Advice on pulling 800 amps.

[kwired]

Thanks, Infinity also touched on my questioning of the original conductor sizing. I think they need to be a size bigger since it's an 800Amp and conductors cannot be smaller than ocp.

Over 800 amps can not be smaller than OCP, but general rule for feeder taps is that the conductor must have ampacity of the overcurrent device it feeds.

 

[milemaker13]

kwired- You've got me now.. Can you explain further about termination temp rating? Honestly it has never come up. Yes, I was doing calcs based on 90* wire. 500mcm=430 amps.

 

[jimC.]

Since I started this....I think that 110.14 restricts us to rating our conductors by the temp rating of terminals at termination devices. Since most terminals are rated 75C, we use that table to begin our calculations. So, to my limited understanding, a 500CM (cu) begins at 380A. I may well be wrong, it's what I use, and so far it's worked for me. Hopefully some of the others will also jump in. This forum helps me remember how little I actually know. And am always grateful how freely everyone shares.

 

[Smart $]

kwired- You've got me now.. Can you explain further about termination temp rating? Honestly it has never come up. Yes, I was doing calcs based on 90* wire. 500mcm=430 amps.

Since I started this....I think that 110.14 restricts us to rating our conductors by the temp rating of terminals at termination devices. Since most terminals are rated 75C, we use that table to begin our calculations. So, to my limited understanding, a 500CM (cu) begins at 380A. I may well be wrong, it's what I use, and so far it's worked for me. Hopefully some of the others will also jump in. This forum helps me remember how little I actually know. And am always grateful how freely everyone shares.

The simple version to comply with termination temperature ratings is you have to use the corresponding temperature column of 310.15(B)(16) to determine minimum conductor size. You can use the column corresponding to the conductor type and insulation rating for other considerations (i.e. derating). The larger size of these two determination is the required minimum size.

See this post for an Excel calculator I made:

http://forums.mikeholt.com/showthread.php?t=150856&p=1460261#post1460261

 

[jimC.]

The simple version to comply with termination temperature ratings is you have to use the corresponding temperature column of 310.15(B)(16) to determine minimum conductor size. You can use the column corresponding to the conductor type and insulation rating for other considerations (i.e. derating). The larger size of these two determination is the required minimum size.

See this post for an Excel calculator I made:

http://forums.mikeholt.com/showthread.php?t=150856&p=1460261#post1460261

Nice, thanks for re-sharing. But on the OP, it's a service, in PVC, so I assumed underground; which would make it THWN, thus limiting us to the 75C column. I'm not trying to be pedantic; I'm studying for my Masters and am wanting to make sure I have this straight.
Thanks

 

[Smart $]

Nice, thanks for re-sharing. But on the OP, it's a service, in PVC, so I assumed underground; which would make it THWN, thus limiting us to the 75C column. I'm not trying to be pedantic; I'm studying for my Masters and am wanting to make sure I have this straight.
Thanks

Hmmm. OP says...

...

I will be doing two 3" emt runs each having (3) 500mcm cables and a ground. The total run will be about 270' with (3) 90's. I plan to place a large J-box about half way.

...

Nevertheless, if the runs were underground PVC using THWN conductors, the minimum size for 75?C terminals would be the same as EMT using THHN. Wiring method has no effect on this determination.

Where you'd run into differences in sizing is conductor ampacity after the application of adjustment and correction factors. With THHN you can make this determination using the 90?C column value, which gives you a little more "padding" to derate and not have to go larger than the minimum size determined for terminations. With THWN you use the 75?C column value and there is very little "padding", if any at all, for decreased ampacity after derating.

 

[kwired]

Nice, thanks for re-sharing. But on the OP, it's a service, in PVC, so I assumed underground; which would make it THWN, thus limiting us to the 75C column. I'm not trying to be pedantic; I'm studying for my Masters and am wanting to make sure I have this straight.
Thanks

Most equipment today has 75C terminations. In the absence of any marking you default to 110.14(C). Which essentially says terminals rated for 100 amp or less or 1AWG or smaller are 60C and everything larger is 75C.

