transformer secondary tap conductors 2006 nec

[david luchini]

Not so, with a 120 secondary voltage, the ratio would be .25.

I must say, I don't quite understand the point you are trying to make. A 45kVA 480 delta-240/120 delta (w/center tap) has a primary rated current of 54.1 Amps and a secondary rated current of 108.2 Amps. The secondary-primary ratio is 0.5, not 0.25.

Sure it does. The secondary conductor amps(150) x secondary to primary ratio (.25) =37.5. A 60 amp primary OCPD is more than 37.5

Again, the secondary-primary ratio for this transformer is 0.5, not 0.25, so your result is incorrect.

The primary Ocpd will always be higher than the secondary conductor amps at the ratio quoted.

The way i understand it, even with 2-wire -2-wire transformers, the secondary conductor amps times the ratio number is what the maximum OCPD is allowed to be set at and still be within the limits of article 450.3.

I don't believe this is correct. The secondary conductor amps times the secondary-primary ratio must be larger than the actual primary OCPD, not the theoretical maximum OCPD per 450.3. In my example, the theoretical maximum is 70A, but I used 60A. You could use 45A on the primary if the load didn't exceed that value. If the example had a 70A primary OCPD (maximum permitted by 450.3) then #1/0 secondary conductors would be protected using the ratio.

With a 120/208 secondary, the 120 volts ratio is .25 with a 480 volt primary voltage. The .25 ratio x secondary amps would be if allowed,what the maximum primary OCPD is allowed to be set at, and if it was set at the typical 125/250% per 450.3, it would be higher than the secondary conductor amperage and now be a tap conductor.

As I have pointed out above, the ratio for the 480-208/120V transformer is 0.43, not 0.25. But you are also misapplying 450.3 here. You wouldn't need to use the 125/250% from 450.3 if you are using primary protection only. You would use the 125% only column for primary protection only. 70A is the max primary OCPD for a 45kVA (480V primary) transformer using primary only protection. #1 Awg for a 240/120V secondary or #1/0 Awg for a 208/120V secondary would be properly protected by a 60A primary OCPD (using the ratio method you describe) and are therefore not tap conductors, and yet they would still be required by code to have and OCPD to protect the secondary conductors. Why would the need the additional secondary OCPD if they are not taps?

To be quite honest, to me it doesn't really matter what the conductors are called. They can be secondary conductors or they can be tap conductors. It is a good debate.

Rick

I agree its a good debate. The forum is a great learning tool.

 

[RUWired]

Dave, i come up with the .25 ratio using basic math. 480/120 is 4 turns primary and 1 turn secondary. The secondary to primary ratio is .25. (1divided by 4). Other than basic math i can't explain it anymore. Maybe with engineering there would be another explaination.

Rick

 

[david luchini]

Dave, i come up with the .25 ratio using basic math. 480/120 is 4 turns primary and 1 turn secondary. The secondary to primary ratio is .25. (1divided by 4). Other than basic math i can't explain it anymore. Maybe with engineering there would be another explaination.

Rick

I would have to agree that basic math would tell us that 480/120 is a 4-1 primary to secondary turns ratio. And that would work great if we were transforming from 480-120V. However, in neither example I gave were we transforming from 480-120V.

The first example was a 45kVA, 480-240/120V delta transformer, and the second example was a 45kVA, 480-208/120V wye transformer.

As you point out, the voltage to turns-ratio relationship is such that: V1=(N1/N2)*V2. The current to turns-ratio relationship is: I1=(N2/N1)*I2.

For the 45kVA, 480-208/120V transformer we have a primary-secondary turns ratio of 480/208= 2.31:1. If we know the primary rated current of this transformer is 54.1A, then we can see that the secondary current is: I2=(N1/N2)*I1, or I2=(2.31)*54.1=124.9A.

If the primary to secondary turns ratio of the 45kVA, 480-208/120V transformer was 4:1 as you suggest, then secondary current would be: I2=(4)*54.1=216.4A. This is clearly not correct. A 45kVA, 480-208/120V transformer has a secondary rated current of 124.9A, and a turns ratio of 2.3:1.

 

[Smart $]

So are not transformer secondary conductor installed per 240.4(F) excluded from this definition?

Not just 240.4(F) but all transformer secondary conductors are excluded, i.e. not tap conductors. Only branch and feeder circuits can be tapped, i.e. they have ocp ahead of the tap. And while primary-protected secondary conductors do have ocp ahead of the connection to the secondary terminals, the ampacity stipulation in 240.4(F) and 240.21(C)(1) prevents secondary conductors from being sized smaller than the secondary protection the primary ocpd affords.

Yet we call transformer secondary conductors "tap conductors" because the ocp requirements are quite similar to actual tap requirements.

 

[RUWired]

Yet we call transformer secondary conductors "tap conductors" because the ocp requirements are quite similar to actual tap requirements.

Interestingly 240.4(E) references all of 240.21 for tap conductor requirements, not just B1-5.

(E) Tap Conductors. Tap conductors shall be permitted to be protected against overcurrent in accordance with the following:

(3) 240.21, Location in Circuit

Rick

 

[Smart $]

Interestingly 240.4(E) references all of 240.21 for tap conductor requirements, not just B1-5.

