480V 800 AMP MLO PARALLEL RUN

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Step by step of electric

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I have a client of mine which invited me to bid for power distribution in his warehouse, his warehouse has a 1900amp siemens switch board which i will need to provide a 500 ft run from the switchboard to designated power distribution, Is it proper for me to install 3 sets of 2 1/2 emt conduit with 400 mcm alumminum wire with a 1/0 copper gu wire .

From the 480v distribution panel i will need to feed a 225kva transformer i know that at 480v i get 271 amps which i will need to multiply 125% for continuous load
which gives me 338 A and my next standard size is 350 amps is this correct because i have a supposed electrical engineer that works in the facilily that says "breaker size of the step-down transformer to 250A. Breaker protection for transformer should not be more than it's capacity"

what primary side size conductor at 75c is necessary to feed a 225kva transfomer in parallel.


thank you!
 
Step by step of electric said:
I have a client of mine which invited me to bid for power distribution in his warehouse, his warehouse has a 1900amp siemens switch board which i will need to provide a 500 ft run from the switchboard to designated power distribution, Is it proper for me to install 3 sets of 2 1/2 emt conduit with 400 mcm alumminum wire with a 1/0 copper gu wire .

From the 480v distribution panel i will need to feed a 225kva transformer i know that at 480v i get 271 amps which i will need to multiply 125% for continuous load
which gives me 338 A and my next standard size is 350 amps is this correct because i have a supposed electrical engineer that works in the facilily that says "breaker size of the step-down transformer to 250A. Breaker protection for transformer should not be more than it's capacity"

what primary side size conductor at 75c is necessary to feed a 225kva transfomer in parallel.


thank you!
Click to expand...

So, to start, I am going to assume that all of this is sufficient to feed the loads. You should start with your load, and move your way back towards the main.

1900A main doesn't make sense. Maybe that is a typo and it is 1600A.

For the 500ft run, I am confused as to what that OCPD's rating is. Is that the "1900A" or is that some other protected size? What is the rating of the power distribution board?

3x 400MCM aluminum is 3x270 = 810A.

If your OCPD is 800A than your EGC is 1/0 Cu.

That is all sufficient.

The transformer is protected by table 450.3(B). You don't need to multiply by 125%. Typical for a 225kVA 480V-120/208V transformer is a 300A or 350A breaker or fused disconnect in line of sight of the transformer. There is no rule that says you can not exceed the current rating of the transformer. In fact, it is typical to overload them to a small degree. What you can not do is exceed the rating of the conductor. That is a no-no.

Your primary side conductor should be suitable for the load on the low side of the transformer. If you wanted to use 400MCM Al is okay with a 300A breaker, so long as the load is less than ( 270 * 480 / 208 ) = 620A. If you go with a 350A breaker or fused disconnect than you need 350A of conductor or rounded up to 350A of conductor. It still must be suitable for the load on the secondary end. 500kcmil AL is an example of one that works.

The low side of the transformer is a little different. You will need to take into account your secondary OCPD. If it is 600A, than you need MORE than 600A of conductor ran. You must be equal to or exceed your secondary's first OCPD. The grounding conductor ran on to the secondary's first OCPD is sized by table 250.102(C) and is referred to as a Supply side bonding jumper.
 
Elect117 said:
So, to start, I am going to assume that all of this is sufficient to feed the loads. You should start with your load, and move your way back towards the main.

1900A main doesn't make sense. Maybe that is a typo and it is 1600A.

For the 500ft run, I am confused as to what that OCPD's rating is. Is that the "1900A" or is that some other protected size? What is the rating of the power distribution board?

3x 400MCM aluminum is 3x270 = 810A.

If your OCPD is 800A than your EGC is 1/0 Cu.

That is all sufficient.

The transformer is protected by table 450.3(B). You don't need to multiply by 125%. Typical for a 225kVA 480V-120/208V transformer is a 300A or 350A breaker or fused disconnect in line of sight of the transformer. There is no rule that says you can not exceed the current rating of the transformer. In fact, it is typical to overload them to a small degree. What you can not do is exceed the rating of the conductor. That is a no-no.

