Sizing Wire

[roger]

As I said earlier, try to consult with a local EE or EC that is experienced with this type of installation. MV work is not forgiving if something does go wrong.

Sorry, but I don't give out my number.

 

[JonathanWaldner]

Ok then. Thanks for your info

 

[roger]

You're welcome and good luck with the project.

 

[JonathanWaldner]

Your transformer will be oversized, suggest looking at standby loss, and then for a smaller transformer

So what transformer would you suggest

 

[Jpflex]

Don't you need a special wire if you go over 600 volts I usually used 4/0 but that is only rated for 600 volts so what kind of wire would you use for that

Good point and yes most wire like THHN is rated for 600 volts max. The high side primary will need specific wire for 7200 volts.

 

[JonathanWaldner]

Good point and yes most wire like THHN is rated for 600 volts max. The high side primary will need specific wire for 7200 volts.

So wich wire do the usually use for 7200 volts

 

[Jpflex]

So wich wire do the usually use for 7200 volts

I’ve only encountered 15k wire but I’m sure anything equal or higher to your rating will suffice. As to which type I’m not an expert on that.

 

[Seven-Delta-FortyOne]

So wich wire do the usually use for 7200 volts

It was posted a couple times: MV cable.

Have you ever done MV terminations? I used to do them a lot when I worked on industrial electrical.

They take about 45 minutes each, there is a learning curve to doing them, special tools and methods.

Like Roger said, it’s unforgiving, and failures are usually dangerous catastrophic and costly

 

[Jpflex]

It was posted a couple times: MV cable.

Have you ever done MV terminations? I used to do them a lot when I worked on industrial electrical.

They take about 45 minutes each, there is a learning curve to doing them, special tools and methods.

Like Roger said, it’s unforgiving, and failures are usually dangerous catastrophic and costly

If this includes 4800 volt line going to transformer load breaks then I have also done this but I do not know what type wire the company/ engineer decided to use

 

[roger]

If this includes 4800 volt line going to transformer load breaks then I have also done this but I do not know what type wire the company/ engineer decided to use

5 KV wil not do it for the OP's situation.

 

[wwhitney]

4000 feet . . .480 from my switch . . . 208/120 . . . 400 amp service

I'm sure others here have done the cost comparisons, but I have the impression that at that distance and service size (particularly as the appropriate load for the controlling voltage drop calculation will be less than 400A), it would be most economical to run the 480V feeder 4000 feet, and use a single transformer at the load end of that run.

Cheers, Wayne

 

[Jpflex]

I'm sure others here have done the cost comparisons, but I have the impression that at that distance and service size (particularly as the appropriate load for the controlling voltage drop calculation will be less than 400A), it would be most economical to run the 480V feeder 4000 feet, and use a single transformer at the load end of that run.

Cheers, Wayne

The wire size also would depend on the actual load amperes that he or she would need to supply as you stated probably less than 400i)

Assuming the possibility of delivering a full load at 400i amperes with 4,000 foot feeders at 480 volts you would see a whopping 173.33i amperes on the 4,000 foot run to the final step down 144.1 KVA transformer

Your new feeder voltage drop limit will be 9.6 volts

And the wire size needed for this 4,000 feet would be 774,535,505 circular mills if not paralleled and therefore is not practical. The largest single conductor shown by NEC is 2,000 kcmil

 

[wwhitney]

Assuming the possibility of delivering a full load at 400i amperes with 4,000 foot feeders at 480 volts you would see a whopping 173.33i amperes on the 4,000 foot run to the final step down 144.1 KVA transformer

But you don't typically expect a 400A service to have 400A of load. Depending on the load calc and future needs, using 100A might be acceptable.

Your new feeder voltage drop limit will be 9.6 volts

I don't see the need to limit voltage drop to 2%, 3% would be fine.

And the wire size needed for this 4,000 feet would be 774,535,505 circular mills

Something is wrong with your calculator, I get 3000 kcm Cu for 4000 feet, 173A, and 9.6V voltage drop.

