Transformer calculations

[ffroh]

I was hoping someone can help with the calculations. I don't normally install transformers. I am not sure about picking out the correct conductor from table 310.16. What column? Bonding jumper? Grounding electrode conductor?

450.3(B)
75 KVA transformer primary 480 volts, 3 phase and secondary 480 volts 3 PH.
I= VA / (E X 1.732)
75,000 VA / (480 V X 1.732) = 90A
90A X 1.25 = 112.5A
240.6 OCPD = 125A

Primary conductor sizing
90A X 1.25 = 112.5A
# 1 Copper THHN run in EMT.

Secondary conductors
90 X 1.25 = 112.5A
MC Cable Copper # 1

250.102(c)(1) Bonding jumper copper # 6
250.66 Grounding electrode conductor copper # 6

41.5 KVA transformer primary 480 volts, 3 phase and secondary 400Y/231, 3 PH.
I= VA / (E X 1.732)
41,500 VA / (480 V X 1.732) = 50A
50A X 1.25 = 63 A
240.6 OCPD 70A

Primary conductor sizing
50A X 1.25 = 63 A
MC Cable Copper THHN # 6

Secondary conductors
41,500 VA / (231V X 1.732) = 104A
Using MC Cable?
250.102(c)(1) Bonding jumper copper # 6
250.66 Grounding electrode conductor copper # 6

 

[augie47]

On your 41.5 kva since you have 125% primary protection, your transformer kva does not limit your secondary conductors so they would be sized based on the size of your secondary over-current device.

 

[jim dungar]

I was hoping someone can help with the calculations. I don't normally install transformers. I am not sure about picking out the correct conductor from table 310.16. What column? Bonding jumper? Grounding electrode conductor?

450.3(B)
75 KVA transformer primary 480 volts, 3 phase and secondary 480 volts 3 PH.
I= VA / (E X 1.732)
75,000 VA / (480 V X 1.732) = 90A
90A X 1.25 = 112.5A
240.6 OCPD = 125A

Primary conductor sizing
90A X 1.25 = 112.5A
# 1 Copper THHN run in EMT.

Secondary conductors
90 X 1.25 = 112.5A
MC Cable Copper # 1

250.102(c)(1) Bonding jumper copper # 6
250.66 Grounding electrode conductor copper # 6

41.5 KVA transformer primary 480 volts, 3 phase and secondary 400Y/231, 3 PH.
I= VA / (E X 1.732)
41,500 VA / (480 V X 1.732) = 50A
50A X 1.25 = 63 A
240.6 OCPD 70A

Primary conductor sizing
50A X 1.25 = 63 A
MC Cable Copper THHN # 6

Secondary conductors
41,500 VA / (231V X 1.732) = 104A
Using MC Cable?
250.102(c)(1) Bonding jumper copper # 6
250.66 Grounding electrode conductor copper # 6

Why are you using the L-L voltage with your 75kVA secondary side calculations and then the L-N voltage for the 41kVA?

 

[Strathead]

I was hoping someone can help with the calculations. I don't normally install transformers. I am not sure about picking out the correct conductor from table 310.16. What column? Bonding jumper? Grounding electrode conductor?

You seem to have a very good grasp of what you need to do. As Augie said, with 125% primary protection and a maximum of 10 feet of secondary conductor, you only need conductors sized to your secondary overcurrent device (no next larger size allowance). For what column in 310.16 it is dictated by conductor insulation (of course) as modified by 110.14. If your conductors are longer than 10 feet let us know.

 

[wwhitney]

I was hoping someone can help with the calculations. I don't normally install transformers. I am not sure about picking out the correct conductor from table 310.16. What column?

Generally 75C; that requires confirming that the terminations at both ends are rated 75C, but that is commonplace these days, even when the NEC specifies that the default is 60C.

Primary conductor sizing
90A X 1.25 = 112.5A
# 1 Copper THHN run in EMT.

Unless you have a continuous load on your transformer, you don't need to multiply the 90A by 1.25. You do, however, need to provide a conductor of a size that is permitted to be protected at your selected 125A primary OCPD. Which under 240.4(B) means it needs an ampacity of 111A. #2 Cu suffices under the 75C column.

Secondary conductors
90 X 1.25 = 112.5A
MC Cable Copper # 1

On the secondary side, you always need to comply with 240.21(C), but you may size the secondary conductors to the load served, not the transformer rating. So again if you have 90A of continuous load, #2 Cu would suffice; for smaller loads, a smaller size would work, down to the limits specified in 240.21(C). For which, BTW, on a delta-wye transformer with identical primary and secondary L-L voltages, the "primary-to-secondary voltage ratio" should be taken to be sqrt(3), not 1.

41.5 KVA transformer primary 480 volts, 3 phase and secondary 400Y/231, 3 PH.

. . .

Secondary conductors
41,500 VA / (231V X 1.732) = 104A

To reiterate Jim's comment, the formula above uses the L-L voltage of 400V, not the L-N voltage of 231V.

Cheers, Wayne

 

[ffroh]

You seem to have a very good grasp of what you need to do. As Augie said, with 125% primary protection and a maximum of 10 feet of secondary conductor, you only need conductors sized to your secondary overcurrent device (no next larger size allowance). For what column in 310.16 it is dictated by conductor insulation (of course) as modified by 110.14. If your conductors are longer than 10 feet let us know.

Why are you using the L-L voltage with your 75kVA secondary side calculations and then the L-N voltage for the 41kVA?

My mistake.

 

[jim dungar]

My mistake.

We've all been there. You are on the right track.

I like to start at my load and then work backwards bro the source. Your secondary conductors are sized to the load and their protection is done per 240.21(C). The transformer primary protection is done per 450.3, as you have done at 125%. The primary conductors are then sized per your primary side protection.

 

[ffroh]

You seem to have a very good grasp of what you need to do. As Augie said, with 125% primary protection and a maximum of 10 feet of secondary conductor, you only need conductors sized to your secondary overcurrent device (no next larger size allowance). For what column in 310.16 it is dictated by conductor insulation (of course) as modified by 110.14. If your conductors are longer than 10 feet let us know.

The secondary size conductors are longer than 10 feet.

 

[augie47]

The secondary size conductors are longer than 10 feet.

Other than a two-wire/two wire or a 3 phase delta delta {see wording in {240.21(C)(1)}, your secondary conductor length must comply with one of the 240.21(C) rules, the 10' and 25' being most common.
(There are exceptions in 240.90 for supervised industrial installations)

 

[wwhitney]

Other than a two-wire/two wire or a 3 phase delta delta {see wording in {240.21(C)(1)}, your secondary conductor length must comply with one of the 240.21(C) rules,

Anyone know why wye-wye isn't also exempted? Simple oversight or perhaps that they are rarely used on the customer side of the service point?

Cheers, Wayne

 

[Strathead]

The secondary size conductors are longer than 10 feet.

Assuming you are inside a building, your only other option is 25 foot rule. There is a whole thread on it here. https://forums.mikeholt.com/threads/transformer-tap-rule.2578067/page-4#post-2885345 but in a nutshell, if you are ending in a breaker that is sized for the whole transformer you are safe. You can't run over 25 feet without overcurrent protection period though.