Feeder Sizing

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jganser

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Okay, to start with, I am fresh out of school, just a junior employee at an Engineering firm. I'm working on a project that the owner wants to add a standby generator to an existing building that has two separate main switchboards (2000A ea.) both fed from the same utility transformer, however metered separately. I have been told that we will do this adding a new MSB that is dual fed from the utility transformer. It will contain (2) 2000A MCB each followed by an ATS which are also fed by the generator. This feeder is the one in question. I am currently showing this is a 4000A feeder from the generator to two 2000A breakers which then feed the ATS's. Ultimately, I want to know how to size this large feeder. Of course I have plenty of supervision, but seeing as how I have just recently started working here, I wanted get some incite on this topic before presenting it to my superior. Other information you may need in helping: 120/208V, 4000A, 3P, 4W, THHN wire...

Thanks for any help you may be able to give me on this project.
 
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jganser said:
I'm working on a project that the owner wants to add a standby generator to an existing building that has two separate main switchboards (2000A ea.) both fed from the same utility transformer, however metered separately. I have been told that we will do this adding a new MSB that is dual fed from the utility transformer. It will contain (2) 2000A MCB each followed by an ATS which are also fed by the generator. This feeder is the one in question. I am currently showing this is a 4000A feeder from the generator to two 2000A breakers which then feed the ATS's. Ultimately, I want to know how to size this large feeder.
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I would first call the utility and get the max demand recorded. This will
be either KVA or KW. Start there.
 
I am having some difficulty understanding your existing and proposed configuration. I gather that the existing system has two separate main service switchboards (MSBs), each with its own metering section. I also gather that each supplies its own set of loads, and that both are supplied from the same utility transformer.

What I don?t get is whether you are replacing the existing two MSBs with a single MSB, or whether you are adding a third MSB and leaving the existing two in place. You say that the new MSB will be dual fed from the utility, so will that make a total of four metered services from the utility transformer? Or are you creating a new ?Main-Tie-Main? configuration?

One more question: Where are you talking about putting the two breakers from the generator? Will they be in separate enclosures near the generator, or will they be in the new MSB?

By the way, welcome to the Forum.
 
Sorry for the confusion. The two existing MSBs will stay. Their 2000A feeds from the utiltiy transformer will be intercepted and sent to the two new ATSs in the new MSB that is dual fed (still confusing, I know). The new MSB will have the following configuration by sections:
1. Meter #1
2. 2000A MCB #1
3. 2000A ATS #1 (existing MSB #1 will be fed from here)
4. (2) 2000A/3P Breakers (this is where the 4000A feeder is coming in from the generator and splitting to both loads via the ATSs)
5. 2000A ATS #2 (existing MSB #1 will be fed from here)
6. 2000A MCB #2
7. Meter #2

Therefore, all metering for both loads will be moved to this new MSB. Also, as shown above, the two breakers from the generator will be inside the new MSB. By the way, originally, yes, we were thinking of utilizing a tie-breaker, but since the entire system is only fed by one utility transformer, that configuration would be useless if the power were to fail.

I hope this helps you understand. I would like to know if you have any other suggestions or approaches to this specific project. I would also like some incite into the sizing aspect of this large 4000A feeder.

Thanks.
 
OK, that helped. Now I think I understand. I also see why you need a single feeder to come from the generator to the new MSB. You need 2000 amps of capacity going to ATS-1, and another 2000 amps of capacity going to ATS-2. But the generator might supply all 4000 amps, so you can?t use two separate feeders.

To get 2000 amps of capacity, you would need six parallel sets of 500 MCM copper. To get 4000 amps of capacity, you need eleven parallel sets of 500 MCM copper. These are based on NEC Table 310.16. The parallel sets of conductors have to be connected in parallel both at the generator and within the new MSB (before splitting off to go to the ATSs).

Exactly what do you need to know about this feeder, in order to proceed with the project?
 
