Transformer secondary protection required?

[david luchini]

You can if it doesn't already have a secondary OCPD. So on a D-D you can choose, but a D-Y already HAS a secondary - the choice was made for you.

Where are you reading anything in Article 450 about D-D or D-Y? It doesn't exist. The Protection required by Table 450.3(B) has NOTHING to do with whether a transformer is D-D or D-Y. The Table applies the same to both types of Transformers. There is nothing in the text that tells you to apply one Protection Method to one type of transformer or the other. You are injecting that requirement yourself.

You design the system first and set sizes second. On a D-D you can elect to add or omit a secondary THEN you look up the appropriate row to select values. On a D-Y it's already been forced on you. You can't omit or ignore it just because you want to use the first row.

You CANNOT elect to omit a secondary OCPD on a D-D transformer if the Primary OCPD is greater than 125% (or next size up.)

This is the fallacy in your argument. You are trying to assign "Primary Only" or "Primary & Secondary" protection of a transformer based on whether the secondary conductors can be protected by the Primary OCPD or whether they need to be protected by an OCPD on the secondary size.

 

[david luchini]

You can if it doesn't already have a secondary OCPD. So on a D-D you can choose, but a D-Y already HAS a secondary - the choice was made for you.

Let me try it this way. I'm going to design an installation that has a 45kVA, 480-208/120V transformer. I know I have to protect the transformer in accordance with Article 450, and the primary and seconary feeder conductors in accordance with Article 240.

I know the primary rated current is 54 Amps, and 54*12%=67.5A. I decide to put a 65A primary OCPD and #6 Awg primary feeder conductors (ampacity of 65.) I have a 200A MCB panel laying around that I want to use on the secondary to save money.

The primary conductors are protected in accordance with their ampacity per 240.4. The Primary OCPD is less than 125% of the primary rated current, so there is no Maximum rating or setting for required on the transformer secondary per Table 450.3(B). I run #3/0 secondary conductors approx. 3 ft and terminate them on the 200A MCB of my panelboard. The #3/0 have an ampacity of 200A. I am properly protecting the secondary conductors in accordance with 240.21(C)(2). My installation meets all the overcurrent protection requirements for the conductors on both sides of the transformer, and the overcurrent protection requirements for the transformer.

 

[T.M.Haja Sahib]

1)Only two wires primary and two wires secondary transformers permit protection of secondary from primary side.
2)The OP may note that provision of primary side protection at 125% of primary current though code compliant may either prevent energisation of transformer on switching on or cause damage to the transformer during fault clearing.

 

[don_resqcapt19]

1)Only two wires primary and two wires secondary transformers permit protection of secondary from primary side. ...

That is not correct. If the transformer primary protection is sized at 125% or less of the rated primary current the transformer secondary does not require protection. It is correct to say that you may be able to use the primary protection to protect the secondary conductors of a two wire to two wire transformer and the secondary conductors of a delta/delta 3 wire system.

2)The OP may note that provision of primary side protection at 125% of primary current though code compliant may either prevent energisation of transformer on switching on or cause damage to the transformer during fault clearing.

Yes, it is possible that the inrush may trip a primary breaker sized at 125% of the rated primary current. I don't understand how this could cause transformer damage during fault clearing.

 

[T.M.Haja Sahib]

That is not correct. If the transformer primary protection is sized at 125% or less of the rated primary current the transformer secondary does not require protection. It is correct to say that you may be able to use the primary protection to protect the secondary conductors of a two wire to two wire transformer and the secondary conductors of a delta/delta 3 wire system.

Yes. I meant the secondary conductors. I do not think the secondary conductors of a delta/delta 3 wire system would be protected by primary side protection for unbalanced over loads on the secondary.

Yes, it is possible that the inrush may trip a primary breaker sized at 125% of the rated primary current. I don't understand how this could cause transformer damage during fault clearing.

A portion of thermal damage curve may lie to left of primary protection device characteristic for L-N fault and thus damage may be done to the transformer before the protection device clears such a fault.

 

[pfalcon]

The broadening of the scope does not mean the conductors. The scope (450.1) say the installation of all transformers. Reading through Article 450, you will see that this includes Overcurrent Protection of Transformers, Guarding of Transformers, Ventilation of Transformers, Accessibility of Transformers, Transformer Vaults, etc.

You will not see anything there about conductors. You will see, however, in the scope of Article 215 "This article covers the installation requirements, overcurrent protection requirements, minimum size, and ampacity of conductors for feeders.."

The scope for conductors is found elsewhere in the Code, not in Article 450. ...

I agree I shouldn't have added the part you colored. But the conductors aren't the issue here. They're a moot point. The discussion is over the presence of the secondary OCPD not the conductors.

 

[pfalcon]

... This is not true. The term "Secondary Overcurrent Protection" does NOT appear anywhere in Article 240. In fact, you will only find that term in Tables 450.3(A), 450.3(B) and in 695.5(B) which relates to Fire Pumps fed via a Transformer ...

... shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 2240.21(C)(1) through (C)(6). ...

I'm gonna have to spend some time staring at this one. You're correct. It doesn't use the phrase "Secondary Overcurrent Protection". I appear to have read that in from the above.

