ocpd

[tower]

i`m doing some studying in stallcups master electrician guide and have come across a calculation that I need a second opinion on. 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. which they state would be 900 amps. but I look in the code book 2002 and there is no 900 amp standard size. the next lower size would be 800 amps. per art. 240.6 .are they quoting a older version of the code or can I find this info some were else in the code book.

 

[roger]

Re: ocpd

You have a good question if there is no more info in the practice question or in the briefing of the practice prep.

The kicker (not necassarily the right answer) is the last sentence in 240.6(A).

Roger

[ November 17, 2003, 08:19 PM: Message edited by: roger ]

 

[templdl]

Re: ocpd

I assume that the 920a is the calculated load, that is 100% on the noncontinuous load plus 125% of the continuous load, size the conductor(s) first , the conductor must rated to carry that current. Then 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.
Remember that the primary purpose of the OCPD is to protect the conductor, the conductor sized to carry the calculated load.

 

[bob]

Re: ocpd

You can not round down below the caculated load. If your caculations are correct the breaker must be greater than 920 amps. The next size is 1000
amps. If you are designing the system you would use a 1200 amps or a 1600 amp depending how much you wanted to add for future load. The breaker is sized to protect the conductor. The conductor is sized to carry the load of 920 amps.

[ November 17, 2003, 11:27 PM: Message edited by: bob ]

 

[pierre]

Re: ocpd

Why not use 3 sets of 400s and protect it with a 1000 amp OCPD?

Pierre

 

[templdl]

Re: ocpd

You may be making a mountain out of a mole hill with regards to sizing the breaker. I don't think it's an issue. It's no big deal with regards to the breaker cost as it's most likely a 1200a frame anyway. But the additional cost of running the cable is significant.
If the "load" is continuous then the actual load is 736a which is less than a 900a breaker anyway. If the load were non-continuous at 920a it is a maximum non-continuous load for less than 3 hours. Remember that a breaker can carry is rating in current continuously at 40degC, that is it will not trip.
Personally I don't think you will ever trip a 1200a frame physically thermal/magnetic breaker with a 900a trip. I took a look at a 1200a frame I2t trip curve that I have and a 900a trip unit may trip after an our at 990a but it also may never trip as the breaker's allowable calibration is +10%. The upper part of that calibration at a one hour time is about 140% of 900a. Remember that these based upon 40degC. If the ambient temp were more like 25degC the current require to trip the breaker in one hour would be higher than 140%.
Electronic breakers do not have thermal elements that respond to the ambient temp so they would be the same at 40degC and 25degC etc. The I2t may also be a bit faster but not significantly.

 

[tower]

Re: ocpd

correct me please. but in art.240.4(c) devices rated over 800 amperes.where the ampacity of conductors do not correspond with a standard device the next lower size must be used if such ocpd is rated over 800 amps.there is no such thing as a 900 amp ocpd. am I correct.

 

[templdl]

Re: ocpd

The C-H 1200a 'N' frame breaker family started out with the NB31200F frame that you could get a NB3900T TM interchangeable trip unit for which has been available for the last 25 years or more. The solid state version, the NC family which the NC31200 was available with a 12NC900 rating plug. The newer ND family, the ND312T33W is available with a 900a 1200NES900T rating plug, etc.
Use, 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.
G-E and Sq-D probably have similar products.

 

[tonyi]

Re: ocpd

An "N" frame Cutler with a 12NES900T fixed rating plug would be 900A.

 

[justin]

Re: ocpd

If I were you I would definitley make sure that I understood the entire process of doing load calcs and exactly how to size your conductors and ocpd. the rest of this info is great but could easily become confusing. if you KNOW the calculation then you should be fine. You defnitely might want to look at sec. 240.4(b)(1),(2),(3) and 240.4(c) and get a better understanding, it will be important if your testing or actually sizing your devices in the field! I am speaking from experience, I too have been confused by basically the same situation and (mis)understanding of the code.

 

[roger]

Re: ocpd

Tower, follow Justins input and forget all of the above "over answers". As I said, the book is lame if no more information is given.

Let's review this thread.

