NEC 240.21(C)(1)

[LarryFine]

The OP said he has two secondary coils. He also said he is using the transformer as a single voltage 2 wire secondary.

One might speculate that he is connecting the coils in parallel, but he could connect them in series and also have a single voltage 2 wire secondary.

To me, either still does not fit the definition of a single-voltage, two-wire secondary, just because that's how it was configured in the field.

 

[david luchini]

To me, either still does not fit the definition of a single-voltage, two-wire secondary, just because that's how it was configured in the field.

How would 120V between 2 wires or 240V between 2 wires not fit the definition of single-voltage, two-wire secondary?

 

[LarryFine]

How would 120V between 2 wires or 240V between 2 wires not fit the definition of single-voltage, two-wire secondary?

Because those describe the as-wired connections of a dual-secondary, dual-voltage transformer, not the physical characteristics of the construction of the transformer. A POCO transformer, for example, can be configured with the dual secondaries wired for 120/240v 1ph with 3 wires out, or 120v as part of a 208/120 3ph wye with 2 wires out.

A genuine single-voltage 2-wire secondary can not be configured for anything except a single voltage because there are only two wires. Even a center tap on a single winding means the transformer does not qualify as a single-voltage secondary with only two wires. To me, the construction, not the field connections, describe the transformer.

Now, I am not claiming to know which example they had in mind when the code was written. I don't know whether the wires "brought out" of the transformer is what determines the characteristics of the transformer.


Added:

The exact wording is "Conductors supplied by the secondary side of a single-phase transformer having a 2 wire (single voltage) secondary."

"Conductors supplied by . . . " is not the same thing as conductors used. It says "a single-phase transformer having a 2 wire (single voltage) secondary." It does not say "as long as you're using it as . . ."

 

[jumper]

This is getting interesting.:thumbsup:

 

[ActionDave]

Because those describe the as-wired connections of a dual-secondary, dual-voltage transformer, not the physical characteristics of the construction of the transformer. A POCO transformer, for example, can be configured with the dual secondaries wired for 120/240v 1ph with 3 wires out, or 120v as part of a 208/120 3ph wye with 2 wires out.

A genuine single-voltage 2-wire secondary can not be configured for anything except a single voltage because there are only two wires. Even a center tap on a single winding means the transformer does not qualify as a single-voltage secondary with only two wires. To me, the construction, not the field connections, describe the transformer.

Now, I am not claiming to know which example they had in mind when the code was written. I don't know whether the wires "brought out" of the transformer is what determines the characteristics of the transformer.


Added:

"Conductors supplied by . . . " is not the same thing as conductors used. It says "a single-phase transformer having a 2 wire (single voltage) secondary." It does not say "as long as you're using it as . . ."

It's a two wire SINGLE VOLTAGE secondary and qualifies for the primary only exception. Maybe you need to parallel a couple of leads to get that single voltage, that's just the way transformers are made.

If this were a big enough transformer then you would use jumpers or bussing on the lugs to get a single voltage secondary. It's just not that often that you need 75Kva 120V transformer. It's common to have a 5Kva used at 120V and those commonly use leads instead of bussing.

 

[LarryFine]

It's a two wire SINGLE VOLTAGE secondary and qualifies for the primary only exception. Maybe you need to parallel a couple of leads to get that single voltage, that's just the way transformers are made.

To me, you are contradicting yourself here, and that brings us to the point of this discussion. It can't be "a two wire SINGLE VOLTAGE secondary" if "you need to parallel a couple of leads to get that single voltage". It's either one or the other.

One is the transformer, the other is how it's connected and used. The rule describes the former, not the latter. Almost every multiple-secondary transformer can be connected as a single-secondary one, but that doesn't make it one.

To say "that's just the way transformers are made" is also the point, as not all transformers are made that way; single-voltage two-wire secondaries certainly are not made that way, but dual-voltage four-wire secondaries are.

 

[jap]

It's a two wire SINGLE VOLTAGE secondary and qualifies for the primary only exception. Maybe you need to parallel a couple of leads to get that single voltage, that's just the way transformers are made.

If this were a big enough transformer then you would use jumpers or bussing on the lugs to get a single voltage secondary. It's just not that often that you need 75Kva 120V transformer. It's common to have a 5Kva used at 120V and those commonly use leads instead of bussing.

