Transformer Primary Conductors

[NClark]

Referencing table 450.3B and primary protection. Assuming you use the maximum 250% allowance for your device rating, how are you calculating the primary conductors? Are they still sized at 125% assuming a continuous load since the primary device at that point is short circuit protection? OR do you match the conductors to the device?

15kVA 480V/120-208V poly phase transformer

15*1000/480*1.73 = 18.1A

18.1 *2.5 = 45.25 (can we round up here??) using a 50A primary protective device?

assuming terminations are 75 degree rated
18.1*1.25 = 22.6 so we could use 10awg
OR
matching the device at 250% so 8awg??

NEC references appreciated.

 

[charlie b]

1. Your math is OK.
2. You can use any value at or below 250%. 250 is a maximum; there is no minimum.
3. No, you can't round up to 50. Note 1 to table 450.3(B) says you can round up from 125%, but it doesn't give you that from 250%.
4. You first select the wire, then select a breaker that can protect that wire.
5. If you select #10 wire, the highest breaker rating is 30 amps.
6. If you select #8 wire, the highest breaker rating is 45 amps.
7. I would usually select #10 with a 30 amp breaker.

 

[NClark]

so to clarify if i want to use the maximum 250%, the protective device being rated at 45A, the conductors must match the 45A rating.

1. Your math is OK.
2. You can use any value at or below 250%. 250 is a maximum; there is no minimum.
3. No, you can't round up to 50. Note 1 to table 450.3(B) says you can round up from 125%, but it doesn't give you that from 250%.
4. You first select the wire, then select a breaker that can protect that wire.
5. If you select #10 wire, the highest breaker rating is 30 amps.
6. If you select #8 wire, the highest breaker rating is 45 amps.
7. I would usually select #10 with a 30 amp breaker.

 

[augie47]

so to clarify if i want to use the maximum 250%, the protective device being rated at 45A, the conductors must match the 45A rating.

yes
and your secondary protection would need to be set at no more than 125% (next size up allowed)

 

[don_resqcapt19]

The size of the primary conductors is one reason that the primary OCPD is almost always less than 150%, and one of my design engineer friends will often specify the primary OCDP below 125%.

 

[electrofelon]

Note the provision in 240.21(B)(3) which may essentially allow you to use smaller size conductors with that 250% ocpd

 

[electrofelon]

The size of the primary conductors is one reason that the primary OCPD is almost always less than 150%, and one of my design engineer friends will often specify the primary OCDP below 125%.

On that topic, it may be good for the OP to read this currently ongoing thread on transformer inrush:

Help solve a debate between my twin brother and I (both electrical engineers) read message below for details

So you're saying if I can temporarily ground both the primary And secondary after power is removed I may have a better chance of measuring max inrush? Laboratories have a hard time measuring the true inrush, I can't imagine you will be able to do it in the field.

forums.mikeholt.com

 

[NClark]

Note the provision in 240.21(B)(3) which may essentially allow you to use smaller size conductors with that 250% ocpd

240.21(B)(3) specifically applies to taps no?

 

[electrofelon]

240.21(B)(3) specifically applies to taps no?

Yes, it refers to supplying a transformer from a feeder tap - in other words the primary conductors have an ampacity less than the primary ocpd. So you're going to have that large primary ocpd you want for inrush but not have to run the corresponding big conductors.

 

[don_resqcapt19]

240.21(B)(3) specifically applies to taps no?

A very specific tap....one where the total length of the primary conductor and the secondary conductor does not exceed 25'. Note that this is a "wire foot" rule and not the distance between the various pieces of equipment in the circuit.

 

[Prototype1]

Referencing table 450.3B and primary protection. Assuming you use the maximum 250% allowance for your device rating, how are you calculating the primary conductors? Are they still sized at 125% assuming a continuous load since the primary device at that point is short circuit protection? OR do you match the conductors to the device?

15kVA 480V/120-208V poly phase transformer

15*1000/480*1.73 = 18.1A

18.1 *2.5 = 45.25 (can we round up here??) using a 50A primary protective device?

assuming terminations are 75 degree rated
18.1*1.25 = 22.6 so we could use 10awg
OR
matching the device at 250% so 8awg??

NEC references appreciated.

**Please correct me if I am wrong** I did not know if it will be helpful to you but here:
Conditions: Primary side of transformer
Overcurrent protection at primary side (Primary > 600V) : Rating of primary fuse at point A = 300 % of primary. FLC or next standard size or Rating of primary circuit breaker at point A = 600%. FLC or next standard size. - Unsupervised

• Rating of Primary Circuit Breaker, so next higher standard size. - Unsupervised

Conditions: Secondary side of transformer
Overcurrent protection at secondary side (Secondary Voltage <= 600V)

• Rating of Sec. Fuse / Circuit Breaker at Point B= 125% of Sec. Full Load Current or Next higher Standard size. -Unsupervised

Overcurrent Protection at Secondary Side (Secondary Voltage >600V):

• Rating of Secondary. Fuse at Point B= 250% of Sec. Full Load Current or Next higher Standard size. -Unsupervised
• Rating of Secondary. Circuit Breaker at Point B= 300% of Sec. Full Load Current. -Unsupervised

Transformer OCPD conditions:
If it is supervised location for the transformer than you are going to select next lower standard size: if it is (primary voltage > 600V and secondary voltage > 600V). But if the (secondary voltage <= 600V), select next Higher standard size, fuse/circuit breaker.)

 

[don_resqcapt19]

**Please correct me if I am wrong** I did not know if it will be helpful to you but here:
Conditions: Primary side of transformer
Overcurrent protection at primary side (Primary > 600V) : Rating of primary fuse at point A = 300 % of primary. FLC or next standard size or Rating of primary circuit breaker at point A = 600%. FLC or next standard size. - Unsupervised

• Rating of Primary Circuit Breaker, so next higher standard size. - Unsupervised

Conditions: Secondary side of transformer
Overcurrent protection at secondary side (Secondary Voltage <= 600V)

• Rating of Sec. Fuse / Circuit Breaker at Point B= 125% of Sec. Full Load Current or Next higher Standard size. -Unsupervised

Overcurrent Protection at Secondary Side (Secondary Voltage >600V):

• Rating of Secondary. Fuse at Point B= 250% of Sec. Full Load Current or Next higher Standard size. -Unsupervised
• Rating of Secondary. Circuit Breaker at Point B= 300% of Sec. Full Load Current. -Unsupervised

Transformer OCPD conditions:
If it is supervised location for the transformer than you are going to select next lower standard size: if it is (primary voltage > 600V and secondary voltage > 600V). But if the (secondary voltage <= 600V), select next Higher standard size, fuse/circuit breaker.)

That is from Table 450.3(A) for transformers of over 1000 volts. Not likely that is the case in this thread. This is a very easy mistake to make as the tables in 450.3 are the only place in the code where the requirements for over 1000 volts appear before those for under 1000 volts.

 

[Prototype1]

That is from Table 450.3(A) for transformers of over 1000 volts. Not likely that is the case in this thread. This is a very easy mistake to make as the tables in 450.3 are the only place in the code where the requirements for over 1000 volts appear before those for under 1000 volts.

Oh yes, got you! I did not notice that. But hey, I think (maybe) he has better idea now. Thanks for notify though.