Transformer secondaries and separately derived systems

[dm9289]

I see this situation often in the 3 very old factories I work and can use some clarification.

Transformer input 2 legs from a 480v system on a circuit breaker (primary protection).

Output 120/240v to feed a panel board main lug.

I believe the secondary conductors should be protected either by a disconnect or a panel main breaker based on info in 240.21(C)
Am I reading this correctly?

Only time no secondary protection is required seems to be for single voltage output if i am correct for example a 120 to 24 volt transformer

Thanks as always for straightening me out
Dave

 

[texie]

Yes, you are correct. As far as I remember this has been in the code for many decades. The physics of this are such that the secondary conductors could be greatly overloaded.

 

[dm9289]

Yes, you are correct. As far as I remember this has been in the code for many decades. The physics of this are such that the secondary conductors could be greatly overloaded.

Thank you that's what i thought but I'm wrong often

 

[jim dungar]

Yes, you are correct. As far as I remember this has been in the code for many decades. The physics of this are such that the secondary conductors could be greatly overloaded.

Yes. However as recently as the 70s these types of installations were still allowed by the NEC and were quite common.

 

[dm9289]

Yes. However as recently as the 70s these types of installations were still allowed by the NEC and were quite common.

Makes sense pre 1950

 

[jim dungar]

Makes sense pre 1950

Some 70 years of successful operation. It appears somebody did not blindly add loads beyond the initial design parameters.

 

[augie47]

I'm not sure when it was 1st introduced in the Code but if the secondary supplies a panelboard, 408.36 requires a main.

 

[jim dungar]

I'm not sure when it was 1st introduced in the Code but if the secondary supplies a panelboard, 408.36 requires a main.

This has been fairly recent. I don't think it has been much more than 20 years or so.

 

[Dennis Alwon]

This is the 2011 to 2017...

(B) Supplied Through a Transformer. Where a panelboard is supplied through a transformer, the overcurrent protection required by 408.36 shall be located on the secondary side of the transformer.

Exception: A panelboard supplied by the secondary side of a transformer shall be considered as protected by the overcurrent protection provided on the primary side of the transformer where that protection is in accordance with 240.21(C)(1).

 

[texie]

Now you guys have me wondering when this became code. When I get a minute I'll go to the basement library where the old code book section is located. My books go back to 1975.

 

[Dennis Alwon]

I'm not sure when it was 1st introduced in the Code but if the secondary supplies a panelboard, 408.36 requires a main.

What about the exception

 

[dm9289]

What about the exception

I think that is for single voltage ex 120 to 24v. 480 to 120/240 i do not believe is single voltage

 

[Dennis Alwon]

I think that is for single voltage ex 120 to 24v. 480 to 120/240 i do not believe is single voltage

Where does it say that. How many panels are fed with a single voltage

 

[david luchini]

Where does it say that. How many panels are fed with a single voltage

It says that in 240.21(C)(1). Think 3 wire delta secondary, like 480-240V delta.

 

[don_resqcapt19]

This has been fairly recent. I don't think it has been much more than 20 years or so.

The oldest code book I have is the 1975 and protection for the secondary conductors was required at that time, and at that time, you only had a 10' rule for the secondary conductors. There was no 25' secondary conductor rule, but there was a 25' feeder tap rule.

 

[jim dungar]

The oldest code book I have is the 1975 and protection for the secondary conductors was required at that time, and at that time, you only had a 10' rule for the secondary conductors. There was no 25' secondary conductor rule, but there was a 25' feeder tap rule.

But my 20 year response was about 408.36, the requirement for a main breaker for a panelboard.

 

[don_resqcapt19]

But my 20 year response was about 408.36, the requirement for a main breaker for a panelboard.so

Sorry

 

[Carultch]

I believe the secondary conductors should be protected either by a disconnect or a panel main breaker based on info in 240.21(C)
Am I reading this correctly?

Only time no secondary protection is required seems to be for single voltage output if i am correct for example a 120 to 24 volt transformer

If you have a qualifying topology of your transformer, the primary OCPD can indirectly protect the secondary conductors. Otherwise, you need a secondary OCPD and compliance with one of the rules in 240.21(C) for how to construct the conductors.

Qualifying topologies are topologies of the transformer that cause the primary current to be directly proportional to the secondary current, no matter what. The examples are 2-wire to 2-wire single phase, and delta-to-delta 3-wire 3-phase. Any wye or centertaps, on either side, make it not a qualifying topology. When you can use the primary OCPD to protect the secondary conductors, you simply multiply the primary OCPD by the primary to secondary voltage ratio. As an example, a 20A breaker on the 480V side of a 480V : 120V transformer with a qualifying topology, would effectively act as an 80A breaker as it protects the secondary.

Center taps and wyes can cause some overloads on the secondary to be in a "blindspot" of the primary OCPD, which is why they require secondary OCPD.

 

[dm9289]

Thank you everyone for your help. Thru your help I'm understanding things better. Sometimes i have trouble interpreting the code