Transformer Primary Conductors Calculation

TheElectrician

Senior Member
Hey Guys,

We have a Solar system installed with the following specs:
- Voltage: 480 V
- Current: 1080 A

This is fed into the secondary of the 1000 kVA transformer with 4kV on the primary whose spec. is:
kVA= (1.732 x 480 x 1080)/1000 = 897.895 kVA (1000 kVA is used)

From this the primary current will be:
I = 897.995*1000/1.732*4000 = 129.6 A

The POCO asked us to use a 2/0 wire for the primary, but I wanted to verify this so, Can someone please provide a step by step process to calculate the conductor size for the primary according to NEC?

Thanks in Advance
 

oldsparky52

Senior Member
Did you ask the PoCo rep why they are requiring this size conductor?

What size conductor do you want to install, and what table would you use for its ampacity rating?
 
Last edited:

sameguy

Senior Member
Location
New York
Occupation
Master Elec./JW retired
Once you provide it, it becomes their property and they would maintain it as the 1000 KVA transformer is theirs and so not covered by NEC. Isn't it?
My thought too.
Perhaps not the owning but the specifying as to being a feeder into primary power.
 

TheElectrician

Senior Member
Did you ask the PoCo rep why they are requiring this size conductor?

What size conductor do you want to install, and what table would you use for its ampacity rating?
Yes, I asked the power company, they just asked me to follow their spec sheet and didnt give any further information on how they came up with it. They just are not giving that information. I was using the the teriplex conductor table 310.60(74) AL for 15kV, for sizing it using the derate factor 1.25
 

TheElectrician

Senior Member
Once you provide it, it becomes their property and they would maintain it as the 1000 KVA transformer is theirs and so not covered by NEC. Isn't it?
You are right, although they provided the t/f the customer owns it, but still they wanted to install the conductor by their specs, and I wanted to figure out how they came up with that number.
 

oldsparky52

Senior Member
Yes, I asked the power company, they just asked me to follow their spec sheet and didnt give any further information on how they came up with it. They just are not giving that information.
Do you think they may be calculating the 1,000KVA transformer at 100% usage instead of using the input amperage like you were? I calculate that to be 144.3-amps.
I was using the the teriplex conductor table 310.60(74) AL for 15kV, for sizing it using the derate factor 1.25
I never did any of this work so please excuse my ignorance, but what is the 1.25 factor for? Temperature, ...? You are multiplying the table ampacity by 1.25 to come up with the allowed ampacity?

W/out any adjustments, it would take a 2/0 cable to meet or exceed (using the table you mentioned) that 144-amp calculation.

I'm just speculating.
 

TheElectrician

Senior Member
Do you think they may be calculating the 1,000KVA transformer at 100% usage instead of using the input amperage like you were? I calculate that to be 144.3-amps.
Yes, I think so too. But is that the right way to do it? I assume we have to take the load current here which is 1080A, using that I get the primary current to be 129.6A. So should we take the name plate detail (1000kVA) to calculate the primary or the kVA (897.89 kVA) calculated from the load current (1080A)?

I never did any of this work so please excuse my ignorance, but what is the 1.25 factor for? Temperature, ...? You are multiplying the table ampacity by 1.25 to come up with the allowed ampacity?

W/out any adjustments, it would take a 2/0 cable to meet or exceed (using the table you mentioned) that 144-amp calculation.

I'm just speculating.
That is alright this is the first time I am designing medium voltage so I might be missing something here too. I thought I have to use the 1.25 factor which is mentioned in the Step 3 in the following link: https://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/TransformerInstallation~20020516.htm

So from this 144A*1.25=180A, cross referencing this with the table 310.60(C)(74) AL, I came up with the 2/0

But do let me know if that is the right way to do it from your experience. Thanks
 

TheElectrician

Senior Member
No, I mean the service disconnect, but the service disconnect and the primary protection could be the same
Yes, initially we didnt have a primary protection for the transformer, then we added it later. But on the same topic, Do I have to have a primary protection for the transformer like a service disconnect designed using the table 430.30 even if I connect the primary of the transformer directly to 4kV line of the utility via manhole?
 
Yes, initially we didnt have a primary protection for the transformer, then we added it later. But on the same topic, Do I have to have a primary protection for the transformer like a service disconnect designed using the table 430.30 even if I connect the primary of the transformer directly to 4kV line of the utility via manhole?
Well I am still not clear where the service point and service disconnect are, and it's hard to establish Much of anything without knowing that. You can't connect a customer transformer directly to the utility. A transformer isn't listed in 230 as something that can be on the line side of the service disconnect.
 

ron

Senior Member
The 4kV is directly connected to the grid which is the service voltage from the POCO, this is an underground run. Hope this helps. Please check attached
View attachment 22982
Seems odd that there is no meter or 4kV disconnect shown on the 4kV side of the transformer.

How do they get reimbursed for the generation or disconnect to maintain the transformer and balance of system?

If the utility wants the wire sized per their standards, we are just guessing how they come up with the size they do because they do not follow the NEC.
 
Hey Guys,

We have a Solar system installed with the following specs:
- Voltage: 480 V
- Current: 1080 A

This is fed into the secondary of the 1000 kVA transformer with 4kV on the primary whose spec. is:
kVA= (1.732 x 480 x 1080)/1000 = 897.895 kVA (1000 kVA is used)

From this the primary current will be:
I = 897.995*1000/1.732*4000 = 129.6 A

The POCO asked us to use a 2/0 wire for the primary, but I wanted to verify this so, Can someone please provide a step by step process to calculate the conductor size for the primary according to NEC?

Thanks in Advance
So assuming there is a 4kv service disconnect somewhere, note that 215.2(B) requires the transformer primary conductors to have an ampacity of at least the nameplate rating of the transformer, so you need at least 144 amps of conductor. The size if your 450.3(A) protection nay come into play too.... You also need the extra 25% on the 130 for continuous load, so your 2/0 looks a hair small, unless it is mv-105.

Edit: just want to say that I am not very experienced in mv design, so I could well be missing something or have something wrong. There always seems to be some weird things with MV that we are not used to in the LV world.
 
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