Transformer.

[infinity]

Actually next size up allowed for the 450 transformer protection, but not for the 240 conductor protection.

Yes, similar meaning but that's a better way of putting it.

 

[yordanys]

125% is the minimum required for the primary but you can go higher (up to 250%) as per Table 450.3(B) since in this case you are required to have secondary protection as well. There is nothing wrong with using 125% for both the primary and secondary. Larger than 125% on the primary gives you more room to avoid nuisance tripping with the inrush current.

Look closely at 450.3(B). IF you have secondary protection at 125%, the primary protection can be as high as 250%. The condcutors would need to be sized to meet the OCP device,

where on the 2017 NEC say the conductor have to meet the conductor size? Thank you in advances.

 

[hillbilly1]

Great! now the thing is the transformer will be approximately 1000 feet away from the power source and after applying VD formula my conductor size will increase to 4/0 awg. I think the best thing is to replace the ckt breaker for a 50 amp at primary side and run a #1 awg. Just thinking about installation cost.

Where did you come up with 4/0? You do not figure voltage drop by breaker size, you figure it by load. Which would be #1 on the primary side.

 

[yordanys]

Where did you come up with 4/0? You do not figure voltage drop by breaker size, you figure it by load. Which would be #1 on the primary side.

Voltage drop formula:

Cmil=1.73 x K x I x d/Vd

Cmil=1.73x21.2x70x1000feet/480x0.03

cmil=178,286.11 table 8 chapter 9 #4/0

 

[hillbilly1]

Voltage drop formula:

Cmil=1.73 x K x I x d/Vd

Cmil=1.73x21.2x70x1000feet/480x0.03

cmil=178,286.11 table 8 chapter 9 #4/0

You don’t use 70 amps, your max load is limited by the secondary ocp. Your load on the primary will the same as using the 50 amp breaker.

 

[yordanys]

Thank you guys for all your responses I appreciate.. have a great week everyone.

 

[don_resqcapt19]

where on the 2017 NEC say the conductor have to meet the conductor size? Thank you in advances.

There is nothing in 2404(A) through (G) that says the main rule in 240.4 does not apply, so the transformer primary conductors must be protected at or below their ampacity.

 

[jackslattc]

I agree, next standard size up rule for the primary just not for the secondary.

Hello Infinity,

In 2017 Code where does it prohibited next size up rule for the secondary?

Thanks for your time.

 

[LarryFine]

Great! now the thing is the transformer will be approximately 1000 feet away from the power source and after applying VD formula my conductor size will increase to 4/0 awg. I think the best thing is to replace the ckt breaker for a 50 amp at primary side and run a #1 awg. Just thinking about installation cost.

With either size wire, why not keep the 70a breaker?

 

[electrofelon]

Hello Infinity,

In 2017 Code where does it prohibited next size up rule for the secondary?

Thanks for your time.

240.21(C)