher standard overcurrent device as long as the actual load doesn't exceed conductor ampacity for 800 amps and less overcurrent devices. This is a primary reason you see many 4/0 aluminum conductors (even when not applying 310.15(B)(7) allowances for dwelling services/feeders. 4/0 has an ampacity of 180 @ 75C, so as long as you don't have a load of 181 - 200 you still can protect it with a 200 amp overcurrent device. If your load is between 181 and 205 you must use at least 250 kcmil minimum conductor, but next standard size overcurrent device in that situation is 225 amps and could be used.
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That seems to contradict what is says in 310.15(B)(7)(1).
 
infinity said:
That seems to contradict what is says in 310.15(B)(7)(1).
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My bad maybe, I meant to say you still see 4/0 aluminum on non dwelling applications because of the fact you still can use the next higher standard overcurrent device, so long as the load is not more then the 180 ampacity of the 4/0 aluminum.

310.15(B)(7) just happens to work out that a 200 amp service or feeder (that qualifies to use (B)(7) will also require minimum of 4/0 aluminum, unless any necessary adjustments cause you to need a larger conductor.
 
Qojoe said:
Does that mean main lugs are on a panel are 90c rated? I ask this because for 200a services the norm out here is 4/0 al but if you use the 75c coluum you would need 250 kc mil (al) and every 200a service ive seen has had either 2/0 cu or 4/0 al for the ungrounded conductors. So im wondering how the 90c columm is used and when to use it.

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It is very common that lugs are marked for 90C, but it is very rare that you can take credit for this rating. They have to be separately installed connectors, usually splices and taps. Manufactured product is rated as a listed assembly, which is usually 75C, occasionally 60C (rare for modern equipment, but officially the default condition for 100A and less).

As others have said, service conductors for single/duplex dwellings have an 83% factor in the algorithm, and often the 240.4(B) "next size up" rule may apply for feeders.
 
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