So for 100A and 14.4V voltage drop, you could use (4) sets of 300 kcm Al. Which is a lot of cable, but you have to compare that with the added cost of the MV transformers and the MV cable and MV installation practices.

Cheers, Wayne

 

[Jpflex]

But you don't typically expect a 400A service to have 400A of load. Depending on the load calc and future needs, using 100A might be acceptable.

I don't see the need to limit voltage drop to 2%, 3% would be fine.

Something is wrong with your calculator, I get 3000 kcm Cu for 4000 feet, 173A, and 9.6V voltage drop.

So for 100A and 14.4V voltage drop, you could use (4) sets of 300 kcm Al. Which is a lot of cable, but you have to compare that with the added cost of the MV transformers and the MV cable and MV installation practices.

Cheers, Wayne

We really don’t know what load(s) are. If this were a 1 or 2 family dwelling and under specific conditions we could also lower the service wires ampacity to 83% below the service rating i believe

I agree that it wouldn’t matter much in real world situations between 2-3% voltage drop for the feeders but i selected 2% as code allows a maximum of 5% drop between feeders and branch circuit and only 3% voltage drop on branch circuits. Mr Mike Holt did a good job explaining the impracticality of this code with all circuits to be treated in this way. He explained that most manufactures merely require voltage to be within 90% of nominal voltage.

Sometimes using my phone as a calculator results in wring figures. Let me re do this and see what went wrong.

Solve for cm

480 V x 0.02 = 1.732 (3 phase) x 12.9 (k copper) x 173.33i (amperes primary) x 4,000’ (feet) / cm (circular mills)

9.6 = 15,490710.10 / cm

Cm = 15,490710.10 / 9.6

Solve for cm = 1,613,615.64

This results in a single 1,750 kcmil conductor per phase at 4,000 feet with 173.33 i amperes (not saying they will load up the wire this much though)

 

[Jpflex]

We really don’t know what load(s) are. If this were a 1 or 2 family dwelling and under specific conditions we could also lower the service wires ampacity to 83% below the service rating i believe

I agree that it wouldn’t matter much in real world situations between 2-3% voltage drop for the feeders but i selected 2% as code allows a maximum of 5% drop between feeders and branch circuit and only 3% voltage drop on branch circuits. Mr Mike Holt did a good job explaining the impracticality of this code with all circuits to be treated in this way. He explained that most manufactures merely require voltage to be within 90% of nominal voltage.

Sometimes using my phone as a calculator results in wring figures. Let me re do this and see what went wrong.

Solve for cm

480 V x 0.02 = 1.732 (3 phase) x 12.9 (k copper) x 173.33i (amperes primary) x 4,000’ (feet) / cm (circular mills)

9.6 = 15,490710.10 / cm

Cm = 15,490710.10 / 9.6

Solve for cm = 1,613,615.64

This results in a single 1,750 kcmil conductor per phase at 4,000 feet with 173.33 i amperes (not saying they will load up the wire this much though)

Perhaps disregard the 83% rule as I’m not sure if it applies to 3 phase to dwelling service

 

[Seven-Delta-FortyOne]

2% as code allows a maximum of 5% drop between feeders and branch circuit and only 3% voltage drop on branch circuits.

The NEC does not stipulate any voltage drop. It’s merely a suggested best practice.

The only Code regarding voltage drop that I am aware of is Californias Energy Code.

 

[Jpflex]

The NEC does not stipulate any voltage drop. It’s merely a suggested best practice.

The only Code regarding voltage drop that I am aware of is Californias Energy Code.

NEC 210.19(A)(1) Informational Note No. 4 limits the voltage drop at the furthest outlet of a load to 3% of the applied voltage. This allows 2% drop in the feeder. Alternatively, the maximum combined voltage drops on the feeder and branch circuits going to the furthest outlet of a load should be limited to 5%.

Maybe here

 

[roger]

I.N.'s are not requirements.

 

[Jpflex]

I.N.'s are not requirements.

Ok so we most likely won’t have to do this for a journeyman’s test?

 

[roger]

Ok so we most likely won’t have to do this for a journeyman’s test?

Correct.