# Size of cable compared to breaker



## 480sparky (Sep 20, 2007)

A feeder with an ampacity of 70a on an 80a breaker is a violation.


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## mbednarik (Oct 10, 2011)

Cable size should be enough to carry the load applied. example, you have a 50 amp continuous load. 50 X 125% = 62.5 amp load. The conductors would have to be large enough to carry 62.5 amps after any/all derating for temp or bundling. Now reference standard OCPD (overcurrent protection device) sizing in 240 to find that would would put that general load on a 70 amp OCPD. If the type of load is any found in table 240.4 G special OCPD dizing applies, refer to the corresponding art.


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## m sleem (May 16, 2012)

480sparky said:


> A feeder with an ampacity of 70a on an 80a breaker is a violation.


but i know we can size the feeder by the previous step of the breaker....but i'm not sure this in regard to all type of loads or only motors.


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## McClary’s Electrical (Feb 21, 2009)

480sparky said:


> A feeder with an ampacity of 70a on an 80a breaker is a violation.


 

Way too broad of a statement. 

What if it's an outside tap? What if it's a tap? What if these are transformer secondary conductors with no overcurrent protection?


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## McClary’s Electrical (Feb 21, 2009)

m sleem said:


> but i know we can size the feeder by the previous step of the breaker....but i'm not sure this in regard to all type of loads or only motors.


 
If your feeders ampacity is not corresponding to a common size breaker, you can got to the next size up as long as you're not over 800 amps. 
There is a standard size 70 amp breaker, so you can't just go to 80.


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## Speedy Petey (Jan 10, 2007)

m sleem said:


> but i know we can size the feeder by the previous step of the breaker....but i'm not sure this in regard to all type of loads or only motors.


Depends on who's code you are referring to. 
I am only familiar with the NEC.


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## 480sparky (Sep 20, 2007)

mcclary's electrical said:


> Way too broad of a statement.............


The OP is way to ambiguous for anything else.


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## Aussielec (Apr 1, 2012)

m sleem said:


> I want to confirm that:
> 
> for example: Breaker size is 80 AT
> 
> ...


 
This statement confuses me(along with everybody else). You size your circuit breaker to be less than the maximum current carrying capacity of your cable. 

Your ciruit breaker is there to protect the cable and nothing else.

If you put an 80amp CB on a cable that is rated to a maximum of 70amps that is violation of any code in any country through out the world period....(sorry drunk)


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## m sleem (May 16, 2012)

mcclary's electrical said:


> If your feeders ampacity is not corresponding to a common size breaker, you can got to the next size up as long as you're not over 800 amps.


could you show an example for this case ?.......our friends deny it.


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## wildleg (Apr 12, 2009)

as I posted in your other thread on almost the same subject, the NEC does not apply to your situation if you are in a Saudi Territory



m sleem said:


> i'm egyptian but i'm working in saudi arabia. ....


 


wildleg said:


> ok, I confirmed that the NEC does not apply, so all the other replies to your motor feeder thread simply do not apply, unless you are not in a Saudi Arabian territory. This is from article 401 of the Saudi Building Code (Electrical)
> 
> 
> *12-3 *
> ...


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## Chris Kennedy (Nov 19, 2007)

m sleem said:


> could you show an example for this case ?.......our friends deny it.


From the 2008 NEC



> 240.4(B) Devices Rated 800 Amperes or Less.
> 
> The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
> 
> ...


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## m sleem (May 16, 2012)

Chris Kennedy said:


> From the 2008 NEC


so clear .....very nice


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## m sleem (May 16, 2012)

Chris Kennedy said:


> From the 2008 NEC


so clear .....very nice :thumbsup:


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## Chris Kennedy (Nov 19, 2007)

m sleem said:


> so clear .....very nice :thumbsup:


Interesting, not something you hear often about the NEC.:laughing:


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## erics37 (May 7, 2009)

wildleg said:


> as I posted in your other thread on almost the same subject, the NEC does not apply to your situation if you are in a Saudi Territory


As you can tell from extensively researching the Saudi electrical code, it is rather lacking, which is probably why the OP is here asking this stuff. Why do you care what guideline they are under?


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## B4T (Feb 10, 2009)

This goes along with post #12.. 

240.6 Standard Ampere Ratings.
(A) Fuses and Fixed-Trip Circuit Breakers. The standard
ampere ratings for fuses and inverse time circuit
breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50,
60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300,
350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000,
2500, 3000, 4000, 5000, and 6000 amperes. Additional
standard ampere ratings for fuses shall be 1, 3, 6, 10, and
601.The use of fuses and inverse time circuit breakers with
nonstandard ampere ratings shall be permitted.


