# Transformer Primary Conductors Calculation

#### TheElectrician

##### Senior Member
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.
Since the power is being generated on site, where does the service disconnect get placed?
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.
I would like to clarify here that initially we did not have a disconnect on the primary side (4kV side) of the transformer. But we realized we needed a primary protection according to NEC 450.30 and the one line was later updated. We also have a main disconnect for the solar system (89L) on the secondary side (480V side) of the transformer. So the setup is basically Inverters -> 52IT -> Meter -> 89L -> Transformer -> Utility

#### electrofelon

##### Senior Member
I would like to clarify here that initially we did not have a disconnect on the primary side (4kV side) of the transformer. But we realized we needed a primary protection according to NEC 450.30 and the one line was later updated. We also have a main disconnect for the solar system (89L) on the secondary side (480V side) of the transformer. So the setup is basically Inverters -> 52IT -> Meter -> 89L -> Transformer -> Utility
Think you meant 450.3...also add to the red "I also realized I needed a service disconnect

#### TheElectrician

##### Senior Member
Think you meant 450.3...also add to the red "I also realized I needed a service disconnect
Yeah, that is right! Thank you!

#### TheElectrician

##### Senior Member
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.
Yes it is MV-105. Yeah that is true, MV is so weird lol!

#### david luchini

##### Moderator
Staff member
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.
I don't see any requirement for the above.

You would need to use the 90deg ampacity unless the terminations are identified for the higher temperature (110.40.) Much the same way as the 75/90 deg rating for LV.

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#### electrofelon

##### Senior Member
I don't see any requirement for the above.
So there is no continuous load adder for feeders over 1000 volts?

#### david luchini

##### Moderator
Staff member
So there is no continuous load adder for feeders over 1000 volts?
I don't see it called for in the Code...unless I am missing it somewhere.

#### topgone

##### Senior Member
So there is no continuous load adder for feeders over 1000 volts?
Please refer to IEEE Std 885 for standard power cable ampacity tables.

#### electrofelon

##### Senior Member
I don't see it called for in the Code...unless I am missing it somewhere.
I didn't look but I'm sure you are correct. Interesting!