Your primary side conductor should be suitable for the load on the low side of the transformer. If you wanted to use 400MCM Al is okay with a 300A breaker, so long as the load is less than ( 270 * 480 / 208 ) = 620A. If you go with a 350A breaker or fused disconnect than you need 350A of conductor or rounded up to 350A of conductor. It still must be suitable for the load on the secondary end. 500kcmil AL is an example of one that works.

The low side of the transformer is a little different. You will need to take into account your secondary OCPD. If it is 600A, than you need MORE than 600A of conductor ran. You must be equal to or exceed your secondary's first OCPD. The grounding conductor ran on to the secondary's first OCPD is sized by table 250.102(C) and is referred to as a Supply side bonding jumper.
Click to expand...
Correct 1600*swb
My distribution panelboard will be 480v 800 amp MLO ,
The 1600* switchboard that will feed the 800a distribution will be a 500ft conduit run
That will be my 3 sets of 2 1/2 emt with 400 mcm aluminum and 1/0 cu ground which 3x270=810A

From the 800 amp distribution i will feed a 480v-120/208v 225kva transformer with copper conductors
I wanted to understand the ocp on the primary and secondary for the 225kva transformer
if i protect my primary will i also need to protect the secondary

thank you!
 
Step by step of electric said:
Correct 1600*swb
My distribution panelboard will be 480v 800 amp MLO ,
The 1600* switchboard that will feed the 800a distribution will be a 500ft conduit run
That will be my 3 sets of 2 1/2 emt with 400 mcm aluminum and 1/0 cu ground which 3x270=810A
Click to expand...

You will need something to protect the 800A of wire. I am still confused if you are saying you have a 800A OCPD or if you are running it from a 1600A breaker / main. That wire size, to the 800A panel, would need to meet or exceed the OCPD's rating when talking about above 800A.

So if your only upstream protection is 1600A, then you need 1600A of wire.

P.S. if it is 1600A, than I suggest making it a tap and putting in a fused disconnect (240.21(B)). I would not suggest just landing 800A of conductor on an MLO. That is perfectly legal, but I would not suggest it. I would put in a fused disconnect. 500ft is a long way to have to run to disconnect the panelboard if something goes wrong in a industrial establishment.


Step by step of electric said:
if i protect my primary will i also need to protect the secondary
Click to expand...


The short answer is yes. You aren't protecting the transformer with the secondary protection but rather the conductors and loads. Transformers are considered separately derived and are not directly electrically connected. They are magnetically coupled. So the wires on the secondary side are rarely protected by the primary side. There are a few instances where they are. A typical 480-208/120 wye is not protected by the primary side.

Some code sections for you to look at would be 240.21(C) and 408.36. They basically say that you need an OCPD on the secondary side unless it is a single phase transformer or a delta - delta transformer.
 
Elect117 said:
You will need something to protect the 800A of wire. I am still confused if you are saying you have a 800A OCPD or if you are running it from a 1600A breaker / main. That wire size, to the 800A panel, would need to meet or exceed the OCPD's rating when talking about above 800A.

So if your only upstream protection is 1600A, then you need 1600A of wire.

P.S. if it is 1600A, than I suggest making it a tap and putting in a fused disconnect (240.21(B)). I would not suggest just landing 800A of conductor on an MLO. That is perfectly legal, but I would not suggest it. I would put in a fused disconnect. 500ft is a long way to have to run to disconnect the panelboard if something goes wrong in a industrial establishment.





The short answer is yes. You aren't protecting the transformer with the secondary protection but rather the conductors and loads. Transformers are considered separately derived and are not directly electrically connected. They are magnetically coupled. So the wires on the secondary side are rarely protected by the primary side. There are a few instances where they are. A typical 480-208/120 wye is not protected by the primary side.

Some code sections for you to look at would be 240.21(C) and 408.36. They basically say that you need an OCPD on the secondary side unless it is a single phase transformer or a delta - delta transformer.
Click to expand...
sorry for the confusion i have a existing 1600 amp switch board where i will install a 800 amp breaker for my 800 amp MLO distribution panel where it will be provided approximate 500 ft away and install my 3 sets of 2 1/2 emt conduit with 4wire 400mcm aluminum and 1/0 cu ground for my feeders. and from there i need to provide a step down 225kva transformer that will sit next to the 800 amp distribution to provide 120/208 600 amp MCB panelboard. 600 amp panel will be next to transformer.
 