I'm glad that made sense. I've actually had my code book open to that table for the past day. I think I'm just having a problem interpreting the data. How did you determine that it would require eleven sets in parallel? If you could explain how to use the table properly, I would be able to intertain the idea of using larger sized cable, plus I would be able to size other feeders in the future. By the way, I have the 2000A feeders sized at 5 sets of 600 MCM which is purly based on a pre-existing feeder schedule from another project.

Thanks for your help Charlie.
 
Use the 75-degree conductor column despite having a 90-degree conductor because the terminal rating of your equipment is rated at 75-degrees (most likely). Charlie came up with the 11 sets of 500 MCM by assuming 500's would be the largest conductor pulled. As the cable becomes larger the increase ampacity increases less do to the skin effect. I also try not to spec larger than 500's because of the difficulty in pulling.

From the table 500 MCM is good for 380 amps.

10 x 380 = 3800 amps. Not enough
11 x 380 = 4180 amps. Enough

See Chapter 9 tables for conduit sizes (C.8 for RMC).

Are the cables going to be installed in dry locations? You state THHN is why I ask.

Regards and welcome.
 
Well that about sums it up. I was using the table properly, but was getting confused when my calculations did not match the pre-existing feeder schedule previously mentioned. This was because I was using the 90-degree conductor column. I mentioned THHN just as an assumption. It will be installed underground in RMC. Thanks.
 
In that case you won't want to use THHN (Dry). Use THWN or XHHW or something rated for wet locations. Depending on your lengths not too long 3-1/2" RMC should do for each set of feeders. There would be 4 no. 500 and I would pull a 1/0 cu gnd if it were me. Best of luck with your new job. Sounds like they got you in the thick of it from the get go.
 
I forgot to say the EGC was sized per Table 250-66. Also, FYI, if you had to derate the conductors for temp. or bundling (see Table 310,15(B)(2)(a)) and you use a 90-degree conductor, you could use the 90 degree column for the derating calculations.
 
spsnyder,
I forgot to say the EGC was sized per Table 250-66.
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The conductors from the generator are feeders and you have to use Table 250.122. 250.122(F) will require a 500 kcmill ECG in each of the parallel raceways for the 4000 amp feeder.
Don
 
spsnyder said:
Charlie came up with the 11 sets of 500 MCM by assuming 500's would be the largest conductor pulled. As the cable becomes larger the increase ampacity increases less do to the skin effect. I also try not to spec larger than 500's because of the difficulty in pulling.
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Just a couple of thoughts after being on the installer end for a while.

I would rather work with 600 cu as apposed to 500 cu.

The stranding is smaller on the 600s, it bends easier.

500s have 37 - 2.95 mm strands

600s have 61 - 2.52 mm strands

Also try to keep it to 10 sets maximum, some utility companies do not allow more then 10 sets to a pad mount transformer.

Another engineer / forum member pointed out that Transformer manufacturers say 10 conductors per phase is the limit on pad mounts.

Less then a month ago I helped disconnect and reconnect a pad mount that had 15 sets of 600 cu per phase.....it was not pretty and a major pain to accomplish.

That said I would recommend 10 sets of 600 cu per phase which results in a capacity of 4,200 amps assuming separate raceways.

Are the cables going to be installed in dry locations? You state THHN is why I ask.
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It seems everything we get is dual rated THWN / THHN -2 meaning wet or dry it is 90 C for the purposes of derating.:cool:
 
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The information you guys have provided is great! I do however have another question regarding the ground conductor. You have refered me to Table 250-66 which seems to makes sense, but could you explain why or why not to apply Table 250.122? If Table 250.122 applies, then the ground conductor required to ground the raceway/equipment would be sized at 500 MCM copper. Seems like overkill, but I still don't understand the difference between these two tables. Again, thanks for this great learning experience!
 
Sorry, my last post was written before I saw don's responce. Assuming don is correct, I will go with table 250.122. Thanks.
 
The short answer is 250.66 applies when there is no upstream overcurrent protection, as is the situation with a service.

250.122 applies when there is over current protection, as is the situation with a feeder.

However 250.122 (F) requires a full size EGC in each parallel raceway.
 
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