Voltage drop Vs Breaker size

[Hameedulla-Ekhlas]

Greeting all,
I have calculated one feeder and breaker size for a project. The total final ampere is 296 ampere included 1.25 for continues.

I have sized breaker 300 ampere with a feeder which has a ampacity capacity of 315 ampere.

Due to voltage drop to compensate the voltage drop the cable size increased one step which has now a ampere capacity of 400.

Now I have got a comment which says " Since your Cable current capacity is 400 you must increase Breaker size too."

I replied back with reference NEC-2008 artical-240, page-179 with Exhibit 240.7.

In this example everything is clear about cable increase size due to voltage drop.

But still I got this comment back to increase the breaker size.

Would anyone please advise me that is there any other reason for this or I have replied right.

 

[Dennis Alwon]

You could increase the breaker size but you don't have too. Perhaps they are referring to art. 250.122(B) for the EGC if there is one.

 

[iwire]

There is no NEC requirement that the the overcurrent protection must be increased in size to match the conductor ampacity.

 

[Hameedulla-Ekhlas]

art. 250.122(B) explains about the ground conductor should be increased with a ratio that the feeder cable has been increased due to voltage drop.
This artical doest say anything about breaker size to be increased.

 

[chris kennedy]

Now I have got a comment which says " Since your Cable current capacity is 400 you must increase Breaker size too."

A comment from whom?

I replied back with reference NEC-2008 artical-240, page-179 with Exhibit 240.7.

In this example everything is clear about cable increase size due to voltage drop.

That exhibit is in reference to tap conductors.

 

[Hameedulla-Ekhlas]

I got this comment from Corps of engineering in Kabul.


NEC-2008 artical-240, page-179 with Exhibit 240.7.

This artical also explains the same situation which a conducture has been increased to compensate the voltage drop and the breaker size has been remained same.

Am I right or not

 

[jwjrw]

Im not aware of a situation in the nec that says you can't underfuse a conductor. It just says you cant overfuse it.

 

[chris kennedy]

Am I right or not

Does your proposed installation meet the definition of 'tap conductors' in 240.2?

 

[Hameedulla-Ekhlas]

Suppose there code nothing says about this issue

you have faced this problem. Will you increase the cable size too or not please.

 

[jwjrw]

I always increase wire size on longer runs and everybody I knows also does

 

[Hameedulla-Ekhlas]

I am completely agree with you and it doest make sense to increase breaker size due to voltage drop


Thanks all guys for information. I really appreciate your information and i really learn alot of things from you all.

 

[Dennis Alwon]

art. 250.122(B) explains about the ground conductor should be increased with a ratio that the feeder cable has been increased due to voltage drop.
This artical doest say anything about breaker size to be increased.

Hameedulla, I realize 250.122(B) was for the EGC, in fact, I stated that. I thought maybe that was what the comment that you received was about. I also stated that the breaker size would not have to be increased but it would not hurt and may be a good idea for future expansion.

 

[chris kennedy]

Suppose there code nothing says about this issue

647.4(D) and 250.122(B) aside, the NEC has no requirements to size conductors for voltage drop. Only non-enforcable FPN's in 210.19, 215.2, 310.15 and 455.6.

you have faced this problem. Will you increase the cable size too or not please.

Yes, often.

 

[Hameedulla-Ekhlas]

Yes, you are right I really appreciate your idea. I know if we increase it is good point for future.

For one thing , it is already included 25% for future

On the other hand, It costs expensive to buy instead of 300 ampere breaker a 400 ampere breaker

I really appreciate your idea.
thanks

 

[RETRAINDAILY]

Greeting all,
I have calculated one feeder and breaker size for a project. The total final ampere is 296 ampere included 1.25 for continues.

I have sized breaker 300 ampere with a feeder which has a ampacity capacity of 315 ampere.

Due to voltage drop to compensate the voltage drop the cable size increased one step which has now a ampere capacity of 400.Now I have got a comment which says " Since your Cable current capacity is 400 you must increase Breaker size too."

I replied back with reference NEC-2008 artical-240, page-179 with Exhibit 240.7.

In this example everything is clear about cable increase size due to voltage drop.

But still I got this comment back to increase the breaker size.

Would anyone please advise me that is there any other reason for this or I have replied right.

you have derated the wire to a lower value it is no longer good for 400 amp
it is good only for the derated value.

 

[Hameedulla-Ekhlas]

I am agree, the cable has been derated but if I increase the breaker size it cost expensive to buy a 400 ampere breaker instead of 300 ampere

 

[gar]

100124-1238 EST

From a logical and theoretical point of view consider the following:

A breaker or fuse at the input end of a wire is there to protect the wire and not the load. However, it could also protect the load. Thus, the design of the breaker in some manner should relate to the failure characteristics of the wire.

The above statement means that the sizing of the breaker can not be larger than that which will protect the wire. It does not say the breaker can not be sized smaller than would be required to protect the wire.

One might also decide that this input breaker is to protect the load and might be sized for this purpose, and be considerably less that required to protect the wire. It could not be sized larger than the requirement to protect the wire.

.

 

[don_resqcapt19]

you have derated the wire to a lower value it is no longer good for 400 amp
it is good only for the derated value.

Increasing the size of the conductor for voltage drop has nothing to do with its ampacity. The ampacity is fixed by the wire size, the adjustment factors in 310.15, the correction factors in T310.16 and the ampacities in T310.16.

 

[Smart $]

... It could not be sized larger than the requirement to protect the wire.

Motor feeders excepted, as they often have an ocpd larger than the conductor's ampacity rating.

 

[RETRAINDAILY]

Increasing the size of the conductor for voltage drop has nothing to do with its ampacity. The ampacity is fixed by the wire size, the adjustment factors in 310.15, the correction factors in T310.16 and the ampacities in T310.16.

Right nec doesn't require any voltage drop calculation but if you do decide to. its to deliver a certain voltage at a certain amperage you would not then put it on a bigger breaker.