Transformer pri & sec OCPD 125 or 250%

[Jpflex]

There has been other post regarding when to use 250% of primary current to protect primary conductors for transformers over 9 amperes or use 125%

I just wanted to make sure I’m understanding this

If my transformer is over 9 amperes (primary side?) then I can size the primary OCPD to to 250% of its rated current? Then size a primar conductor to this ampacity after temperature correction and derating or at 125 % of continuous load?

Then I can size the secondary OCPD to 125% of its rated current? Then size the secondary conductor for this ampacity as above?


However primary only protection at 125 percent only applies to single phase 2 wires in and 2 out or delta delta?

Is this correct?

 

[david luchini]

If my transformer is over 9 amperes (primary side?) then I can size the primary OCPD to to 250% of its rated current? Then size a primar conductor to this ampacity after temperature correction and derating or at 125 % of continuous load?

Size the primary conductors to be protected by the primary OCPD.

However primary only protection at 125 percent only applies to single phase 2 wires in and 2 out or delta delta?

Primary only protection can apply to any transformer.

 

[Jpflex]

Size the primary conductors to be protected by the primary OCPD.

Primary only protection can apply to any transformer.

The code says primary only protection only applies to 2 types of transformers

 

[david luchini]

The code says primary only protection only applies to 2 types of transformers

No it doesn't.

 

[ptonsparky]

Sizing the Transformer primary protection at no more than 125% allows for no secondary overcurrent protection for transformer. See note 2. You still need to protect the secondary conductors and tap rules apply as well.

Good luck at getting some transformers to start up on that 125%.

 

[jim dungar]

The code says primary only protection only applies to 2 types of transformers

No, it says lack of transformer secondary conductor protection applies to two types of transformers, per article 240.

Article 450.3 requires transformer primary protection at 125%. The size of the primary protection can optionally be increased if suitable protection is provide on its secondary.

 

[wwhitney]

Good luck at getting some transformers to start up on that 125%.

So for any particular transformer, how do you tell ahead of time what the minimum size primary OCPD required to allow it to start up would be?

Thanks, Wayne

 

[jim dungar]

So for any particular transformer, how do you tell ahead of time what the minimum size primary OCPD required to allow it to start up would be?

Thanks, Wayne

Look at manufacturer data for transformer inrush and compare it to the OCPD time current curve.
I have always been able to get these values from the major manufacturers of stock transformers.

 

[ptonsparky]

So for any particular transformer, how do you tell ahead of time what the minimum size primary OCPD required to allow it to start up would be?

Thanks, Wayne

Good question. Lower impedance of transformers has increased the peak current over the last decade or so.

 

[Jpflex]

No, it says lack of transformer secondary conductor protection applies to two types of transformers, per article 240.

Article 450.3 requires transformer primary protection at 125%. The size of the primary protection can optionally be increased if suitable protection is provide on its secondary.

For the two types of transformers protected by primary only protection, 240.4 (F) protection of conductors says:

Single phase (other than 2 wire) and multiphase (other than delta delta 3 wire) SHALL NOT have their secondary conductors considered protected by the primary over current device

 

[wwhitney]

For the two types of transformers protected by primary only protection, 240.4 (F) protection of conductors says:

Single phase (other than 2 wire) and multiphase (other than delta delta 3 wire) SHALL NOT have their secondary conductors considered protected by the primary over current device

There's three different things in a transformer installation that need protection: primary conductors, the transformer itself, and secondary conductors.

The question of what protection is required for the transformer itself is governed by 450.3, which is what the OP is about. Protection of the secondary conductors is a separate question, which 240.4(F) bears on.

The upshot of 450.3 and 240.4(F) is that for the two types of transformers listed as exceptions in the sentence you quoted, if you size your primary OCPD at no more than 125% of the transformer rating, and size the primary conductors accordingly, then your primary OCPD suffices for transformer protection, and if you size your secondary conductors accordingly, then it also suffices for secondary conductor protection.

However, this is the exception rather than the rule; usually you'll have both primary and secondary OCPD. Then you'll need to ensure your primary conductors are protected by the primary OCPD; the combination of primary and secondary OCPD protect the transformer; the secondary OCPD protects the conductors on its load side, and importantly, that the secondary conductors on the supply side of the secondary OCPD meet one of the requirements of 240.21(C).

Cheers, Wayne

 

[david luchini]

For the two types of transformers protected by primary only protection, 240.4 (F) protection of conductors says:

240.4(F) is titled Transformer Secondary Conductors. It has nothing to do with whether a transformer is primary only protected.

 

[Jpflex]

There's three different things in a transformer installation that need protection: primary conductors, the transformer itself, and secondary conductors.

The question of what protection is required for the transformer itself is governed by 450.3, which is what the OP is about. Protection of the secondary conductors is a separate question, which 240.4(F) bears on.

The upshot of 450.3 and 240.4(F) is that for the two types of transformers listed as exceptions in the sentence you quoted, if you size your primary OCPD at no more than 125% of the transformer rating, and size the primary conductors accordingly, then your primary OCPD suffices for transformer protection, and if you size your secondary conductors accordingly, then it also suffices for secondary conductor protection.

However, this is the exception rather than the rule; usually you'll have both primary and secondary OCPD. Then you'll need to ensure your primary conductors are protected by the primary OCPD; the combination of primary and secondary OCPD protect the transformer; the secondary OCPD protects the conductors on its load side, and importantly, that the secondary conductors on the supply side of the secondary OCPD meet one of the requirements of 240.21(C).

Cheers, Wayne

My NEC book is badly worn so some of the sections of the 450 are torn


What I don’t get is why table 450.3 B for transformers over 9 amperes with primary and secondary shows overcurrent protection at 250% but 450.3 says no more than 125%

 

[Jpflex]

240.4(F) is titled Transformer Secondary Conductors. It has nothing to do with whether a transformer is primary only protected.

Yes, 240.4 (F) is for protection of conductors, but it does say that PRIMARY OCPD cannot protect secondary wires unless it is one of two transformers mentioned so this section is about whether a transformer is primary only protected as it says this specifically

 

[jim dungar]

My NEC book is badly worn so some of the sections of the 450 are torn


What I don’t get is why table 450.3 B for transformers over 9 amperes with primary and secondary shows overcurrent protection at 250% but 450.3 says no more than 125%

If you have protection on the secondary side set at not more than 125% of the secondary full load then you are permitted, but not required, to increase the required primary side up to 250%.

 

[Jpflex]

If you have protection on the secondary side set at not more than 125% of the secondary full load then you are permitted, but not required, to increase the required primary side up to 250%.

O ok but I can’t seem to find this wording in NEC maybe my page is torn there but do you know the exact code for this?

 

[ptonsparky]

O ok but I can’t seem to find this wording in NEC maybe my page is torn there but do you know the exact code for this?

Look at the table and it’s footnotes in 450

 

[jim dungar]

O ok but I can’t seem to find this wording in NEC maybe my page is torn there but do you know the exact code for this?

It doesn't use these exact words.
The table has one row that says the primary can be 125% max, without any restriction on the secondary size at all.
It has a second row which shows the primary can be higher if the secondary is limited.

 

[augie47]

Here is a neat little online calculator you can use......

NEC Table 450.3(B)

 

[Jpflex]

It doesn't use these exact words.
The table has one row that says the primary can be 125% max, without any restriction on the secondary size at all.
It has a second row which shows the primary can be higher if the secondary is limited.

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

Or 250% primary protection if secondary 125%