# Thread: Transformer Primary Conductors Calculation

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Originally Posted by TheElectrician
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.

2. Originally Posted by TheElectrician
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.
No, I mean the service disconnect, but the service disconnect and the primary protection could be the same

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Originally Posted by oldsparky52
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)?

Originally Posted by oldsparky52
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/mojonewsarc...n~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

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Originally Posted by electrofelon
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?

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Originally Posted by TheElectrician
Thank you for that, I should have known that. duh

6. Originally Posted by TheElectrician
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.

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Originally Posted by electrofelon
No, I mean the service disconnect, but the service disconnect and the primary protection could be the same
Since the power is being generated on site, where does the service disconnect get placed?

8. Originally Posted by oldsparky52
Since the power is being generated on site, where does the service disconnect get placed?
The requirements are not any different for a "generation only" electrical service.

9. Originally Posted by TheElectrician
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
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.

10. Originally Posted by TheElectrician
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?

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.