Transformer Secondary

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knifeswitch

Member
Location
Mill City, Oregon
Hello,

A 25 kva transformer is being installed. When sizing my OCPD for the secondary conductors and the single phase 120/240 panel board (70' from transformer) I multiplied secondary current by 125% (104 x 125% = 130 amps). If I use the next size up OCPD, I get a 150 amp OCPD. I was looking at a single phase chart from Mike Holt that states the 25kva single phase transformer should have a secondary OCPD of 125 amps. Would anyone be willing to help clarify which OCPD (125a or 150a) would be correct and why?

Chart: https://www.mikeholt.com/files/PDF/Transformer_Chart.pdf

Thank you,

Carl
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
knifeswitch said:
That would mean that whoever created the chart arbitrarily picked 125 as the acceptable ocpd. Does anyone else have any helpful information or reasoning?
Click to expand...

it is inherently an arbitrary choice. The chart you linked to has other arbitrary choices in it as well such as wire and conduit sizes.

In fact, conduit is an arbitrary choice as there are other wiring methods that could be employed and still meet code.
 
Last edited:
T

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
knifeswitch said:
Hello,

A 25 kva transformer is being installed. When sizing my OCPD for the secondary conductors and the single phase 120/240 panel board (70' from transformer) I multiplied secondary current by 125% (104 x 125% = 130 amps). If I use the next size up OCPD, I get a 150 amp OCPD. I was looking at a single phase chart from Mike Holt that states the 25kva single phase transformer should have a secondary OCPD of 125 amps. Would anyone be willing to help clarify which OCPD (125a or 150a) would be correct and why?

Chart: https://www.mikeholt.com/files/PDF/Transformer_Chart.pdf

Thank you,

Carl
Click to expand...

70 feet of secondary conductor does not comply with 240.21(C).
 
K

knifeswitch

Member
Location
Mill City, Oregon
texie said:
Maybe I'm not understanding where your secondary OCPD is. Is it at the transformer and the panel is 70 feet away?
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No, it's in the panel. I have primary protection in the mcc. Transformer next to the mcc. Up and over 70' to main breaker panel. I'm just wondering if I'm missing something on ocpd sizing after looking at the chart. The 37.5kva secondary is the max at 200 butt the 25kva doesn't give the max..... Seemed odd. We were going with a 150amp main in the panel and I'm just trying to double check that. I thought I might get helpful answers here...
 
david luchini

david luchini

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Staff member
Location
Connecticut
Occupation
Engineer
knifeswitch said:
No, it's in the panel. I have primary protection in the mcc. Transformer next to the mcc. Up and over 70' to main breaker panel. I'm just wondering if I'm missing something on ocpd sizing after looking at the chart. The 37.5kva secondary is the max at 200 butt the 25kva doesn't give the max..... Seemed odd. We were going with a 150amp main in the panel and I'm just trying to double check that. I thought I might get helpful answers here...
Click to expand...

You can't run 70' from the xfmr to the first OCPD, as Texie noted. 240.21(C) limits you to 25' to the first OCPD, unless it is in a "Supervised Industrial Installation" per 240.90.

The 150A breaker would be fine. It looks like the chart uses the nearest breaker to the 125% rating. 25kva rounds down, 37.5kva rounds up, 50kva rounds down.
 
K

knifeswitch

Member
Location
Mill City, Oregon
david luchini said:
You can't run 70' from the xfmr to the first OCPD, as Texie noted. 240.21(C) limits you to 25' to the first OCPD, unless it is in a "Supervised Industrial Installation" per 240.90.

The 150A breaker would be fine. It looks like the chart uses the nearest breaker to the 125% rating. 25kva rounds down, 37.5kva rounds up, 50kva rounds down.
Click to expand...

It limits running farther than 25' without ocpd on the secondary. The last sentence states we can't use 240.4(B).
 
david luchini

david luchini

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Location
Connecticut
Occupation
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knifeswitch said:
You can run up to 25' without ocpd and longer with ocpd. Where does it limit the run? I'm looking at it now....
Click to expand...

That's not what 240.21(C) says. It permits you to connect conductors to the secondary transformer, without an OCPD at the point the receive their supply (the transformer secondary terminals), under the conditions listed in 240.21(C)(1) through (C)(6).

(C)(1) doesn't apply to your three wire secondary.
(C)(2) limits the conductors to 10'
(C)(3) limits the conductors to 25' in an industrial application
(C)(4) has no length limit where the secondary conductors are outside
(C)(5) limits the primary and secondary to 25' for feeder tapped transformers
(C)(6) limits the conductors to 25'.

You can't run the conductors from the transformer 70' to the panel main circuit breaker. You need and OCPD to comply with the length limit requirements of 240.21(C).
 
T

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
knifeswitch said:
. I thought I might get helpful answers here...
Click to expand...

We are trying to help by explaining the requirements of 240.21(C) in addition to your OCPD sizing question.
You can solve this by locating the secondary OCPD at the transformer. Then use a MLO panel or just keep the panel with the (now redundant) main breaker if you wish.
 
K

knifeswitch

Member
Location
Mill City, Oregon
texie said:
We are trying to help by explaining the requirements of 240.21(C) in addition to your OCPD sizing question.
You can solve this by locating the secondary OCPD at the transformer. Then use a MLO panel or just keep the panel with the (now redundant) main breaker if you wish.
Click to expand...

I was on a lift all day and now I have some time. These are feeders and are addressed in 240.21(B) [240.21(C) is considering a single load]. I'm in a mill so 240.21(B)(4) applies.

As for the difference in the chart "logic", I received an answer I can live with.
 
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packersparky

Senior Member
Location
Wisconsin
Occupation
Inspector
knifeswitch said:
I was on a lift all day and now I have some time. These are feeders and are addressed in 240.21(B) [240.21(C) is considering a single load]. I'm in a mill so 240.21(B)(4) applies.

As for the difference in the chart "logic", I received an answer I can live with.
Click to expand...


Your original post asked about transformer secondary conductors. They are not feeders. The overcurrent protection would have to be in accordance with 240.21(C).
 
david luchini

david luchini

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Location
Connecticut
Occupation
Engineer
knifeswitch said:
These are feeders and are addressed in 240.21(B) [240.21(C) is considering a single load]. I'm in a mill so 240.21(B)(4) applies.
Click to expand...

240.21(B) deals with "feeder taps." That doesn't apply to the transformer secondary conductors that you have described.

240.21(C) is the proper section for your installation.
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE

Not addressing the OP's question, but in table 450.3(B) if a transformer has a rated primary current less than 2 amperes then the maximum overcurrent protection allowed on the primary side goes down if you add protection on the secondary (it goes down from 300% to 250% of the primary rated current). That seems kind of strange.
 
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