THWN is 75C insulation, THWN-2 is 90C insulation.

Then there is NM cable that is limited to be used at its 60C rating but you can start with 90C values for ampacity adjustments (if it is NM-B, which all new NM cable is)

 

[milemaker13]

Still working thru this info.. But I think I'm following (at a distance, lol). I don't have my code book with me today, and finding the info online is more than a little confusing (bits and pieces).

Basically, unless I can prove otherwise, my terminals are rated at 75*. I don't have the actual table 110.14., but does the terminal amp ratings differ from conductor amp ratings at the same temp (75*). For example: 500mcm @ 75*= 380a.. is this the same for the terminals?

I guess so. Basically, the terminal on a 400a breaker would have to be large enough to accept at least 600mcm. Is this the basic idea here?


Jim C-
This forum helps me remember how little I actually know. And am always grateful how freely everyone shares.

I fully agree. Thanks guys.

 

[jimC.]

Still working thru this info.. But I think I'm following (at a distance, lol). I don't have my code book with me today, and finding the info online is more than a little confusing (bits and pieces).

Basically, unless I can prove otherwise, my terminals are rated at 75*. I don't have the actual table 110.14., but does the terminal amp ratings differ from conductor amp ratings at the same temp (75*). For example: 500mcm @ 75*= 380a.. is this the same for the terminals?

I guess so. Basically, the terminal on a 400a breaker would have to be large enough to accept at least 600mcm. Is this the basic idea here?


Jim C-
This forum helps me remember how little I actually know. And am always grateful how freely everyone shares.

I fully agree. Thanks guys.

I was essentially questioning on the 800Amp, and was reminded that the key word is "over 800". For your 400Amp, your load calc's may dictate a 400mcm and your next size up be a 400A breaker.

 

[milemaker13]

The equipment grounding conductor is based on the size of the overcurrent device. Table 250.122 Each conduit must have an equipment grounding conductor based on T. 250.122

Let me ask this about conduit fill: 3" Emt can have (4) 500 cables in it, so my ground wire would fit- no problem. But 3" can only fit (3) 600 cables. Does this prevent me from adding my ground wire? The ground will be 1/0 thhn.

I think it would fit, but will it be allowed?

If not, is it allowed to use the pipe as the EGC?

Again, I wish I had my book so I could look some of this up my self. I may have to run home to grab it..

 

[milemaker13]

I was essentially questioning on the 800Amp, and was reminded that the key word is "over 800". For your 400Amp, your load calc's may dictate a 400mcm and your next size up be a 400A breaker.

Can you guys explain the "next size up rule"? I did see some mention of that "over 800a" business.

 

[jimC.]

Can you guys explain the "next size up rule"? I did see some mention of that "over 800a" business.

230.90(A) ex. 2 is next size up. - if the ampacity of an ungrounded conductor does not correspond to a standard rating of an ocp, then the next size up can be used ...
240.4(B) - if it does not EXCEED 800 Amps

 

[milemaker13]

230.90(A) ex. 2 is next size up. - if the ampacity of an ungrounded conductor does not correspond to a standard rating of an ocp, then the next size up can be used ...
240.4(B) - if it does not EXCEED 800 Amps

So, I don't think there is a standard 760 amp breaker avalible... So does that allow me to go back to using the 500 cable? Its at 800a, not over.

 

[milemaker13]

unless you are prohibited from using aluminum wire, i'd pull 700MCM
aluminum simpull in 4" conduit,..

Going to the 75* tables, I'd need 900 mcm cable. I don't even have conduit fill tables that list 900 mcm.

Now if the "next size up rule" applies to this situation... maybe the AL would be do-able.

 

[kwired]

Still working thru this info.. But I think I'm following (at a distance, lol). I don't have my code book with me today, and finding the info online is more than a little confusing (bits and pieces).

Basically, unless I can prove otherwise, my terminals are rated at 75*. I don't have the actual table 110.14., but does the terminal amp ratings differ from conductor amp ratings at the same temp (75*). For example: 500mcm @ 75*= 380a.. is this the same for the terminals?

