Transformer Primary Conductors

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NClark

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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.
 
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.
 
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.
charlie b said:
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.
Click to expand...
 
NClark said:
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.
Click to expand...
yes
and your secondary protection would need to be set at no more than 125% (next size up allowed)
 
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%.
 
don_resqcapt19 said:
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%.
Click to expand...
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 said:
240.21(B)(3) specifically applies to taps no?
Click to expand...
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.
 
NClark said:
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.
Click to expand...
**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.)
 
Prototype1 said:
**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.)
Click to expand...
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.
 
don_resqcapt19 said:
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.
Click to expand...
Oh yes, got you! I did not notice that. But hey, I think (maybe) he has better idea now. Thanks for notify though.
 
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