 

[pfalcon]

... Article 240 does not lay claim to the Transformer primary OCPD to avoid a secondary OCPD. 240.4(F) and 240.21(C)(1) permit transformer secondary conductors to be protected by the primary OCPD on a D-D transformer, under the conditions specified therein. Both of those two section specify that the protection of the Transformer must be in accordance with 450.3. In other words, the Transformer Protection and the Conductor Protection are separate issues. ...

(C)(1) permits using the primary OCPD for both issues. Phrase it how you want, the point is that the conductor protection is using a device that's distinctly part of the transformer protection and installation.

 

[ggunn]

It appears to me that electricians and engineers need to start going to law school so that they can correctly interpret the NEC.

 

[pfalcon]

... Again, not true. Table 450.3(B) is TELLING us how the Transformer OCPD becomes part of the installation. 450.3(B) says "Transformers 600V, Nominal, or Less: Overcurrent protection shall be provided in accordance with Table 450.3(B)." This is a Mandatory rule in the Code. Protection shall be provided in accordance with Table 450.3(B), not shall be provided in accordance with Article 240.

I have to leave this one alone right now. You're logic is flawed but I can't phrase it.

The words "NOT REQUIRED" are right there in black and white in Table 450.3(B). But you're telling us to just ignore them.

As you pointed out - Note 2 only applies AFTER you determine there is secondary protection. In short, the "Secondary Protection (See Note 2.)" column doesn't get looked at until AFTER the row is selected. When secondary protection is already present you don't get to look across row 1.

 

[pfalcon]

Where are you reading anything in Article 450 about D-D or D-Y? It doesn't exist. The Protection required by Table 450.3(B) has NOTHING to do with whether a transformer is D-D or D-Y. The Table applies the same to both types of Transformers. There is nothing in the text that tells you to apply one Protection Method to one type of transformer or the other. You are injecting that requirement yourself.

You CANNOT elect to omit a secondary OCPD on a D-D transformer if the Primary OCPD is greater than 125% (or next size up.)

This is the fallacy in your argument. You are trying to assign "Primary Only" or "Primary & Secondary" protection of a transformer based on whether the secondary conductors can be protected by the Primary OCPD or whether they need to be protected by an OCPD on the secondary size.

You're going backward in the discussion here. We've already agreed 450 doesn't say anything about D-D or D-Y. You're implying that I've claimed it did. That's false. The term I used is CHOICE which presumes you'd make the conductor sizing required to make that choice.

 

[pfalcon]

Thanks David for your time and patience.

I've discovered my false presumption that existed in Article 240 that led to all this. :slaphead:
I'd not have found it except for piecing different things together from all our posts.

 

[don_resqcapt19]

Yes. I meant the secondary conductors. I do not think the secondary conductors of a delta/delta 3 wire system would be protected by primary side protection for unbalanced over loads on the secondary. ...

The NEC says that they are so protected.

240.4(F) Transformer Secondary Conductors. Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device. ...

 

[david luchini]

Thanks David for your time and patience.

I've discovered my false presumption that existed in Article 240 that led to all this. :slaphead:
I'd not have found it except for piecing different things together from all our posts.

Glad you finally got there...I thought you were close.

What was your epiphany?

 

[pfalcon]

Glad you finally got there...I thought you were close.

What was your epiphany?

:ashamed: Somewhere in looking at and answering between posts 59 and 62 I realized I was ignoring (C)(2) through (C)(6).

 

[T.M.Haja Sahib]

The NEC says that they are so protected.

The NEC 1999 at least does not include that provision. I do not know from which code cycle onwards the secondary conductors of delta-delta transformers not considered as tap conductors.

 

[don_resqcapt19]

The NEC 1999 at least does not include that provision. I do not know from which code cycle onwards the secondary conductors of delta-delta transformers not considered as tap conductors.

It is 240-21(c)(1) in the 99 code. It was added in the 96 code in 240-3(i).

 

[T.M.Haja Sahib]

It is 240-21(c)(1) in the 99 code. It was added in the 96 code in 240-3(i).

Thanks, Don.

 

[T.M.Haja Sahib]

That is not correct. If the transformer primary protection is sized at 125% or less of the rated primary current the transformer secondary does not require protection. It is correct to say that you may be able to use the primary protection to protect the secondary conductors of a two wire to two wire transformer and the secondary conductors of a delta/delta 3 wire system.

That the secondary does not require protection means that it is already protected by primary protection. If inrush current does not permit sizing of primary protection at 125% or less of transformer full load current, the primary protection device size may be increased up to 250%. In such cases, the transformer secondary can not be considered to be protected by the primary protection.

 

[pfalcon]

1)Only two wires primary and two wires secondary transformers permit protection of secondary from primary side.
2)The OP may note that provision of primary side protection at 125% of primary current though code compliant may either prevent energisation of transformer on switching on or cause damage to the transformer during fault clearing.

... as specified in 240.21(C)(1) through (C)(6) ...

If the secondary conductors are 3m or less you can use (C)(2) to protect them instead of the primaries. Note that (C)(2) protects the secondary conductors without using OCPD. Example: 240.21(C)(2)(1)b mentions the poster's 200A OCPD as "the overcurrent-protective device at the termination of the secondary conductors." Here they've avoided calling the OCPD at the far end of the run "Secondary OCPD" because it's not being used for the transformer protection.