Choosing maximum OCPD size for transformer secondary (newbie question)

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unsaint33

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When sizing transformer 2ndary side fuses, 450.3 tables shows the maximum OCPD sizes. Here is my newbie electrician question... Why would I want the maximum OCPD? Regardless of where the 2ndary side OCPD is installed (in that transformer or in a panel the 2ndary conductors supply to), a larger 2ndary OCPD means larger 2ndary conductors required. My only explanation is that, I generally want the largest OCPD I am allowed to install so I can serve more loads. Am I thinking right? Thank you.
 
unsaint33 said:
When sizing transformer 2ndary side fuses, 450.3 tables shows the maximum OCPD sizes. Here is my newbie electrician question... Why would I want the maximum OCPD? Regardless of where the 2ndary side OCPD is installed (in that transformer or in a panel the 2ndary conductors supply to), a larger 2ndary OCPD means larger 2ndary conductors required. My only explanation is that, I generally want the largest OCPD I am allowed to install so I can serve more loads. Am I thinking right? Thank you.
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Often you will not use the max for the secondary protection. For example, probably the most common transformer in the low voltage world is a 75kva 480 delta to 208y/120. This would have a secondary current of 208 amps (and a 50kva single phase with 240 secondary is the same secondary current and also super common). Most of the time the secondary conductors will feed a 200A MB panelboard. Of course you could go 225 or even 300 using the next size up allowance in note 1, but 200 is just super common and you rarely need the extra 8 amps. Remember that also the secondary protection most of the time is also meeting the requirements of 408.36 for panelboard protection and 125% of the common transformer sizes are oddball main breaker panelboard sizes.

If you had a 200 amp continuous load (or any other continuous load that is about the rating of the transformer) you would need that extra 25% for the secondary OCPD.

The primary OCPD on the other hand, many people like to size that as large as possible for inrush, but you need larger conductors to go along with it.
 
electrofelon said:
Often you will not use the max for the secondary protection. For example, probably the most common transformer in the low voltage world is a 75kva 480 delta to 208y/120. This would have a secondary current of 208 amps (and a 50kva single phase with 240 secondary is the same secondary current and also super common). Most of the time the secondary conductors will feed a 200A MB panelboard. Of course you could go 225 or even 300 using the next size up allowance in note 1, but 200 is just super common and you rarely need the extra 8 amps. Remember that also the secondary protection most of the time is also meeting the requirements of 408.36 for panelboard protection and 125% of the common transformer sizes are oddball main breaker panelboard sizes.

If you had a 200 amp continuous load (or any other continuous load that is about the rating of the transformer) you would need that extra 25% for the secondary OCPD.

The primary OCPD on the other hand, many people like to size that as large as possible for inrush, but you need larger conductors to go along with it.
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So if you had a 45kva 3Ø 480v to 120/208 trans fed with a 70 amp primary breaker. Load is 120 amps. Do you need a secondary overcurrent for the panel. I read you need a main for the secondary panelboard if it is lighting and appliance.

If so what size? could you go with a 125 at the 120/208 panelboard?
 
Sierrasparky said:
So if you had a 45kva 3Ø 480v to 120/208 trans fed with a 70 amp primary breaker. Load is 120 amps. Do you need a secondary overcurrent for the panel. I read you need a main for the secondary panelboard if it is lighting and appliance.

If so what size? could you go with a 125 at the 120/208 panelboard?
Click to expand...

Panel needs OCPD. 125A breaker okay.
 
unsaint33 said:
When sizing transformer 2ndary side fuses, 450.3 tables shows the maximum OCPD sizes. Here is my newbie electrician question... Why would I want the maximum OCPD? Regardless of where the 2ndary side OCPD is installed (in that transformer or in a panel the 2ndary conductors supply to), a larger 2ndary OCPD means larger 2ndary conductors required. My only explanation is that, I generally want the largest OCPD I am allowed to install so I can serve more loads. Am I thinking right? Thank you.
Click to expand...

