Reasoning for "next size up"?

[cppoly]

Example: 225 A continuous load * 1.25 = 281 A. Conductors sized at 300 kcmils with 285 A ampacity. Circuit breaker sized at next size up = 300 A.

From 285 A to 300 A, the breaker would never trip. So wouldn't this cause overheating for the conductors?

 

[iwire]

Example: 225 A continuous load * 1.25 = 281 A. Conductors sized at 300 kcmils with 285 A ampacity. Circuit breaker sized at next size up = 300 A.

From 285 A to 300 A, the breaker would never trip. So wouldn't this cause overheating for the conductors?

The code allows the next size up but does not allow us to load the conductor above its ampacity.

If it were to be overloaded the NEC recognizes there is enough of a safety factor in the ampacity charts that 15 more amps on a 285 amp conductor will not be a problem.

 

[david luchini]

You have a load of 225A on conductors with an ampacity of 285. Why would the conductors overheat?

 

[cppoly]

You have a load of 225A on conductors with an ampacity of 285. Why would the conductors overheat?

Good point, I was thinking of the 1.25 factor. But if I replace 225A and say 281 A of non continuous load, then this is what I mean by my example.

The code allows the next size up but does not allow us to load the conductor above its ampacity.

This would be where in the code?

Safety factor make sense too.

 

[iwire]

This would be where in the code?

Perhaps?

310.15 Ampacities for Conductors Rated 0?2000 Volts.
(A) General.
(1) Tables or Engineering Supervision. Ampacities for
conductors shall be permitted to be determined by tables as
provided in 310.15(B) or under engineering supervision, as
provided in 310.15(C).

 

[david luchini]

Good point, I was thinking of the 1.25 factor. But if I replace 225A and say 281 A of non continuous load, then this is what I mean by my example.

Then you have a load of 281A on conductors with an ampacity of 285. Same question - why would the conductors overheat?

The code allows the next size up but does not allow us to load the conductor above its ampacity.

This would be where in the code?

For branch circuits: 210.19(A)(1) "Branch-circuit conductors shall have an ampacity not less than the maximum load to be served."

For feeders: 215.2(A)(1) "Feeder conductors shall have an ampacity not less than require to supply the load as calculated in Parts III, IV and V of Article 220."

 

[pfalcon]

Good point, I was thinking of the 1.25 factor. But if I replace 225A and say 281 A of non continuous load, then this is what I mean by my example.

Then you have a load of 281A on conductors with an ampacity of 285. Same question - why would the conductors overheat?

Cause Jimmy Joe Bob figured out how to add his wife's hair blower to the circuit? +12A?

 

[cppoly]

Then you have a load of 281A on conductors with an ampacity of 285. Same question - why would the conductors overheat?"

Well I'm thinking there are reasons for overloading. Equipment problems, motor problems, people plugging in additional equipment, etc. Something that wasn't intended in the design case.

I'm thinking of an abnormal condition that would cause an overload.

 

[Gregg Harris]

Good point, I was thinking of the 1.25 factor. But if I replace 225A and say 281 A of non continuous load, then this is what I mean by my example.




This would be where in the code?

Safety factor make sense too.

Take a look at 240.4 (B)

 

[cppoly]

Take a look at 240.4 (B)

This was from the handbook's explanation of 240.4 (B):

"For example, a 500-kcmil THWN copper conductor has an allowable ampacity of 380 amperes from Table
310.16. This conductor can supply a load not exceeding 380 amperes and, in accordance with 240.4(B), can be protected by a 400-ampere overcurrent protective device."

My concern is that a load can be designed for 380 A. But, lets say the feeder circuit is fully loaded to that 380A, and then let's say a copier, printer, and fax machine is installed and now the feeder current is 395 A. The 400 A overcurrent device doesn't protect the feeder.

 

[don_resqcapt19]

...
My concern is that a load can be designed for 380 A. But, lets say the feeder circuit is fully loaded to that 380A, and then let's say a copier, printer, and fax machine is installed and now the feeder current is 395 A. The 400 A overcurrent device doesn't protect the feeder.

The ampacity tables are on the conservative side. It is very unlikely that the cable would be damaged by running at 400 amps. Keep in mind that the UL standard for OCPDs would permit that 400 amp device to supply 536 amps into the circuit forever without tripping.

 

[jaggedben]

My concern is that a load can be designed for 380 A. But, lets say the feeder circuit is fully loaded to that 380A, and then let's say a copier, printer, and fax machine is installed and now the feeder current is 395 A. The 400 A overcurrent device doesn't protect the feeder.

This is what iwire means by a safety factor. The real ampacity of the conductors before they would actually melt in most conditions is higher than 380 and higher even than 400A. The safety factor is the laboratory measurement divided by the nominal rating. For example, if conductors melt at 600A in laboratory conditions, and the NEC allows 400A, then the safety factor is 1.5. I don't know what safety factors the NEC actually uses but I would guess at least 1.33 if not much higher (such that a 15.1 amp load can be on a 20A breaker.)

Safety factors exist for all kinds of ratings of engineered products. For lifting products (rope, chain, eyebolts, shackles, etc.) safety factors can be in the range of 5 or 10 to allow for extreme dynamic loading and other such things.

 

[iceworm]

The ampacity tables are on the conservative side. It is very unlikely that the cable would be damaged by running at 400 amps. Keep in mind that the UL standard for OCPDs would permit that 400 amp device to supply 536 amps into the circuit forever without tripping.

You're thinking UL 489, 135% trip test?

ice

 

[don_resqcapt19]

You're thinking UL 489, 135% trip test?
ice

Yes. It has to trip within an hour at 135%, but it is never required to trip at less than 135%. As I understand it, the standard for fuses is the same.

 

[cppoly]

Thanks

 

[jwelectric]

Last Fall I got to see a short video of a short length of #6 and a 5/8 ground rod in a dead fault situation.
The 5/8 ground rod turned red and was gone but the #6 was still intact.

As to the 135% thing, I heard, I thought, that a breaker would carry 135% for two hours. Do I have the wrong information?

Then we have this 10,000 RMS Sym. Stuff that is wrote on 15 amp breakers connected to a #14 conductor. Does this mean something?

I don?t know that this will shed any light on the discussion but I wanted to get into the conversation so this is what I posted.

 

[jim dungar]

As to the 135% thing, I heard, I thought, that a breaker would carry 135% for two hours. Do I have the wrong information?

Then we have this 10,000 RMS Sym. Stuff that is wrote on 15 amp breakers connected to a #14 conductor. Does this mean something?

The 10,000 Amps is the amount of fault current the device can interrupt.

 

[don_resqcapt19]

...As to the 135% thing, I heard, I thought, that a breaker would carry 135% for two hours. Do I have the wrong information?
...

My understanding is that the breaker must trip in one hour or less at 135% of its rating.

 

[iceworm]

My understanding is that the breaker must trip in one hour or less at 135% of its rating.

Yes. Paraphrasing UL489, section on normal (temp ratings) stuff
At 135% the CB shall trip within 1 hour.

At 100% the CB shall not trip. The test is run until the CB reaches a stable operating temp.

Between 100% and 135% there is no spec

Examples that meet spec:
If the CB never tripped at 100%, but tripped in one minute at 101%

If the CB never tripped at 134%, but tripped in one minute at 135%

On small CBs it doesn't matter much. However, when one is doing a coordination study, all of the CB (and fuse) curves have a definite thickness. Left side is "shall not trip". Right side is "shall trip". In between is unknown.

ice