Transformer secondary lenghth

[mbrooke]

In a delta wye 480 to 208Y/120 volt transformer, what is the maximum length the secondary feeders are allowed to travel?

 

[Pharon]

If your primary OCPD is sized at 125% or less, you don't need secondary OCPD at all.

Is that what you're asking? How far can the secondary conductors travel before you need OCPD?

 

[david luchini]

If your primary OCPD is sized at 125% or less, you don't need secondary OCPD at all.

Is that what you're asking? How far can the secondary conductors travel before you need OCPD?

If your primary is sized at 125% of less, you don't need secondary protection for the Transformer. The secondary conductors will still need protection, requiring an OCPD.

Section 240.21(C) gives the maximum secondary conductor length for several different installation situations.

 

[Pharon]

Ah, that makes sense - good point. I knew the tap rule had to come into play there somehow...

 

[mbrooke]

If your primary OCPD is sized at 125% or less, you don't need secondary OCPD at all.

Is that what you're asking? How far can the secondary conductors travel before you need OCPD?

Only for delta delta or 2 wires. This has a wye secondary.

 

[augie47]

Noted and addressed in Post #3

If your primary is sized at 125% of less, you don't need secondary protection for the Transformer. The secondary conductors will still need protection, requiring an OCPD.

Section 240.21(C) gives the maximum secondary conductor length for several different installation situations.

 

[electrofelon]

If your primary is sized at 125% of less, you don't need secondary protection for the Transformer. The secondary conductors will still need protection, requiring an OCPD.

Section 240.21(C) gives the maximum secondary conductor length for several different installation situations.

.....and for completeness and those who may be researching this later on, if the xformer is feeding a panelboard, it needs to be protected at its rating (408.36). So we need to protect three things: the transformer, the conductors, and the panelboard. Articles 450, 240, and 408 respectively.

 

[mbrooke]

Noted and addressed in Post #3

I know for a fact that is incorrect. The 125% primary rule applies only to a delta delta or a 2 wire to 2 wire.

Delta wye as is the case (as well as delta detla center tap or single phase 120/240) needs secondary protection no matter what.


The issue here is a how long the secondary conductors can travel to the panel board and under what conditions.

 

[Smart $]

I know for a fact that is incorrect. The 125% primary rule applies only to a delta delta or a 2 wire to 2 wire.

Delta wye as is the case (as well as delta detla center tap or single phase 120/240) needs secondary protection no matter what.


The issue here is a how long the secondary conductors can travel to the panel board and under what conditions.

That is incorrect.

Primary-only protection at 125% transformer rated-current can be applied to all transformers up to 600V [1,000V under 2014 NEC].

Secondary conductor protection is required regardless, whether the transformer is primary-only or primary and secondary protected.

What comes into play is whether it is advantageous to use a primary-only transformer protection scheme when you're going to have OCPD for the secondary conductors anyway. Secondary conductor protection can also be the transformer secondary protection, and additionally panelboard protection. One OCPD can satisfy all three requirements.

 

[Smart $]

I should have also stated that conductors of delta-delta and 2-wire secondaries can be considered protected by a primary-only transformer protection within the stipulations of 240.21(C)(1). In these cases, there still remains the question whether there is a limit to the secondary conductor length. None is stated in (C)(1)... but the general statement states "as specified in 240.21(C)(1) through (C)(6)", rather than one of them. :huh:

 

[augie47]

and, I reiterate, david's post #3 is accurate !

"If your primary is sized at 125% of less, you don't need secondary protection for the Transformer. The secondary conductors will still need protection, requiring an OCPD.

Section 240.21(C) gives the maximum secondary conductor length for several different installation situations."

The key word is underlined. The 125% rule comes from Art 450 which covers transformer protection.

Art 240, on the other hand, and 240.21(C) in particular, covers conductor protection as noted by others. The Art 450 125% rule has little if anything to do with the conductor length or protection.
'
In answer to your OP:
"In a delta wye 480 to 208Y/120 volt transformer, what is the maximum length the secondary feeders are allowed to travel?"
the maximum length is infinite...... per 240.21(C)(4)

 

[david luchini]

In answer to your OP:
"In a delta wye 480 to 208Y/120 volt transformer, what is the maximum length the secondary feeders are allowed to travel?"
the maximum length is infinite...... per 240.21(C)(4)

:thumbsup:

 

[mbrooke]

Ok Im starting to make sense of it.

Just confused why parts of the code say a secondary breaker is needed while others let it go...



Either way, does anyone know of any Mike Holt videos on the subject? Im a visual learner sometimes.

 

[david luchini]

Ok Im starting to make sense of it.

Just confused why parts of the code say a secondary breaker is needed while others let it go...

I think people get confused on the fact that the Transformer Protection and the Conductor Protection are separate items (probably because one OCPD most often provides both functions.)

You can have a transformer that has "Primary Only Protection" but still need an OCPD on the secondary side to protect the secondary conductors. In this case, the secondary OCPD will not be providing Transformer Protection as far as the Code is concerned, only the secondary conductor protection.

 

[mbrooke]

I think people get confused on the fact that the Transformer Protection and the Conductor Protection are separate items (probably because one OCPD most often provides both functions.)

You can have a transformer that has "Primary Only Protection" but still need an OCPD on the secondary side to protect the secondary conductors. In this case, the secondary OCPD will not be providing Transformer Protection as far as the Code is concerned, only the secondary conductor protection.

Ok its beginning to make sense. Im getting the two mixed up as one concept.

Thanks!

 

[augie47]

Ok its beginning to make sense. Im getting the two mixed up as one concept.

Thanks!

Happens a lot.. that's why they added a reference note in Art 450 to Art 240 for conductor protection.