Overcurrent protection for transformer

[New EE]

I have a distribution panel that is rated at 208/120V, 3PH, 4W and need to provide a new 75KVA transformer to feed a new 480/277, 3PH, 4W panelboard. Since I'm coming off a 208/120V distribution panel and increasing the voltage to 480/277V am I sizing the primary and secondary overcurrent protection correctly?

Primary overcurrent protection - 75KVA/360 (208x1.732)= 208 x 1.25 = 260A
Use a 3P300A breaker for the primary protection

Secondary overcurrent protection - 75KVA/831 (480x1.732) = 90 x 1.25 = 11.5A. Use a 3P125A breaker for the secondary protection

 

[charlie b]

Other than your typo (11.5 versus 112.5), I think you have it right.

 

[jim dungar]

Today's energy efficient transformers, usually have high inrush currents, 75kVA units often exceed 12-14X. I would be prepared to size the primary breaker to have a magnetic/instantaneous setting able to handle this current.

 

[lmichael310]

I have a distribution panel that is rated at 208/120V, 3PH, 4W and need to provide a new 75KVA transformer to feed a new 480/277, 3PH, 4W panelboard. Since I'm coming off a 208/120V distribution panel and increasing the voltage to 480/277V am I sizing the primary and secondary overcurrent protection correctly?

Primary overcurrent protection - 75KVA/360 (208x1.732)= 208 x 1.25 = 260A
Use a 3P300A breaker for the primary protection

Secondary overcurrent protection - 75KVA/831 (480x1.732) = 90 x 1.25 = 11.5A. Use a 3P125A breaker for the secondary protection

Wouldnt the transformer KVA be divided by the phase to neutral voltage to give the largest current for the OCPD. i.e 75KVA/(120*1.732) and 75 KVA/(277*1.732).

 

[david luchini]

Wouldnt the transformer KVA be divided by the phase to neutral voltage to give the largest current for the OCPD. i.e 75KVA/(120*1.732) and 75 KVA/(277*1.732).

No, you use the phase-to-phase voltage to determine the transformer rated currents.

For a 75kVA transformer, it would be rated 208A on the 208V side and 90A on the 480V side.

 

[lmichael310]

No, you use the phase-to-phase voltage to determine the transformer rated currents.

For a 75kVA transformer, it would be rated 208A on the 208V side and 90A on the 480V side.

Does the sizing of the primary and secondary ocpd comply with Art 450.3 (B) which requires the ocpd to be sized at 250% for the primary and 125% for the secondary?

 

[jim dungar]

Does the above comply with Art 450.3 (B) which requires the ocpd to be sized at 250% for the primary and 125% for the secondary?

You are reading the requirement as a 'must be', when it is actually just a limit/maximum.

450.3(B) allows the primary device to be sized up to 250% if the secondary is sized not more than 125%.

 

[beanland]

Coordinate

Coordinate

You also need to make sure the 480V breaker trips before the 208V breaker. That is also a reason to size the 208V breaker at 250% and the 480V breaker at 125%. That guarantees that the 208V breaker will let the 480V breaker trip first for overloads. The 208V breaker will only trip for transformer faults.

Also, I assume this transformer is 208V delta to 480Y/277V wye? it does not impact the breaker but does affect the required protection. If the transformer is delta-delta, you don't need secondary protection, size the primary at 125%.

 

[don_resqcapt19]

You also need to make sure the 480V breaker trips before the 208V breaker. That is also a reason to size the 208V breaker at 250% and the 480V breaker at 125%. That guarantees that the 208V breaker will let the 480V breaker trip first for overloads. The 208V breaker will only trip for transformer faults. ...

I see no reason to corrdinate between the primary and secondary devices of the same transformer. If either device trips, the result is the same.

 

[beanland]

Coordination

Coordination

I agree, the effect on the load is the same.

However, if a breaker trips, it is nice to know whether the fault is on the transformer secondary or in the transformer. By having slower response on the transformer primary, troubleshooting is easier.

Also, if inrush is a problem, you need a slower breaker on the transformer primary than the secondary.

 

[jim dungar]

I... it is nice ...is easier.

Nice and easy are not code requirements, they are design choices.

 

[lmichael310]

I have a distribution panel that is rated at 208/120V, 3PH, 4W and need to provide a new 75KVA transformer to feed a new 480/277, 3PH, 4W panelboard. Since I'm coming off a 208/120V distribution panel and increasing the voltage to 480/277V am I sizing the primary and secondary overcurrent protection correctly?

Primary overcurrent protection - 75KVA/360 (208x1.732)= 208 x 1.25 = 260A
Use a 3P300A breaker for the primary protection

Secondary overcurrent protection - 75KVA/831 (480x1.732) = 90 x 1.25 = 11.5A. Use a 3P125A breaker for the secondary protection

I have a related question. I was attempting to find out the wire size of the conductors that connect the transformer primary and secondary to the panelboards. Using Table 310.15(B)(16) and 250.122, I get a 300 kcmil conductor for the primary based off the 260 A current and I get a #2 AWG conductor for the secondary based off the 112 A current. Using Table 250.122 I get a #4 AWG and #6 AWG CU GND conductor for the primary and secondary respectively. Are these wire sizes correct?

 

[david luchini]

I have a related question. I was attempting to find out the wire size of the conductors that connect the transformer primary and secondary to the panelboards. Using Table 310.15(B)(16) and 250.122, I get a 300 kcmil conductor for the primary based off the 260 A current and I get a #2 AWG conductor for the secondary based off the 112 A current. Using Table 250.122 I get a #4 AWG and #6 AWG CU GND conductor for the primary and secondary respectively. Are these wire sizes correct?

These wire sizes are partially correct and partially incorrect based on the stated circuit breaker sizes:

Primary overcurrent protection - 75KVA/360 (208x1.732)= 208 x 1.25 = 260A
Use a 3P300A breaker for the primary protection

Secondary overcurrent protection - 75KVA/831 (480x1.732) = 90 x 1.25 = 11.5A. Use a 3P125A breaker for the secondary protection

300 MCM is OK for the 300A primary c/b, using the next standard size up rule, and the #4G is correct for the 300A c/b.

#2 Awg on the secondary, is too small for the 125A secondary c/b per 240.21(C). It needs to be at least #1 Awg. #6G on the secondary is correct, but not from 250.122, but from 250.66 (via 250.30.)