ocpd

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Re: ocpd

Originally posted by tower:. . . after calculating the amps on a feeder that came out to 920 amps, to size the ocpd you would round down to the next smallest size.
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I think that this portion of the original posting is the cause of a bit of confusion here. Several members have already pointed out the error in this statement. The error comes from trying to round, up or down, from the calculated load.

Step One:
Calculate the load. Not having the original problem at my disposal, I will presume that you did that part right. I interpret the original posting as saying that the calculate load is 920 amps. I must then further presume (as templdl has already pointed out) that this result correctly accounts for continuous and non-continuous loads.

Step Two:
Find a set of conductors that can take every bit of that 920 amps. You can get a bigger conductor, but this is not the time to ?round down.?

Step Three:
Find an OCPD that is higher than 920, but no higher than the ampacity of your selected conductors.

Step Four:
At this point, you discover THE TRAP! If in Step Two you picked, for example a set of 4 conductors of size 4/0, you have an ampacity of 920 amps. You must then take the next lower size of OCPD, which is 800 amps. This will not carry the calculated load, and would cause you to fail the test question.

The Solution:
You want to be able to select a 1000 amp OCPD. This means that the conductors must be sized for at least 1000 amps. Lacking any other information on design constraints, I would select a set of 4 conductors sized 250 MCM.
 
Re: ocpd

charlie b, Why did you use 800a when 900a is an available trip unit size? Doesn't this apply? NEC 240.6, Standard Ampere rating, does not specifically include 900a as standard, but, "The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted.

Dave
 
Re: ocpd

Temp
He chose 800 amps because it is listed as the standard size in the book. 900 amps is not the correct size if you caculated 920 amps. If you can get a trip unit set at 920 amps then you can use it. If not you use the next size greater than 920 amps.
 
Re: ocpd

Bob,
Then, this doesn't apply: "The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted."
That it shoulsd be an 800a and that a 900a is not an option even though it is available as standard from the manufacturers.
You then stated that "If you can get a trip unit set at 920 amps then you can use it. If not you use the next size greater than 920 amps" when
the size the OCPD per NEC art 240.4(C) where the OCPD can be equal to but not more than the ampacity or the rating of the rated wire.
What part of the NEC applies then?? Are you saying that, because there is no other choice but an 800a trip breaker that you must increase the size of the cable tin order to use a 1000a breaker?
Dave
 
Re: ocpd

Dave read Charlie's post again it says it all.

If the load is 920 amps the conductors and the breaker must match or exceed that amount.

A 800 or 900 amp breaker can not be used they are to small.

If you could get a 920 amp breaker you could use it. :)

In reality you will not get a 920 amp breaker, you are stuck with a 1000 amp breaker and therefore the conductors will also have to be rated at least 1000 amps.
 
Re: ocpd

OK, I'm going to go out on a limb here and say the actual load is 724 amps This is because there was no mention of load in Towers first post in this thread.

What Tower is asking, is why the 900 amp breaker is shown for the answer when he has figured (4) 4/0 conductors.

Here is the real problem he is trying to answer from this thread


posted November 16, 2003 07:44 PM
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say you have a continuous feeder loaded of 724 amps and the conductors are paralleled 4 times per phase to 75 degree terminals. after the calculations you come up with the phase conductors being sized at 4/0 awg. with out going into calculations, my question is. how is it that the amperage is assumed to be evenly divided amongst the paralleled conductors.say 230 amps per phase for a total of 920 amps.
Click to expand...
:D

Roger
 
Re: ocpd

Temp
If you have a load of 920 amps then you need to size the conductor and breaker to carry this load.
Which do you size first, the breaker or the wire.If you choose the breaker first, then the size must be 920 amps. The conductor must also be 920 amps or greater. If you can not find a 920 amp breaker then you must go up to the next size. I
would think this would be 1000 amp. If so, then the conductor must be equal to or greater than 1000 amps. You can use non standard breakers as
per the NEC. I don't think you can go down to 900 amps to get there when the load has been stated to be 920 amps.