I guess so. Basically, the terminal on a 400a breaker would have to be large enough to accept at least 600mcm. Is this the basic idea here?


Jim C-
This forum helps me remember how little I actually know. And am always grateful how freely everyone shares.

I fully agree. Thanks guys.

When selecting conductor the ampacity must meet two conditions.

First it must meet the terminal temperature rating. Your terminals are 75C. So no matter what other adjustments apply your minimum conductor size can never be smaller than size determined using 75C ampacity table.

Second if you are using 90C conductors you can start ampacity adjustments for number of conductors in raceway or ambient temperature using 90C ampacity tables, but if your final conductor selection @ 90C ends up being smaller than the conductor necessary for termination temp rating you must still use the larger conductor necessary for termination temp.

So, I don't think there is a standard 760 amp breaker avalible... So does that allow me to go back to using the 500 cable? Its at 800a, not over.

The next size up rule is not part of conductor ampacity selection - it is part of overcurrent device selection. This information is in 240.4 The over 800 amp level is where we have to have a conductor with an ampacity equal or greater than the overcurrent device setting.

If we have an overcurrent device rated 800 amps or less, we only need to size conductors to the load at minimum and can protect them at the next higher standard overcurrent device rating. These standard device ratings are mentioned in 240.6

A fairly common example of this is 4/0 aluminum conductor. It has an ampacity of 180 amps @ 75C. It can be protected by a 200 amp overcurrent device because that is the next higher standard device size, but the load on the conductor can not be greater than 180 amps to do this. If the load were 185 amps then you must use a larger conductor even though you can still protect it with a 200 amp device.

You have a similar situation with your 500 KCMIL copper conductors. They are only good for 380 amps, but can be protected by a 400 amp device as long as the load on the conductors is 380 or less, and in the parallel situation you have 760 amps of conductor - next size up is 800, but load on the conductor can not be over 760.

To parallel three 500's for a 1200 amp service is not permitted though - because of the "over 800 amp rule".

3 500's = 1140 ampacity and there is no standard rating of 1140 standard overcurrent devices are 1000 and 1200. So we either need larger conductors for 1200 or can protect these conductors at 1000. Second option is not going to work if the load is over 1000 though.

 

[eHunter]

When selecting conductor the ampacity must meet two conditions.

First it must meet the terminal temperature rating. Your terminals are 75C. So no matter what other adjustments apply your minimum conductor size can never be smaller than size determined using 75C ampacity table.

Second if you are using 90C conductors you can start ampacity adjustments for number of conductors in raceway or ambient temperature using 90C ampacity tables, but if your final conductor selection @ 90C ends up being smaller than the conductor necessary for termination temp rating you must still use the larger conductor necessary for termination temp.



The next size up rule is not part of conductor ampacity selection - it is part of overcurrent device selection. This information is in 240.4 The over 800 amp level is where we have to have a conductor with an ampacity equal or greater than the overcurrent device setting.

If we have an overcurrent device rated 800 amps or less, we only need to size conductors to the load at minimum and can protect them at the next higher standard overcurrent device rating. These standard device ratings are mentioned in 240.6

A fairly common example of this is 4/0 aluminum conductor. It has an ampacity of 180 amps @ 75C. It can be protected by a 200 amp overcurrent device because that is the next higher standard device size, but the load on the conductor can not be greater than 180 amps to do this. If the load were 185 amps then you must use a larger conductor even though you can still protect it with a 200 amp device.

You have a similar situation with your 500 KCMIL copper conductors. They are only good for 380 amps, but can be protected by a 400 amp device as long as the load on the conductors is 380 or less, and in the parallel situation you have 760 amps of conductor - next size up is 800, but load on the conductor can not be over 760.

To parallel three 500's for a 1200 amp service is not permitted though - because of the "over 800 amp rule".

3 500's = 1140 ampacity and there is no standard rating of 1140 standard overcurrent devices are 1000 and 1200. So we either need larger conductors for 1200 or can protect these conductors at 1000. Second option is not going to work if the load is over 1000 though.

