Over current protection

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philly

Senior Member
charlie b said:

I was not saying that it is OK. It's not OK. I know that it's not OK. I was asking if anyone knew exactly which code article it would violate.

(Edited to add: Wasasparky answered that question for me in post #31, as I acknowledged in post #32.)
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I apoligize, I misunderstood your post.

I thought you were trying to defend your example rather than purposely give an incorrect example and question others why it was wrong.

The code on this issue is clear to me now.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Just to clarify, if the main breaker is specified, and the known load will be well under the service size, then the smallest service/feeder conductor size that can be used is one with an ampacity higher than the next-lower breaker size, correct?

For example, let's say the EE specifies a 300a main breaker, and the load will be under 200a, then the wire ampacity must be greater than the next-lower breaker size, a 250a. So, I'd need at least 250cu or 400al, presuming 75deg terminals.

Have I got it correct?
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
LarryFine said:
Just to clarify, if the main breaker is specified, and the known load will be well under the service size, then the smallest service/feeder conductor size that can be used is one with an ampacity higher than the next-lower breaker size, correct?

For example, let's say the EE specifies a 300a main breaker, and the load will be under 200a, then the wire ampacity must be greater than the next-lower breaker size, a 250a. So, I'd need at least 250cu or 400al, presuming 75deg terminals.

Have I got it correct?
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Bumped, because I really need to know today.

Thanx,
 
S

short circuit faulted

Member
Location
Columbia SC
Seems we are awash in Ariticle 240 and 215 and lacking one other piece of information regarding the OCPD as IMHO most breakers unless specified as 100% rated open at approximately 80% of rating. So if you had a calculated continous load of 761 it is not the conductors that you need to worry about but the OCPD. Regarding the 600KCMIL versus 500 apparently electrical engineers finally found ampacity tables in NEC, which is a shock given they normally only use the NEC to thier advandage when it behooves them otherwise they quickly point out it is only a minimum standard. Regardless they have begun specifing single sets 600KCMIL in 400A applications and step it up throughout the possible circuit sizes. Only issue I've ever seen in past 30 years with 500KCMIL have been computer room applications on the neutral due to harmonics. Otherwise I have not seen a problem.

Have a great day all!

Steve
 
weressl

weressl

Esteemed Member
Location
Texas by the Gulf, geat crab, great shrimp
short circuit faulted said:
Seems we are awash in Ariticle 240 and 215 and lacking one other piece of information regarding the OCPD as IMHO most breakers unless specified as 100% rated open at approximately 80% of rating. So if you had a calculated continous load of 761 it is not the conductors that you need to worry about but the OCPD. Regarding the 600KCMIL versus 500 apparently electrical engineers finally found ampacity tables in NEC, which is a shock given they normally only use the NEC to thier advandage when it behooves them otherwise they quickly point out it is only a minimum standard. Regardless they have begun specifing single sets 600KCMIL in 400A applications and step it up throughout the possible circuit sizes. Only issue I've ever seen in past 30 years with 500KCMIL have been computer room applications on the neutral due to harmonics. Otherwise I have not seen a problem.

Have a great day all!

Steve
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The breakers are designed to be applied at 80% of their nameplate rating. They do NOT open when the current reaches 100%.

Nice potshot at engineers. You're one of those never miss an oppurtunity:cool:
 
S

short circuit faulted

Member
Location
Columbia SC
Actually I learned a great deal of VERY good electrical pratices from several Electrical Engineers - Men who took pride in thier work and noted job aspects in such through fashion that you could take off the job by going through thier notes. Most of them unfortunately are a dead or retired breed that we may never see again. There are still those few who aren't so wet behind the ears that they will listen to EC - however the majority of what I see as engineered drawings anymore are at best a 70% complete with a brief narrative to cover the missing drawing portions or - Architectural drawings with absolutely nothing electrical and a four page narrative which supposedly covers the electrical system for a twelve story building - It is pathetic.
 
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markstg

Senior Member
Location
Big Easy
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
This will be the last bump. If it doesn't work, I'll start a new thread.

LarryFine said:
Just to clarify, if the main breaker is specified, and the known load will be well under the service size, then the smallest service/feeder conductor size that can be used is one with an ampacity higher than the next-lower breaker size, correct?

For example, let's say the EE specifies a 300a main breaker, and the load will be under 200a, then the wire ampacity must be greater than the next-lower breaker size, a 250a. So, I'd need at least 250cu or 400al, presuming 75deg terminals.

Have I got it correct?
Click to expand...
 
Pierre C Belarge

Pierre C Belarge

Senior Member
Location
Westchester County, New York
LarryFine said:
Just to clarify, if the main breaker is specified, and the known load will be well under the service size, then the smallest service/feeder conductor size that can be used is one with an ampacity higher than the next-lower breaker size, correct?

For example, let's say the EE specifies a 300a main breaker, and the load will be under 200a, then the wire ampacity must be greater than the next-lower breaker size, a 250a. So, I'd need at least 250cu or 400al, presuming 75deg terminals.

Have I got it correct?
Click to expand...


Larry
For services, there are 2 different sections to reference.

1. 230.42
With one service disconnect, the conductor is sized according to this section.

2. 230.90exception 3
With more than one service disconnect, the conductor is sized according to this section number.
 
S

short circuit faulted

Member
Location
Columbia SC
Mark, again IMHO the only CB I've known to hold at and above thier rated value were Federal Pacific Stablok and SQD QO prior to vis-a-trip windows. Other breakers I've seen begin to heat up at 80% and trip prior to 90% - perhaps that is a deep south ambient heat problem.


Have a good day!
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Pierre C Belarge said:
Larry
For services, there are 2 different sections to reference.

1. 230.42
With one service disconnect, the conductor is sized according to this section.

2. 230.90exception 3
With more than one service disconnect, the conductor is sized according to this section number.
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Can my previous question be answered with a "yes" or a "no"?
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
short circuit faulted said:
Mark, again IMHO the only CB I've known to hold at and above thier rated value were Federal Pacific Stablok and SQD QO prior to vis-a-trip windows. Other breakers I've seen begin to heat up at 80% and trip prior to 90% - perhaps that is a deep south ambient heat problem.
Click to expand...
It must be something.:)

All UL489 breakers have been tested at 100% current in open air. Once they are put into an enclosure they become '80% rated' based on the temperature in the enclosure being 40?C (104?F).

Even in enclosures, breakers and fuses, will carry quite a bit more than 100-150% for many minutes and 200-300% for many seconds (and up to minutes).

And FWIW, QO breakers without the Visi-Trip feature are more than 45 years old.
 
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