# 1 You bring up some confusion in an instructional text, (You only thought there was confusion)

# 2 I agree that there is something wrong unless there is some info missing. I also point to the last sentence of 240.6(A)

#3 Now the third post gives you a math lesson that is assumed you haven't considered.

#4 The fourth post points out what you would do if you were designing.

#5 You are asked in the fifth post why you didn't change the whole example.

#6 Here in the sixth post economics are brought into the equation. (If it's not out of you monetary reach all is remedied)

#7 You post the correct point of the original question.

#8 You are given a manufacturers commercial here, and reminded of the last sentence in 240.6(A)

#9 Another commercial which is actually a reminder of the last sentence in 240.6(A)

#10 Justin indicates you should avoid misunderstanding the code, and I agree.

Hope you are clear on the answer to you question. :roll:

Good luck Tower

Roger

[ November 18, 2003, 09:40 PM: Message edited by: roger ]

 

[hurk27]

Re: ocpd

To me it sounds like you calculated the feeder load at 920 now you size your wire to this load then you see what the max OCPD that the nec allows to protect this feeder. The feeder has to be able to carry the calculated load it can not be under. then the feeder has to be protected. If the OCPD is not a standard size you are allowed to go to the next size up OCPD to protect the feeder.
Is my thinking right?

 

[hurk27]

Re: ocpd

After rereading his post I think he calculated the amount of current that the feeder is allowed to handle and he now needs to size the breaker to protect the feeder which would require what the feeder were feeding?

 

[ryan_618]

Re: ocpd

I've been silent on this because I left my NEC at home, but I'll jump in now.

In my opinion, the conductors must have the ampacity to handle the calculated load. Once you have determined that, you need to protect the conductors. You are not allowed to round up after 800 amps, so I go with Pierre's reply: You will have to increase the size of both the conductors and the OCPD. It might help to work backwards on this, and start with the breaker that can handle the load, then determine the smallest size of wire that that breaker will protect. I wish I had my book here, I want to know what the last sentance Roger is referring to is.

 

[roger]

Re: ocpd

Ryan, the sentence states, "The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted

Roger

 

[bob]

Re: ocpd

Towers Post

Correct me please but Article 240.4C(c) Devices rated over 800 amperes. Where the ampacity of the conductors with a standard device the next the next lower size must be used if such ocpd is rated over 800 amps.

You need to read article 240.4(c) again It does not read as quoted.
240.4(c) says

If the device is over 800 amps then the conductors
it protects shall be equal to or greater than the ampacity of the overcurrent device.

[ November 20, 2003, 11:38 PM: Message edited by: bob ]

 

[don_resqcapt19]

Re: ocpd

The calculated load is 920 amps so the minimum ampacity of the conductors is 920 amps. As this circuit is over 800 amps you are not permitted to use the next larger standard size OCPD. If you can find a non-standard trip OCPD rated at 920 amps you can use 4 sets of 4/0 for this load. If not, use 4 sets of 250 with a 1000 amp OCPD. If this a 480/277 wye system the 1000 amd OCPD may have to have a ground fault trip.
Don

 

[roger]

Re: ocpd

Everybody is trying to solve a problem they don't have the information for. This could turn into a ridiculously long thread over an exam question.

The real item with the info we have is rounding down to 900 amps.

from the original post:
to size the ocpd you would round down to the next smallest size. which they state would be 900 amps.

Let's forget about actual or mythical calculated loads, these numbers we don't have and any assumption is speculation at best.

Roger

[ November 20, 2003, 08:36 PM: Message edited by: roger ]

 

[don_resqcapt19]

Re: ocpd

Roger,
The original question says that the calculated load is 920 amps. How can we round down to 900 amps?
Don

 

[roger]

Re: ocpd

Don, Tower said,

the amps on a feeder

he doesn't directly say load.

Does he mean conductor ampacity?

In a test or exam practice question I could see this asking what size OCPD would be required for (4) sets of 4/0 THW copper conductors at 75 deg F

A 1000 Amp
B 950 Amp
C 900 Amp
D none of the above

The question might ask for (2) sets of 700 MCM.

There is just not enough information, If we could see the exact wording from James Stallcups book, I'm sure we would understand why 900 is shown as the answer.

Roger