Medium Voltage to Low Voltage Transformer

[kentwc]

I am installing a 12,470 volt down to a 480/277 volt transformer rated for 5051.8 amps for LV and 194.5 amps for the HV. I plan on running 1/0awg MV-90 220 mil 133% insulation and a number 6 awg ground for the HV side. On the LV side I am going to a 2000 amp breaker. I plan on running 600 kcmils parallel run of 6 wires each phase. For the neutral side i plan on running 6 runs of 250 kcmils and also my grounding wire of 6 runs of 250 kcmils. I am running the LV side in 6 runs of 3 1/2" emt conduits. Am I doing my calculations right or am I missing something here? FYI I am coming from a HV transfer switch inside the building to outside the building to the transformer than back up inside the building to the 2000 amp breaker.

 

[Julius Right]

Sorry, kentwc, you post, it seems to me ,it is very inconsistent.
At first I think your transformer is 2400 kVA rated so the LV current has to be 2400/sqrt(3)/0.48= 2886.8 A. If you forgot to divide by sqrt(3) you got 5000 A.
Second, ampacity of a cable of 600 MCM copper according to NEC Tb.310.15(B)(16)
-let's say the breakers terminals are 90oC rated- will be 475 A so 6*475=2850 only .
The breaker has to be of 1.25*2886.8=3600 A minimum, rated, so you need now 8*475=3800A.
I think will be better to use busbar instead of cables.

 

[kentwc]

Sorry, kentwc, you post, it seems to me ,it is very inconsistent.
At first I think your transformer is 2400 kVA rated so the LV current has to be 2400/sqrt(3)/0.48= 2886.8 A. If you forgot to divide by sqrt(3) you got 5000 A.
Second, ampacity of a cable of 600 MCM copper according to NEC Tb.310.15(B)(16)
-let's say the breakers terminals are 90oC rated- will be 475 A so 6*475=2850 only .
The breaker has to be of 1.25*2886.8=3600 A minimum, rated, so you need now 8*475=3800A.
I think will be better to use busbar instead of cables.

Sorry if it seems inconsistent. So i am replacing a delta delta transformer to a delta wye transformer. The transformer we purchased is rated for 5051.8 amps on the LV side. We currently have our transformer feeding a 2000 amp breaker for our main distribution panel. Right now they only want me to run enough cable to feed the 2000 amp breaker. The transformer will be placed outside the building, so i will have to run my HV from the disconnect switch inside the building down to the transformer outside. Than with my LV I will have to run the cables from the transformer back into the building to my 2000 amp breaker. I was unaware that i can use busbar for that type of application?? Also i was going off the 75oC column which would be 420*6=2520 amps. But i will have a neutral ran with my 3 current carrying conductors ran in conduit for the last 10 ft so i will have to deduct by 80% which puts me at 2016 amps.

 

[kentwc]

View attachment W40981-IP.pdf

 

[steve66]

You don't have to derate for the neutral unless the majority of the load is non-linear and creates harmonics. Unlikely with a 2000 amp service. We just went through this on another thread.

Also, even if you had to derate, you would probably use a conductor rated for 90 deg C in a wet location, so you would be derating from 475 amps per run (the ampacity of the cable at 90 deg C.) (Wire terminations are normally limited to the 75 deg C ampacity, but for derating for more than 3 CCC in a conduit, you can use the 90 deg. ampacity if you have the right type of wire.)

So I would probably use (5) sets of 600KCM if the voltage drop isn't too high. How far is the run?

The 3.5" conduit sounds OK.

The 1/0 primary is good for 225 amps. Will you fuse the primary run at its source at 225 amps or less?

How did you get 250 KCM for the neutral? Are there quite a few three phase loads that won't have any neurtal current?

Also, I'm not sure about the ground size. There is a rule about the ground being a minimum of 12.5% of the total phase conductor area, but I can't find it right now. I can't remember if it only applies to 250.122, or 250.66.