Are you asking about tranny loading and efficiency by chance?
 
Sierrasparky said:
So if you had a 45kva 3Ø 480v to 120/208 trans fed with a 70 amp primary breaker. Load is 120 amps. Do you need a secondary overcurrent for the panel. I read you need a main for the secondary panelboard if it is lighting and appliance.

If so what size? could you go with a 125 at the 120/208 panelboard?
Click to expand...

As jumper said, panel still needs OCPD per 408.36. Note the lighting and appliance panelboard thing is no longer. OF course you could have separate transformer secondary and panelboard protection, but combining them into one is usually the most economical.
 
unsaint33 said:
When sizing transformer 2ndary side fuses, 450.3 tables shows the maximum OCPD sizes. Here is my newbie electrician question... Why would I want the maximum OCPD? Regardless of where the 2ndary side OCPD is installed (in that transformer or in a panel the 2ndary conductors supply to), a larger 2ndary OCPD means larger 2ndary conductors required. My only explanation is that, I generally want the largest OCPD I am allowed to install so I can serve more loads. Am I thinking right? Thank you.
Click to expand...

In fact, note that usually you will have one OCPD meeting 3 requirements: the transformer secondary protection, panelboard protection, and transformer secondary conductor protection. Typicaly youwould coordinate and size everything so one OCPD does them all.
 
Sierrasparky said:
So if you had a 45kva 3Ø 480v to 120/208 trans fed with a 70 amp primary breaker. Load is 120 amps. Do you need a secondary overcurrent for the panel. I read you need a main for the secondary panelboard if it is lighting and appliance.

If so what size? could you go with a 125 at the 120/208 panelboard?
Click to expand...
You could however protect the secondary with the primary device if the secondary were 3 wire delta.
 
jumper said:
Panel needs OCPD. 125A breaker okay.
Click to expand...

electrofelon said:
As jumper said, panel still needs OCPD per 408.36. Note the lighting and appliance panelboard thing is no longer. OF course you could have separate transformer secondary and panelboard protection, but combining them into one is usually the most economical.
Click to expand...

electrofelon said:
In fact, note that usually you will have one OCPD meeting 3 requirements: the transformer secondary protection, panelboard protection, and transformer secondary conductor protection. Typicaly youwould coordinate and size everything so one OCPD does them all.
Click to expand...

thanks
Knowing that. If the trans is fed from a breaker sized for the OCPD what is the max distance of the secondary Feeder to the panelboard before a Disco or OCPD is if required to be installed before the panelboard
 
jumper said:
Not sure I understand the question clearly.
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In another article it appears that there might be a maximum feeder length from trans to panelboard where a OCPD might be necessary before a main breaker in the panel.
 
I think there was a 10 foot rule that was spoken of. I don't have my code book at the in Laws.
 
jumper said:
There is a 10’ and 25’ rule for inside transformer secondary conductors.
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The 10' or 25' rules (240.21 (C)) are not just for secondary conductors within the transformer, right? For example, for the 10' rule, as long as the secondary conductor ampacity is not less than 1/10th of the primary OCPD, and the total secondary conductor length is not over 10' (regardless of whether the OCPD is in the transformer or in a panelboard after the transformer) we do not need an OCPD on the secondary side (not considering the 408.36 rule).
 
unsaint33 said:
The 10' or 25' rules (240.21 (C)) are not just for secondary conductors within the transformer, right? For example, for the 10' rule, as long as the secondary conductor ampacity is not less than 1/10th of the primary OCPD, and the total secondary conductor length is not over 10' (regardless of whether the OCPD is in the transformer or in a panelboard after the transformer) we do not need an OCPD on the secondary side (not considering the 408.36 rule).
Click to expand...
AFAIK, 10 and 25 foot rules is length of conductor from transformer secondary terminal to overcurrent device, similar to feeder tap rules in (B).
 
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