Nec ?

[Tigger08]

Hello, This is a NEC ? I know that a type TW cable containing two No 12 copper conductors has a maximum overcurent protection of 20 amps. If you look at the NEC table 4, it shows #12* copper max overcurent protection of 25 with 60^ (140f) ... with the * the foot notes show unless specifically permitted in NEC Section 240-3 the overcurrent protection shall not exceed 20 amperes for No. 12. for ambient temperature. could someone shed some light on 25amps?

 

[infinity]

Hello, This is a NEC ? I know that a type TW cable containing two No 12 copper conductors has a maximum overcurent protection of 20 amps. If you look at the NEC table 4, it shows #12* copper max overcurent protection of 25 with 60^ (140f) ... with the * the foot notes show unless specifically permitted in NEC Section 240-3 the overcurrent protection shall not exceed 20 amperes for No. 12. for ambient temperature. could someone shed some light on 25amps?

Where is table 4?

 

[roger]

Can you explain what reference material you are using? Type TW is not a cable assembly and your use of 240-3 would be an older version of the NEC.

Roger

 

[Tigger08]

Yes Rob, Hi ... It is just a reprinted from nfpa 70-1999

 

[Tigger08]

It is also in the 2008 NEC as well.

 

[roger]

You will need to invest in a recent NEC, I doubt there any exams given on the 99 at this time.

Roger

 

[roger]

It is also in the 2008 NEC as well.

Where? What page are you looking at?

Roger

 

[Twoskinsoneman]

Hello, This is a NEC ? I know that a type TW cable containing two No 12 copper conductors has a maximum overcurent protection of 20 amps. If you look at the NEC table 4, it shows #12* copper max overcurent protection of 25 with 60^ (140f) ... with the * the foot notes show unless specifically permitted in NEC Section 240-3 the overcurrent protection shall not exceed 20 amperes for No. 12. for ambient temperature. could someone shed some light on 25amps?

To answer your question and not just give you a hard time...
From the 08:
240.4(D) Small Conductors. Unless specifically permitted in
240.4(E) or (G), the overcurrent protection shall not exceed
that required by (D)(1) through (D)(7) after any correction
factors for ambient temperature and number of conductors
have been applied.

There is a list, of places where the OCPD can exceed those listed in (D)(1)-(D)(7) The list is in "....(E) or (G)...."

Read through 240.4(E) and especially (G).

 

[Tigger08]

Just a sec. I am looking it up.

 

[Tigger08]

page 310.15 of the 2008 nec.

 

[roger]

page 310.15 of the 2008 nec.

310.15 is an article section, it is not a page number and I'm not trying to give you a hard time. You'll have to get a lot more savvy with your code book before you take an exam.

Roger

 

[infinity]

page 310.15 of the 2008 nec.

Do you mean Table 310.16?

 

[Tigger08]

Thank You ..But on article 310.15 Table 310.16 under awg of #12* out to the right it shows under types tw, uf 25 amps ... temperature rating of 60^ c (140^ f ) why is over the 20 amps...

 

[Tigger08]

Yes .... ........

 

[roger]

Thank You ..But on article 310.15 Table 310.16 under awg of #12* out to the right it shows under types tw, uf 25 amps ... temperature rating of 60^ c (140^ f ) why is over the 20 amps...

Read the article sections twoskins pointed you to.

Roger

 

[winnie]

Table 310.16 gives the _ampacity_ of conductors. It doesn't tell you the current that the conductor is permitted to carry in all circumstances; it simply tells you one feature that may limit the installation.

Ampacity is the maximum current that a conductor can carry without overheating, given the specified ambient conditions and conductor insulation temperature limits. #12 THWN can carry 25A without overheating, and if you have 90C insulation, then it can carry 30A without overheating.

Other parts of the code place _additional_ restrictions on #12 conductors. In general, #12 may only be protected by a 20A or smaller OCPD. I believe that this is for reasons of short circuit protection, not overload protection...but a side effect is that the continuous loading is reduced.

Hmm, copper.org would hate it, but I wonder if 'small conductor' OCPD could be designed which would safely permit small conductors to be used to their full ampacity.

-Jon

 

[Tigger08]

Thanks All ,, have a good night..

 

[nakulak]

table 310.16 has a note at the bottom which refers you to 240.4. Different gauge conductors have different ampacities (read the definition in section 100 of ampacity), but constraints are put on those ampacities by the code, and you have just discovered that. simply put for your case, the ampacity of 12 awg is 30 amps, but 12 awg cannot have ocpd above 20 except for the uses described in 240.4. kapish ?

 

[jim dungar]

...but I wonder if 'small conductor' OCPD could be designed which would safely permit small conductors to be used to their full ampacity.

'Small conductors" can be used above the OCPD limit placed on #14, #12, and #10 branch circuits; motor loads is the first thing that comes to mind.

 

[winnie]

To clarify,

I wonder if it is possible to design a circuit breaker with trip characteristics which would permit small conductors to be used at their thermal ampacity in general purpose and receptacle circuits.

-Jon