Step by step of electric said:
sorry for the confusion i have a existing 1600 amp switch board where i will install a 800 amp breaker for my 800 amp MLO distribution panel where it will be provided approximate 500 ft away and install my 3 sets of 2 1/2 emt conduit with 4wire 400mcm aluminum and 1/0 cu ground for my feeders. and from there i need to provide a step down 225kva transformer that will sit next to the 800 amp distribution to provide 120/208 600 amp MCB panelboard. 600 amp panel will be next to transformer.
Click to expand...

Sounds good. Then everything I said above should make sense for the install. Just make sure the load calcs are good and a quick voltage drop calc as raberding suggested above.

P.S. I didn't check conduit fill because even if it is less than the fill, you are the one that has to pull it. So going smaller on the conduit might make the pull harder. Also depends on # of bends and pull direction. But that part is the installers fun. :)
 
The 3 sets of 400 aluminum my VD calc says you're good up to 700 amps of load with less than 3% VD. Feeding a 600 amp panelboard with a 225 kva transformer is fine also. You'll need 600 conductors and a 600 amp OCPD on the secondary.
 
If 480 V it is primary rating and if the rated apparent power it is 225 kVA, then the primery rated current it is 225/1.73/0.48=271 A. According to Table 450.3(B) -for primery protection only-will be 125% .So 271*1.25=339 then 350 A.
As per Table 310.16 for 300 mcm aluminum conductor 75oC 230 A[parallel 2 is 460 A].
According to Table 8.2(7)[ Annex B] for 90 ohm.m Earth 3 ducts in concrete duct-bank
[Detail 2] the ampacity of 300 mcm aluminum is still 204 A.
Using Table 9 for 300 mcm aluminum the impedance per kft is 0.088 ohms then using only 2 parallel cables voltage drop it is 1.73*350*0.088*500/1000/2=13.321 V[2.78%]
 
Julius Right said:
If 480 V it is primary rating and if the rated apparent power it is 225 kVA, then the primery rated current it is 225/1.73/0.48=271 A. According to Table 450.3(B) -for primery protection only-will be 125% .So 271*1.25=339 then 350 A.
As per Table 310.16 for 300 mcm aluminum conductor 75oC 230 A[parallel 2 is 460 A].
According to Table 8.2(7)[ Annex B] for 90 ohm.m Earth 3 ducts in concrete duct-bank
[Detail 2] the ampacity of 300 mcm aluminum is still 204 A.
Using Table 9 for 300 mcm aluminum the impedance per kft is 0.088 ohms then using only 2 parallel cables voltage drop it is 1.73*350*0.088*500/1000/2=13.321 V[2.78%]
Click to expand...
Sounds like you've confused the underground feeder with the transformer conductors. The underground feeder is 800 amps.
 
Julius Right said:
If 480 V it is primary rating and if the rated apparent power it is 225 kVA, then the primery rated current it is 225/1.73/0.48=271 A. According to Table 450.3(B) -for primery protection only-will be 125% .So 271*1.25=339 then 350 A.
As per Table 310.16 for 300 mcm aluminum conductor 75oC 230 A[parallel 2 is 460 A].
According to Table 8.2(7)[ Annex B] for 90 ohm.m Earth 3 ducts in concrete duct-bank
[Detail 2] the ampacity of 300 mcm aluminum is still 204 A.
Using Table 9 for 300 mcm aluminum the impedance per kft is 0.088 ohms then using only 2 parallel cables voltage drop it is 1.73*350*0.088*500/1000/2=13.321 V[2.78%]
Click to expand...

infinity said:
Sounds like you've confused the underground feeder with the transformer conductors. The underground feeder is 800 amps.
Click to expand...

I don't think the OP said anything about underground? Did I miss that?
 
Now it is clearer but still confusing a bit.480 V 800 A feeder.jpg
Why do I need to know the transformer protection switch?
 
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