Transformer.

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yordanys

yordanys

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Location
New Jersey
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Electrician
Good Morning Everyone,

I'm running a job where the project manager want me to uses an existent 70 amp circuit breaker on de primary side for a 30 KVA transformer. After I did my calculation the maximum over current device I can put in the primary side is 50 amp. What explanation can I gave him in order to get the approval of the right material for the installation of my 30 kva transformer? Thank you in advance.

Stay Safe,

Yordanys.
 
Have you taken a look at Table 450.3(B) for primary and secondary protection? Also just a note the secondary voltage is 208Y/120.

BTW. Welcome to the Forum. :)
 
I did, primary and secondary protection are more than 9 amp, need to be sized 125 percent of primary and secondary current. 450.3(b) note 1. I also check on Mike calculation app and is confirming my calculation. What would happens if I use a bigger size of OCD on the primary side?

Thank you for give me this great support and opportunity of learning.
 
USER INPUTS:
System Phase: Three Phase
Primary Volts: 480
Secondary Volts: 120/208
Transformer kVA: 30


PRIMARY RESULTS
1. Transformer Size: 30 kVA
2. Transformer Protection Size: 50 Amperes
3. Conductor Size: 8 AWG Copper, rated 50A at 75°C
4. Equipment Grounding Conductor: 10 AWG
5. Raceway will be SER cable.


SECONDARY RESULTS
1. Transformer Size: 30 kVA
2. Conductor Protection Size: 100 Amperes
3. Conductor Size: 3 AWG Copper, rated 100A at 75°C
4. System/Supply-Side Bonding Jumper: 8 AWG
5. Raceway will be SER cable.
 
electrofelon said:
You may go 250% of nameplate for primary protection if you also have secondary protection. Do you not want to provide secondary protection?
Click to expand...
My thought as well, with the required secondary protection the 70 OCPD (and conductors) on the primary should be code compliant.
 
USER INPUTS:
System Phase: Three Phase
Primary Volts: 480
Secondary Volts: 120/208
Transformer kVA: 30


PRIMARY RESULTS
1. Transformer Size: 30 kVA
2. Transformer Protection Size: 50 Amperes
3. Conductor Size: 8 AWG Copper, rated 50A at 75°C
4. Equipment Grounding Conductor: 10 AWG
5. Raceway will be SER cable.


SECONDARY RESULTS
1. Transformer Size: 30 kVA
2. Conductor Protection Size: 100 Amperes
3. Conductor Size: 3 AWG Copper, rated 100A at 75°C
4. System/Supply-Side Bonding Jumper: 8 AWG
5. Raceway will be SER cable.


PRIMARY CALCULATIONS
1. Transformer kVA Size [NEMA]
NEMA 30 kVA
2. Primary Protection Size [240.6(A) and 450.3(B)]
Primary protection is sized to 125% of the primary current.
Primary Current = Transformer VA/(480V x 1.732)
Primary Current = 30,000/(480 x 1.732) = 36A
Primary Protection Amperes = 36A x 125% = 45A
Primary Protection Size = 50A
3. Conductor Size [210.20(A), 240.4, and Table 310.15(B)(16)]
Primary conductors are sized to 125% of 36A primary current.
Primary Conductor Ampacity: 36A x 125% = 45A
Primary Conductor Size: 8 AWG, rated 50A at 75°C
4. Equipment Grounding Conductor [Table 250.122]
Primary EGC sized to the rating of the 50A protection device.]


SECONDARY CALCULATIONS
1. Transformer kVA Size [NEMA]
NEMA 30 kVA
2. Conductor Protection Size [215.3, 240.6(A), and 240.21(C)]
Secondary protection should be sized about 125% of the secondary current.
Secondary Current = Transformer VA/(208V x 1.732)
Secondary Current = 30,000/(208 x 1.732) = 83A
Secondary Conductor Protection Amperes = 83A x 125% = 104A
Secondary Conductor Protection Size: 100A
3. Conductor Size [110.14(C)(1)(a)(3)/(b)(1), 240.21(C), Table 310.15(B)(16)]
CAUTION: Secondary conductors must terminate in an overcurrent protection device, have an ampacity no less than the secondary OCPD, and have a maximum length of 25 feet [240.21(C)(6)].
4. System/Supply-Side Bonding Jumper [250.30(A) and 250.102(C)]
Sized to the secondary 3 AWG conductor.
5. Raceway Size [Chapter 9, Table 1]
Based on a at 40% fill, with an equipment grounding conductor.
 
Hey guys, on the transformer calculation app both sides primary and Secondary protection are sized at 125%. Why 250%?
 
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,
 
yordanys said:
Hey guys, on the transformer calculation app both sides primary and Secondary protection are sized at 125%. Why 250%?
Click to expand...

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.
 
so I can use the 70 amp over-current(existent ckt breaker) device on the primary side for my 30 kva transformer and #4 Awg conductors(310.15(b)(16) on the primary side?
 
infinity said:
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.
Click to expand...
infinity said:
I agree, next standard size up rule for the primary just not for the secondary. :)
Click to expand...
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.
 
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