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Joined: Mar 2005
Posts: 1,213
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Tesla, OCP does not protect against the type of transient spikes you're talking about, because they happen far too quickly for a fuse or breaker to interrupt, and are over. You need a surge suppressor for this. Suffied to say, NEC does not require secondary OCP if primary is fed from an appropriately sized OCP. The 450.3(B) tables and secondary OCP are related more to bus taps and such.

Joined: Jun 2004
Posts: 1,273
T
Member
2005 NEC 240(F)...

TRANSFORMER SECONDARY CONDUCTORS:

Single-phase (other than 2-wire) and multi-phase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary over-current device.....

This phrase has been in the Code for years -- for the reasons detailed above.


Tesla
Joined: Jul 2002
Posts: 241
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SJT Offline OP
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Tesla, 450.2 talks about any transformer, single or polyphase. 450.3(B) is table for Primary and secondary protection.

Joined: Jun 2004
Posts: 1,273
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You are still stuck with OPCD on the Secondary under 450.3(B)...

Primary only protection is disallowed by 240(F) which means that you must use line 2 from the table: PRIMARY & SECONDARY PROTECTION.

I have in my day installed a ton of dry-type delta-wye transformers. Without exception the AHJ looked for the secondary OCPD --- RIGHT OFF THE BAT EVERY TIME.

It remained on their hot list of common electrician errors.

I can't speak for your AHJ, but out here they are on this like white on rice. You'll get red-tagged every time.



Tesla
Joined: Jul 2001
Posts: 599
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JBD Offline
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Originally Posted by Tesla
You are still stuck with OPCD on the Secondary under 450.3(B)...

Primary only protection is disallowed by 240(F) which means that you must use line 2 from the table: PRIMARY & SECONDARY PROTECTION.

I have in my day installed a ton of dry-type delta-wye transformers. Without exception the AHJ looked for the secondary OCPD --- RIGHT OFF THE BAT EVERY TIME.

It remained on their hot list of common electrician errors.

I can't speak for your AHJ, but out here they are on this like white on rice. You'll get red-tagged every time.



Wrong, it is not about 3-phase transformers, it is about transformers with multiple output voltages.

Yes, 240.4(F) says the secondary conductors need protection except for single voltage secondaries.
But it is 240.21(C)that says where the conductor over current protection must be located on the secondary of a transformer and what size that protection can be.

450.3(B) only says the transformer primary must be protected at not more than 125% except when a properly sized secondary protection is provided.

Joined: Nov 2000
Posts: 2,148
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There are two issues here. The protection of the transformer as found in 450.3 and the protection of the secondary conductors as found in 240.21(C). These are two independent (in most cases) issues. If the primary protection of the transformer is 125% or less of the rated primary current, there is no code requirement to provide protection for the secondary of the transformer. The conductors connected to the secondary of the transformer will require protection based on the rules found in 240.21(C).


Don(resqcapt19)
Joined: Jun 2004
Posts: 1,273
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I must refer you to the NEC Handbook for elaboration.

2005 NEC Handbook page 162 explaining 240.4(F):

The fundamental requirement of 240.4 specifics that conductors are to be protected against over-current in accordance with their ampacity, and 240.21 requires that the protection be provided at the point the conductors receive their supply. Section 240.4(F) permits the secondary circuit conductors from a transformer to be protected by over-current devices in the primary circuit conductors of the transformer only in the following two special cases:

1. A transformer with a 2-wire primary and 2-wire secondary, provided the transformer primary is protected in accordance with 450.3

2. A 3-phase, delta-delta-connected transformer having a 3-wire, single-voltage secondary, provided its primary is protected in accordance with 450.3

Except for these two special cases, transformer secondary conductors must be protected by the use of over-current devices, because the primary over-current devices do not provide such protection....


-----

The basic math of 4:1 current multiplication of primary-to-primary vs line-to-neutral is a cooker.

Again and again I must proclaim: AHJ -- handbook in hand -- will be shooting down delta-wye transformers without OCP at the secondary.


Last edited by Tesla; 02/20/10 10:09 PM.

Tesla
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SJT Offline OP
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In the 09 NEC, 240.21(C) talks about a set of conductors feeding a single load. The two 100Amp panels do not have the conductors going directly back to the transformer. If they did, It might be compliant. I'm gonna talk to the AHJ on this one. Thanks for your inputs.

Joined: Nov 2000
Posts: 2,148
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Originally Posted by Tesla
...
Again and again I must proclaim: AHJ -- handbook in hand -- will be shooting down delta-wye transformers without OCP at the secondary.


And again I will say that the secondary of the transformer itself does not require protection if the primary protection is sized at 125% or less of the rated primary current. There is no question that protection of the secondary conductors is required, except in the two special cases you cited. My issue is only with the choice of words, as the end result is the same...there will be an OCPD on the secondary side of the transformer, but it is often only for the protection of the secondary conductors and not for the protection of the secondary winding. Article 450 only applies to the transformer itself. Article 240 only applies to the protection of the conductors and not to the protection of the transformer. Even your quote from the handbook states "transformer secondary conductors must be protected by the use of over-current devices".


Don(resqcapt19)
Joined: Jan 2001
Posts: 1,044
Tom Offline
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I've re-read the original post and the OCPD size on the primary was not mentioned and I don't believe that SJT posted it in any of his follow ups.

That being the case, I'll stand by my answer if the primary side OCPD exceeds 60 amps. If the primary OCPD is 60 amps or less, I definitely agree that there can be as many OCPDS of any size on the secondary side as you can afford to buy, keeping in mind that all conductors must be protected.

I believe that Don's answer is correct, there will always be an OCPD on the secondary of a delta-wye connection and that it is there for the protection of the conductors , not the transformer, as long as the primary side OCPD does not exceed 125%.

In a left-handed way (no insult meant to you southpaws), Tesla is correct that an inspector will write a violation if there is no OCPD device on the secondary side of a delta-wye, but it isn't required to be there for the protection of the transformer.


Last edited by Tom; 02/21/10 08:52 PM.

Few things are harder to put up with than the annoyance of a good example.
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