 

[Julius Right]

In my opinion, it is another problem here:
I think-according to NEC Art.240.21(C) if the cable from transformer to circuit breaker 2000 A it is more than 7.5 m you have a transformer secondary circuit breaker of 3*5051 A[15 kA rated].
Since the transformer connection is Delta-Yn art.240.21(C)(1) Protection by Primary Overcurrent Device is not permissible.
However, in any case, the ampacity of the secondary conductors has to be not less than the secondary current rating of the transformer, and the sum of the ratings of the overcurrent devices does not exceed the ampacity of the secondary conductors.

 

[Julius Right]

Sorry! wrong factor. I said: “you have a transformer secondary circuit breaker of 3*5051 A[15 kA rated]” Factor is only 1.25 [Table 450.3 secondary less than 1000 V]
Then actually it has to be 6500 A[However, it’s not important: maximum rated circuit
breaker I know it is only 4000 A]:ashamed1:

 

[Smart $]

... The transformer we purchased is rated for 5051.8 amps on the LV side. ...

Let's back up a little bit here. That 5051.8 nameplate rated amps is dependent on how you operate the transformer... it is based on fan cooled and 65°C temperature rise. If you are not using fan cooling you have to use the left of slash kVA rating to determine rated amperes... and if only 55°C temperature rise, the left set of kVA ratings.

Also, secondary protection at 125%, next size up if not a standard ocpd rating, is the maximum. This can be increased to 250% if in a supervised location. If tranny has coordinated thermal overload protection, secondary protection can be omitted*. This is according to Table 450.3(A).

* Does not relieve secondary conductor protection required by 240.21(C).


When you follow 240.21(C), I believe you only have to size your conductors to the OCPD at the first disconnecting means (2000A). And you only have to derate for 4 conductors per raceway if your loads are more than 50% nonlinear (electronic power systems). Otherwise, the neutral does not count as a current-carrying conductor.

 

[Smart $]

... For the neutral side i plan on running 6 runs of 250 kcmils and also my grounding wire of 6 runs of 250 kcmils. ...

If there are no metallic paths between transformer frame/enclosure and grounding system at building, you can install a system bonding jumper at both transformer and first disconnecting means and omit the supply-side bonding jumper (aka grounding wire)... use the grounded conductor (X0) to land required GE/GEC and SBJ frame/enclosure. And SBJ neutral to EGC at building.

 

[kentwc]

You don't have to derate for the neutral unless the majority of the load is non-linear and creates harmonics. Unlikely with a 2000 amp service. We just went through this on another thread.

Also, even if you had to derate, you would probably use a conductor rated for 90 deg C in a wet location, so you would be derating from 475 amps per run (the ampacity of the cable at 90 deg C.) (Wire terminations are normally limited to the 75 deg C ampacity, but for derating for more than 3 CCC in a conduit, you can use the 90 deg. ampacity if you have the right type of wire.)

So I would probably use (5) sets of 600KCM if the voltage drop isn't too high. How far is the run?

The 3.5" conduit sounds OK.

The 1/0 primary is good for 225 amps. Will you fuse the primary run at its source at 225 amps or less?

How did you get 250 KCM for the neutral? Are there quite a few three phase loads that won't have any neurtal current?

Also, I'm not sure about the ground size. There is a rule about the ground being a minimum of 12.5% of the total phase conductor area, but I can't find it right now. I can't remember if it only applies to 250.122, or 250.66.

On the primary run I am coming off a switchboard that has multiple disconnects feeding 3 different plants straight down to the transformer so it will not have protection on the primary side. On the neutral for 250 kcmil i went off 220.61 which says take 70%+200/6 runs of conduit which gave me 243.3 which the next size up would be 250 kcmils and yes their is a lot of three phase loads that won't have any neutral current. I am thinking I do not need to run a ground wire if I use a system bonding jumper at both sides like Smart $ has said. Also the run is only 100 Feet