(E) Tap Conductors. Tap conductors shall be permitted to be protected against overcurrent in accordance with the following:

(3) 240.21, Location in Circuit

Rick

Yes, but 240.4(E) specifies "Tap Conductors". Where in 240.21 is a transformer secondary conductor deemed a tap conductor?

 

[RUWired]

Its not mentioned anywhere in 21C, but it clearly says in (E)....shall be permitted to be protected against overcurrent in accordance with 21. That to me would cover all of 21.
But i see your point
Rick

 

[RUWired]

I was looking up secondary tap conductors on the net and i see a lot of IAEI news clips ,and some from CMP 10 that refer to them as tap conductors.
http://www.iaei.org/magazine/2002/03/feeder-and-transformer-secondary-tap-conductors/
Rick

 

[Smart $]

I was looking up secondary tap conductors on the net and i see a lot of IAEI news clips ,and some from CMP 10 that refer to them as tap conductors.
http://www.iaei.org/magazine/2002/03/feeder-and-transformer-secondary-tap-conductors/
Rick

I don't deny the industry calls them tap conductors. There currently just isn't anything in the NEC that specifically calls them tap conductors.

 

[dana1028]

I don't deny the industry calls them tap conductors. There currently just isn't anything in the NEC that specifically calls them tap conductors.

Well as is often said...everyone has their own opinion 'bout things...and as is often seen in this forum, no amount of discussion or argument will dissuade a person from their point of view.

Seems to me 240.4(E) makes it pretty clear that the conductors in 240.21 are considered [by the NEC] to be tap conductors.

But I don't have to worry the point because every jurisdiction I've worked in the AHJ has considered transformer secondaries to be taps.

Additionally, the last sentence in 240.21 has been interpreted [in this forum] to mean, 'you can't tap a tap.'...and I've never heard an argument about that.

 

[david luchini]

Seems to me 240.4(E) makes it pretty clear that the conductors in 240.21 are considered [by the NEC] to be tap conductors.

Seems to me that this is quite a leap...and not supported by the code text.

240.4(E) says that tap conductors shall be permitted to be protected again overcurrent in accordance with the following: (3) 240.21, Location in Circuit.

The title of section 240.21 is "Location in Circuit" not "Tap conductors" or some variation, thereof.

240.21 includes eight sections, (A) thru (H).

(A) Branch Circuit Conductors,
(B) Feeder Taps
(E) Busway Taps
(F) Motor Circuit Taps,

all relate to Taps.

(C) Transformer Secondary Conductors
(D) Service Conductors
(G) Conductors from Generator Terminals
(H) Battery Conductors

all do NOT relate to Taps.

If 240.4(E) means that Transformer Secondary Conductors are taps, then 240.4(E) also meant that Service Conductors are taps, or Battery Conductors are taps, etc.

 

[Smart $]

Well as is often said...everyone has their own opinion 'bout things...and as is often seen in this forum, no amount of discussion or argument will dissuade a person from their point of view.

Not an absolute. There are occasions where persons change their point of view.

Seems to me 240.4(E) makes it pretty clear that the conductors in 240.21 are considered [by the NEC] to be tap conductors.

An implicit reference is far from an explicit definition.

I'll even give you an example. Look at 240.21(A). It states:

Branch-circuit tap conductors meeting the requirements of 210.19 shall be permitted to have overcurrent protection as specified in 210.20.

So would you say branch-circuit tap conductors meeting the requirements of 210.19's general statement qualify for ocp per 210.20, or do they have to meet the requirements of the exceptions to 210.19?

But I don't have to worry the point because every jurisdiction I've worked in the AHJ has considered transformer secondaries to be taps.

Additionally, the last sentence in 240.21 has been interpreted [in this forum] to mean, 'you can't tap a tap.'...and I've never heard an argument about that.

It doesn't matter whether they are or are not tap conductors. They are without question, transformer secondary conductors, and that is what the NEC calls them.

 

[Smart $]

Seems to me that this is quite a leap...and not supported by the code text.

240.4(E) says that tap conductors shall be permitted to be protected again overcurrent in accordance with the following: (3) 240.21, Location in Circuit.

The title of section 240.21 is "Location in Circuit" not "Tap conductors" or some variation, thereof.

240.21 includes eight sections, (A) thru (H).

(A) Branch Circuit Conductors,
(B) Feeder Taps
(E) Busway Taps
(F) Motor Circuit Taps,

all relate to Taps.

(C) Transformer Secondary Conductors
(D) Service Conductors
(G) Conductors from Generator Terminals
(H) Battery Conductors

all do NOT relate to Taps.

If 240.4(E) means that Transformer Secondary Conductors are taps, then 240.4(E) also meant that Service Conductors are taps, or Battery Conductors are taps, etc.

Great point !!!

 

[elohr46]

this was an interesting discussion but the bottom line is that electricians and electrical inspectors will still call xfmr sec. cond. "tap conductors", just like when they call a panelboard a "subpanel" when the NEC does not use either term.