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## wildleg (Apr 12, 2009)

erics37 said:


> As you can tell from extensively researching the Saudi electrical code, it is rather lacking, which is probably why the OP is here asking this stuff. Why do you care what guideline they are under?


Don't you think that it is appropriate to apply the code that specifically applies to the situation ? the Saudi code was initiated in 2007, and the section I quoted specifically references three other codes to use if the letter of their Article 401 isn't clear. The IEC is one of the codes that applies, but nowhere is the NEC referenced. Would you be so quick to quote European codes for work in the US ? I doubt it. I can't for the life of me understand why you would be so quick to defend the quoting of a code that doesn't even apply to the installation, given the information that was stated ? If there is no reference in their code to the NEC, doesn't it make you consider the notion that there is a reason for that ?


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## 1capybara (Feb 27, 2009)

mbednarik said:


> Cable size should be enough to carry the load applied. example, you have a 50 amp continuous load. 50 X 125% = 62.5 amp load. The conductors would have to be large enough to carry 62.5 amps after any/all derating for temp or bundling. Now reference standard OCPD (overcurrent protection device) sizing in 240 to find that would would put that general load on a 70 amp OCPD. If the type of load is any found in table 240.4 G special OCPD dizing applies, refer to the corresponding art.


ok but what about 20% spare capacity for the feeder and the breaker size.
50A continuous load x 125% = 62.5 amps
20% spare capacity = 75 amps
use 80 amp breaker and size feeder to carry not less than 80 amps= 4 awg
[not counting deration for temp or bundling]


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## erics37 (May 7, 2009)

1capybara said:


> ok but what about 20% spare capacity for the feeder and the breaker size.
> 50A continuous load x 125% = 62.5 amps
> 20% spare capacity = 75 amps
> use 80 amp breaker and size feeder to carry not less than 80 amps= 4 awg
> [not counting deration for temp or bundling]


Nothing in the code requires that you size for spare capacity. If you have a calculated continuous load of 62.5 amps and you could find a 62.5 amp overcurrent device then you could use that. Otherwise you can adjust the OCPD size to one of the sizes on the standard list in 240.6. Some feeders (like ones supplying motors for instance) don't allow you to size to the next standard OCPD up, so in those cases you're stuck using the next size down.


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## Magnettica (Jan 23, 2007)

240.4(B)

*Overcurrent Devices Rated 800 Amperes or Less.*
The next higher standard overcurrent device rating (above
the ampacity of the conductors being protected) shall be
permitted to be used, provided all of the following conditions
are met:
(1) The conductors being protected are not part of a branch
circuit supplying more than one receptacle for cordand-
plug-connected portable loads.
(2) The ampacity of the conductors does not correspond
with the standard ampere rating of a fuse or a circuit
breaker without overload trip adjustments above its rating
(but that shall be permitted to have other trip or
rating adjustments).
(3) The next higher standard rating selected does not exceed
800 amperes.


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## 1capybara (Feb 27, 2009)

Agreed, "spare capacity" isnt mentioned in NEC, However : 
1. branch ckts for convenience outlets 15-30A [table 210.21B2] 
cannot exceed 80% of OCPD
2. three (3) phase feeder ckts with neutrals [table 310.15B2a] shall not exceed 80% of ampacity


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## erics37 (May 7, 2009)

1capybara said:


> Agreed, "spare capacity" isnt mentioned in NEC, However :
> 1. branch ckts for convenience outlets 15-30A [table 210.21B2]
> cannot exceed 80% of OCPD
> 2. three (3) phase feeder ckts with neutrals [table 310.15B2a] shall not exceed 80% of ampacity


There are a lot more types of loads requiring circuits to be sized that way if they are continuous. The NEC doesn't actually say "you can load this circuit up to 80%" it only says (in most applications), "Size your circuit at 125% of continuous loads plus 100% of non-continuous."

I think it has more to do with the cumulative heating effect of running a load through a conductor continuously than anything else. Everything you're saying is correct but it's misleading and confusing to stray from standard NEC terms when in an NEC discussion. It shouldn't be construed that the 20% of wiggle room is "spare capacity."

However I can't say I haven't used it that way before :whistling2:


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## 1capybara (Feb 27, 2009)

@erics37: 
agreed


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