240.4(B) Can someone explain why?

[elec_eng]

I am having a hard time understanding this.

240.4 (B) permits the use of the next larger standard overcurrent device. Does this mean that 240.4(B) permits no overload protection for device rated 800A or less?

For example, 500kcmil cu conductor has an allowable ampacit of 380A. Per 240.4(B), this conductors can be protected by a 400A overcurrent protective device. But what if, some reason, this conductor is overloaded to 390A, then this conductor is not going to be protected by 400A cb. Correct?

That said, my questions are

1) why does the code permit the next larger standard overcurrent device?

2) Why not go with the next smaller overcurrent devices?

3) Why this is only permitted to devices with 800A or less?

4) Why not with devices with 800A or more?

Another fact that I am having hard time understanding is the rating of the circuit breaker. Most of circuit breakers are rated 80% of its rating, expect the ones with 100% reated ones. 400A MCCB is only rated 320A.

So go back to our example, a 400A circuit breaker is protecting a 500Kcmil condutor (380A). But if your loads exceed 320A, your 400A circuit breaker is not going to provide overcurrent protection anymore because it exeeds its rating. What am I missing here? Please help.

 

[benaround]

ElecEng,

What you are doing is jumping into the middle of this and trying to make
sense out of it.

Take your example, the 500mcm conductor was chosen for a reason, the NEC

never lets you size the conductor for less than the load it is to serve. So, say

the load to be served is 350a, then go to 310.16 tables, 400mcm is good for

335a so no good, next 500mcm is good for 380a that will work,but there is no

380a c.b., you can't go to 300a your load is larger than that, so, the NEC

allows the use of the next higher standard rating OCPD, and saftey has not

been violated. The conductors can handle the load, the OCPD can handle

the conductors.

Next: 400a c.b. is rated for 320 amps continous ( more than 3 hours) and is

rated 400a non-continous.

Next: Why not over 800a ? Because the increase in the standard rating jumps

200a to 1000a then 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000.

Under 800a is a lot smaller gap between standard ratings.

I hope this helps you out. The examples may not be 100% correct,but do make the point.

 

[iwire]

I had a long reply ready to go and I lost it.

The short story...

• The NEC expects the people calculating the load not to overload the conductors
  
• The 310.16 ampacity ratings are conservative.
  
• The Article 220 calculations are conservative.
  
• All breakers can be loaded to 100% Non-continuously.

240.4(B) is not the only oddity you can find.

You might want to look at table 310.15(B)(6) where the NEC allows the use of smaller then normal conductors for services and 230.90(A) Exception 3 where the NEC allows service conductors to be 'protected' far above their rating providing the calculated load is under the conductor amapcity.

 

[Platinum]

I had a long reply ready to go and I lost it.
/

Find that long reply iwire!
Good topic.....i often questioned this myself.

 

[charlie b]

But what if, some reason, this conductor is overloaded to 390A, then this conductor is not going to be protected by 400A cb. Correct?

In case the other answers were not clear enough on this point, if the calculated load is 390 amps, you cannot use 500 MCM conductors. The conductors you select must have an ampacity at least as high as the calculated load.

 

[steve66]

Just to make things more confusing, a lot of people seem to think a 400 amp breaker will trip for a current a little over 400 amps. That's just not the case. Without looking at a curve, you probably need 450-500 amps or even more before a 400 A breaker will trip.

So the idea that a 400 amp breaker limits the current on a conductor to 400 amps just isn't true.

Steve

 

[don_resqcapt19]

Just to make things more confusing, a lot of people seem to think a 400 amp breaker will trip for a current a little over 400 amps. That's just not the case. Without looking at a curve, you probably need 450-500 amps or even more before a 400 A breaker will trip.

So the idea that a 400 amp breaker limits the current on a conductor to 400 amps just isn't true.

Steve

Even without looking at the time current curves, you need to understand that the UL standard for both breakers and fuses permits them to hold 135% of the rated current forever.

 

[charlie]

. . . you need to understand that the UL standard for both breakers and fuses permits them to hold 135% of the rated current forever.

Don, I'm shooting from the lip, I don't believe that is a true statement. It seems that OC devices are run at 100% in open air for an extended period of time and they must not let go. The same test is run with a modest OL and they must open the circuit inside of a prescribed period of time.

I would like someone to verify my statements since I am not sure at this point. :-?

 

[jim dungar]

Looking at test standards from 1987 (NEMA AB-1), for inverse-time-delay characteristics:

Breakers at ambient air of 25?C
1) At 200% breakers should trip at different times based on their current rating

2) Breakers should not trip within a fixed time at 100% and must trip at an additional fixed time when raised to 135%, again based on current rating.

So for a 400A breaker
100% no trip for 2 hours, then
135% trip within 2 additional hours
200% trip within 12 minutes

And for a 20A breaker
100% no trip for 1 hour, then
135% trip within 1 additional hour
200% trip within 2 minutes

 

[charlie]

Thanks Jim, that was what I wanted. :smile:

 

[iwire]

So for a 400A breaker
100% no trip for 2 hours, then
135% trip within 2 additional hours
200% trip within 12 minutes

It sounds to me like it is within the specifications for 134% to result in a no trip.

 

[jim dungar]

It sounds to me like it is within the specifications for 134% to result in a no trip.

Actually I think you would have to draw the curve of Never @ 100%,Target @ 135%, and Target @ 200% to determine the actual trip points.

But in any case the time will be measured in Hours not Seconds like many people assume. I believe these values would also apply to fuses.

 

[don_resqcapt19]

The following is from Square D catalog 0730CT9801R1/08 for QO/QOB breakers. This is where I got my 135% from.

The tripping characteristics of QO and QOB circuit breakers can be represented by a characteristic
tripping curve that plots tripping time versus current level. The curve shows the amount of time
required by a circuit breaker to trip at a given overcurrent level. The curve has a performance band that
is bound by a minimum and a maximum value of clearing time. Total clearing time is the sum of the
sensing time, unlatching time, mechanical operating time and arcing time of the circuit breaker. For
currents in excess of 135% of the circuit breaker rating at rated ambient temperature (40?C), the circuit
breaker will automatically open the circuit within limits specified by the band.

 

[elec_eng]

In case the other answers were not clear enough on this point, if the calculated load is 390 amps, you cannot use 500 MCM conductors. The conductors you select must have an ampacity at least as high as the calculated load.

Charlie,

You could have a situation where loads were added after the fact over the years without considering the size of the conductor. I am talking hypothetically here.

 

[elec_eng]

Looking at test standards from 1987 (NEMA AB-1), for inverse-time-delay characteristics:

Breakers at ambient air of 25?C
1) At 200% breakers should trip at different times based on their current rating

2) Breakers should not trip within a fixed time at 100% and must trip at an additional fixed time when raised to 135%, again based on current rating.

So for a 400A breaker
100% no trip for 2 hours, then
135% trip within 2 additional hours
200% trip within 12 minutes

And for a 20A breaker
100% no trip for 1 hour, then
135% trip within 1 additional hour
200% trip within 2 minutes

Won't the wire insulation be damaged during this time delay?

 

[LarryFine]

Won't the wire insulation be damaged during this time delay?

Now you know why the NEC conductor ampacity ratings are so conservative.

 

[jim dungar]

Won't the wire insulation be damaged during this time delay?

UL standards and NEC installation/sizing requirements are designed to protect conductors, including insulation. Insulation is damaged based on "heat over time".


Don,
The text you quoted does not directly address my old NEMA standard. For example; your text says for currents in excess of 135% without describing previous loading or what happens at 135%. My quoted standard says 100% for time T then immediately 135% for another time T.