[ November 23, 2003, 08:53 AM: Message edited by: bob ]
 
Re: ocpd

Originally posted by tower:
after calculating the amps on a feeder that came out to 920 amps,
Click to expand...
Originally posted by Roger:
OK, I'm going to go out on a limb here and say the actual load is 724 amps This is because there was no mention of load in Towers first post in this thread
Click to expand...
I take "amps on a feeder" to mean the load connected to a feeder.


We are all just guessing but no where in this thread did tower mention 724 amps. :)

Bob

[ November 23, 2003, 03:23 AM: Message edited by: iwire ]
 
Re: ocpd

ok i will jump into the fray here also. forget about Towers computations in whether they are correct or incorrect. at this stage it has become a mute point. also forget about 240.4(b) as it does not apply since we are dealing with amperage OVER 800.

240.4(c) is the whole key to solving this equation. for those of you who do not know what it says, it states:

"Devices rated over 800 amperes. Where the overcurrent device is rated over 800 amperes, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device defined in 240.6.

the key here is the ampacity of the conductors that we have to protect. you first have to figure out what your load is. then select an ocpd that is at least equal to the load, NEVER less. once you have selected the ocpd that is equal to or greater than the computed load you can now select the wire size to serve such load, sized according to the size of the ocpd. remember 240.4(c)? "the ampacity of the conductors shall be = to or greater than the rating of the overcurrent protective device"
 
Re: ocpd

dkstein,
What you've stated is the point I have been making all along.
Calculate load, size wire to carrry laod, apply breaker to protect wire.
 
Re: ocpd

Temp
Your statement is totally opposite that of Mr. Stein's. In his last paragraph he says select the OC device first and then select the conductor to match. Your statement says to select the conductor first and then match the OC to the wire capacity.
In other sections of the code it allows you to do as you stated. 240.3C does not allow you to do this.
 
Re: ocpd

Bob, You stated "240.3C does not allow you to do this." with regards to calculating load, sizing the conductor, selecting the OCPD.
The way I interpret it is that you must protect the cable at its rated ampacity which is often not practical. However you are allowed to select and OCPD that is the closest standard rating above the cable ampacity for cables rated up to and including 800a. For example(2) 500MCM cables have an ampacity rating of 760a. Since 760a is not a commonly available OCPD rating you are allowed to apply the next higher standard rating of 800a per 240.3(B).
If you were to select (2)600MCM cables with a rated ampacity of 840a which, again, is not a commonly available ampacity rating you are not allowed to select the next higher standard rating but must select the next lower rating,an 800a OCPD, ref 240.3(C).
Selecting (4)4/0 cable with an ampacity of 920a would be protected with a 900a OCPD, again ref 240.3(C).
It's a factor of being up to 800a or 801a and above. 240.3(B) allows to to go up where 240.3(c) states that you can't.
Or, am I missing what you are referring to.
Dave
 
Re: ocpd

"after calculating amps on a feeder that came out to 920 amp'

Like Bob has said, that seems to be a calculation for some kind of load.
If I calculated 920 amps, I read 240.6(A) and see the next size OCPD is 1000, because an 800 amp OCPD will not hold. Now I read 240.4(C) and I read that I need conductors to be = to or > than 1000 amp OCPD. I read 310.16 and see that 3 sets of 400 kcmil will conduct 1005 amps :D I do not understand what is so hard about this :confused:

Pierre
 
Re: ocpd

Pierre, nothing is hard about understanding these numbers or code sections, the problem is the 920 amps Tower is refering to and James Stallcups answer. With the answer being 900 amps this would indicate the 920 amp number would be the condutor ampacity not a load to be served.


Roger
 
Re: ocpd

When I had a James Stallcup booklet and had a question, I wrote him and he always answered.

Why don't the book holder write Mr. Stallcup ?
 
Re: ocpd

I don't know if this will help.

I have Stallcup's 1999 Electrical Design book and he gives a diagram and explanation for applying 240-3(c)[240.4(c)in 2002].

In his example he used 4 x 4/0 conductors to carry the calculated load (the load was not 920A, but the conductors capability was 920A which satisfied the actual load of 847A)....he then showed how you would use the 900A OCPD device (actual load was 847A, conductors capability = 920A...so the OCPD = 900A).

Again, we're just speculating here, but I think maybe the original post gave us slightly incorrect information....Stallcup's Electrical Design example sure resembles this discussion.
 
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