He's saying actually it is not because there are more than (2) physical conductors sticking out of the insulating material of the transformer on the primary and secondary side.

May be odd way of looking at it but it is a valid point per the way it's written.

Strictly 2 transformer input primary conductors and 2 transformer output secondary conductors to connect the field wiring to only are how some may be looking at this.

I tend not to see it that way myself, but, Oh well.

JAP>

 

[LarryFine]

As another example, look at the typical buck-boost transformer with two 120v windings and two 12v windings. With every installed configuration, it's connected to the outside world with only two primary and two secondary connections. But, it certainly is not a single-voltage, 2-wire primary to a single-voltage, 2-wire secondary transformer. It is most definitely a dual-voltage, 4-wire primary to a dual-voltage, 4-wire secondary transformer.

 

[don_resqcapt19]

The OP said he has two secondary coils. He also said he is using the transformer as a single voltage 2 wire secondary.

One might speculate that he is connecting the coils in parallel, but he could connect them in series and also have a single voltage 2 wire secondary.

David, I almost always agree with your comments, but this is one where I don't. I see the "two wire secondary" as referring to the physical construction of the transformer itself, and not to how the transformer is connected when it is installed. At least that is how I read the code language.

 

[wwhitney]

So what is the definition of "having"? Cheers, Wayne

 

[jap]

So what is the definition of "having"? Cheers, Wayne

I agree.

The transformer does "have" that particular output along with "having" another one also.

JAP>

 

[jap]

I have a transformer I am trying to determine meets this provision in the code for omitting secondary protection. Its a 480/240 to 240/120 transformer with 2 hots and a GND on the primary side and 1 hot, a neutral, and a GND on the secondary side. I think this scenario is not sufficient for this rule because of the neutral coming off the secondary but i am not sure. Any insight would be greatly appreciated!

What size transformer are you working with?

JAP>

 

[ActionDave]

I see it as being hung up on nomenclature instead of what is going on electrically.

 

[wwhitney]

I see it as being hung up on nomenclature instead of what is going on electrically.

It would be nice if the NEC had some language to the effect of "when the rules apply ambiguously to a situation, use the principles of physics to determine the proper interpretation."

Cheers, Wayne

 

[GoldDigger]

I see it as being hung up on nomenclature instead of what is going on electrically.

:thumbsup:
The physics behind the code rule is that for a split voltage secondary, as long as the center tap is brought out of the transformer to feed loads it is possible to overload one half of the secondary by a factor of two or more without tripping the primary OCPD.
That will not happen with a single two-wire connection on the secondary side of the transformer, regardless of how many unused taps there are on the secondary side.
However, if the center tap is grounded, even if no grounded conductor is brought to the loads, then a ground fault in a strict two wire load connected to the hot wires could still end up drawing excess current only from one side of the secondary. A bolted ground fault will surely trip the primary OCPD, but a higher resistance fault mechanism could in principle overload the secondary without tripping the primary OCPD.

 

[kwired]

As another example, look at the typical buck-boost transformer with two 120v windings and two 12v windings. With every installed configuration, it's connected to the outside world with only two primary and two secondary connections. But, it certainly is not a single-voltage, 2-wire primary to a single-voltage, 2-wire secondary transformer. It is most definitely a dual-voltage, 4-wire primary to a dual-voltage, 4-wire secondary transformer.

It is also not a separately derived system - there is no "secondary" to protect.

:thumbsup:
The physics behind the code rule is that for a split voltage secondary, as long as the center tap is brought out of the transformer to feed loads it is possible to overload one half of the secondary by a factor of two or more without tripping the primary OCPD.
That will not happen with a single two-wire connection on the secondary side of the transformer, regardless of how many unused taps there are on the secondary side.
However, if the center tap is grounded, even if no grounded conductor is brought to the loads, then a ground fault in a strict two wire load connected to the hot wires could still end up drawing excess current only from one side of the secondary. A bolted ground fault will surely trip the primary OCPD, but a higher resistance fault mechanism could in principle overload the secondary without tripping the primary OCPD.

:thumbsup::thumbsup:

Opposite end of the spectrum - a 10 hp motor with dual voltage 230/460 rating is still a 10 hp motor at either voltage.

A set of "parallel conductors" is still considered one conductor with an overall greater ampacity then the components that make it up.