# Thread: Transformer Primary Conductors Calculation

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## Transformer Primary Conductors Calculation

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?

2. Are you taking service at primary voltage? Where does does the 4kv go? Where is the service disconnect? Those answers will help.

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Originally Posted by electrofelon
Are you taking service at primary voltage? Where does does the 4kv go? Where is the service disconnect? Those answers will help.
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
Last edited by TheElectrician; 05-23-19 at 02:52 PM.

4. Where is the service disconnect?

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Originally Posted by electrofelon
Where is the service disconnect?
Do you mean the primary protection of the transformer? if so it is not added in this one line, but there is a disconnect before (on the secondary side) of the transformer.
Last edited by TheElectrician; 05-23-19 at 05:28 PM.

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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 by oldsparky52; 05-24-19 at 07:04 AM.

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Originally Posted by TheElectrician

The POCO asked us to use a 2/0 wire for the primary,

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?

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Originally Posted by Sahib
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.

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Originally Posted by oldsparky52
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

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Originally Posted by Sahib
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.