Transformer pri & sec OCPD 125 or 250%

[david luchini]

Yes I only see the table that shows primary at 125% with no secondary protection

So any transformer that has primary protection at not greater than 125%, doesn't need transformer secondary protection.

 

[xformer]

So any transformer that has primary protection at not greater than 125%, doesn't need transformer secondary protection.

IMHO... That is correct, but if you are installing a panelboard on the secondary of the transformer, then OCPD for the panelboard is required.

 

[jim dungar]

IMHO... That is correct, but if you are installing a panelboard on the secondary of the transformer, then OCPD for the panelboard is required.

Yes. Secondary conductor protection is also required.
Either of these may be able to be the optional secondary side protection asked about in the OP.

 

[ptonsparky]

Yes I only see the table that shows primary at 125% with no secondary protection

Or maximum of 250% primary protection if secondary 125%

 

[don_resqcapt19]

IMHO... That is correct, but if you are installing a panelboard on the secondary of the transformer, then OCPD for the panelboard is required.

In most cases the rules in 240.21(C) will require protection for the transformer secondary conductors, or as you said 408.36 will apply.
It is just that that the rules in 450 only apply to the protection of the transformer itself.

 

[jim dungar]

In most cases the rules in 240.21(C) will require protection for the transformer secondary conductors, or as you said 408.36 will apply.
It is just that that the rules in 450 only apply to the protection of the transformer itself.

Correct.
You could have a1000A panelboard with a 600A main breaker fed with 800A of conductor on the 208Y/120V secondary of a 75kVA as long as the primary ptotection is not more than 125%. Nothing like provisions for future growth.

 

[Jpflex]

Correct.
You could have a1000A panelboard with a 600A main breaker fed with 800A of conductor on the 208Y/120V secondary of a 75kVA as long as the primary ptotection is not more than 125%. Nothing like provisions for future growth.

But the NEC code book says to use the table which says you can protect the primary to 250% ampacity if secondary is protected and system is over 9 amperes.

Most I see use only 125% on primary OCPD however

 

[jim dungar]

But the NEC code book says to use the table which says you can protect the primary to 250% ampacity if secondary is protected and system is over 9 amperes.

Most I see use only 125% on primary OCPD however

You can go all the way up to 250%, Only if you have secondary protection sized at no more than125%. The example I gave does not meet this requirement.

Most people do not research the predicted inrush of the transformer and then select the appropriate protective devices. Instead they simple jump to the maximum allowed per their installation.

 

[ptonsparky]

You can go all the way up to 250%, Only if you have secondary protection sized at no more than125%. The example I gave does not meet this requirement.

Most people do not research the predicted inrush of the transformer and then select the appropriate protective devices. Instead they simple jump to the maximum allowed per their installation.

It's similar to testing ground rod resistance. Easier to put in two and go home.

 

[jim dungar]

It's similar to testing ground rod resistance. Easier to put in two and go home.

Except the transformer current can usually be determined with an email before the material is bought and simply oversizing may waste, space, resources, and money.

 

[ptonsparky]

Except the transformer current can usually be determined with an email before the material is bought and simply oversizing may waste, space, resources, and money.

There are almost always exceptions.

 

[Jpflex]

In most cases the rules in 240.21(C) will require protection for the transformer secondary conductors, or as you said 408.36 will apply.
It is just that that the rules in 450 only apply to the protection of the transformer itself.

Isn’t the OCPD for the panel on the secondary also overload protection or OCPD for secondary side and would have to be protected at 125% of secondary current?

 

[augie47]

Isn’t the OCPD for the panel on the secondary also overload protection or OCPD for secondary side and would have to be protected at 125% of secondary current?

Oftentimes if the transformer is feeding a single that is the case but not necessarily so.
The panel OCPD must meet the requirements of 408.36; the transformer OCPD must meet the requirements of 450 and the provisions of 240.21 have to be taken into consideration. One device might meet all three Sections but not necessarily.