Nicely done and clearly stated.:thumbsup:

 

[milemaker13]

Thank you Kwired. I understand now. I have my code book today and will be spending some time reading.

I need more info from upstairs on the future equipment being installed that is to be fed from this service. That way I can make a determination on using 500's vs. 600's.

I'd still like to know about the conduit fill. If I was to use 600mcm, 3 will fit into 3" (which I have on hand) Will I be allowed to also include my 1/0 ground wire?

 

[kwired]

Thank you Kwired. I understand now. I have my code book today and will be spending some time reading.

I need more info from upstairs on the future equipment being installed that is to be fed from this service. That way I can make a determination on using 500's vs. 600's.

I'd still like to know about the conduit fill. If I was to use 600mcm, 3 will fit into 3" (which I have on hand) Will I be allowed to also include my 1/0 ground wire?

I didn't do the calculations and don't know what will fit off the top of my head.

The fill tables in annex C are for when all conductors are same size. And is technically not part of the code, it is just an informative table included in the book.

If you want to know what area a specific set of conductors (whether all same size or mixed) needs for fill, then use tables 4 and 5 in chapter nine to do the calculations.

 

[Smart $]

When selecting conductor the ampacity must meet two conditions.

First it must meet the terminal temperature rating. Your terminals are 75C. So no matter what other adjustments apply your minimum conductor size can never be smaller than size determined using 75C ampacity table.

Second if you are using 90C conductors you can start ampacity adjustments for number of conductors in raceway or ambient temperature using 90C ampacity tables, but if your final conductor selection @ 90C ends up being smaller than the conductor necessary for termination temp rating you must still use the larger conductor necessary for termination temp.



The next size up rule is not part of conductor ampacity selection - it is part of overcurrent device selection. This information is in 240.4 The over 800 amp level is where we have to have a conductor with an ampacity equal or greater than the overcurrent device setting.

If we have an overcurrent device rated 800 amps or less, we only need to size conductors to the load at minimum and can protect them at the next higher standard overcurrent device rating. These standard device ratings are mentioned in 240.6

...

Actually, the next size up rule affects conductor selection indirectly. Conductor sizing rules may yield a conductor too small to be protected by an OCPD determined by the noncontinuous plus 125% continuous rule. In this case, we have to increase the conductor size to one with an ampacity greater than the next smaller standard OCPD rating when 800A or less, or to a size with ampacity not less than the OCPD rating for over 800A.

Here's the four step process I wrote in another thread:

Generally (with emphasis), conductor sizing is a four-step process...

1) Minimum size of conductor for terminal temperature limitations (using calculated load value with 125% factoring for continuous loads).

2) Minimum ampacity of conductor before adjustment and correction must equal or exceed 125% continuous plus noncontinuous sum of calculated load.

3) Minimum ampacity of conductor after adjustment and correction must equal or exceed calculated load (does not include 125% factoring for continuous loads).

4) The overcurrent protection device must have a rating not less than 125% continuous plus noncontinuous sum of calculated load. The adjusted and corrected conductor ampacity must at a minimum be greater than next smaller standard overcurrent device rating [240.4(B); does not apply in all instances]. Where 240.4(B) does not apply, the adjusted and corrected conductor ampacity must equal or exceed overcurrent protection rating.

Ultimately, the chosen conductor must meet all four requirements.

 

[milemaker13]

Let me throw another one at ya- The breaker we have installed is a standard 80% breaker. Meaning I should not load it beyond 640 amps, right?

Basically what this looks like to me is that this breaker will only serve this one press line. There won't be enough left over to do much else.

And if this is true, then isn't the 500 cable at 760 amps (75*) sufficent for the calculated load(640 max)?

Here is what I have worked out so far: The press, straightener and feeder are fused at a combined 387 amps (200+175+12). If they are continuous and/or motor loads then x 125%= 484 amps.

640-484=156 amps left avalible.

Way off base so far? Let me know what you think. Thanks guys.