Not More than 600V: Transformer Primary Conductors

[publicgood]

215.2(B) describes size of feeders supplying transformers. Is there anywhere in the code describing sizing of the primary conductors for less than 600V applications? Specifically, when using article 450 to up-size primary for in-rush. 240 has lots of references for the secondary conductors and primary also if your application allows primary protection only.

 

[publicgood]

No one?

 

[JFletcher]

What more specifically are you looking for besides the code sections you mentioned? Do you have an exact transformer in mind?

 

[kwired]

conductors need to have minimum ampacity of 100% of non continuous load plus 125% of continuous load in most instances.

overcurrent protection may possibly be higher to allow for inrush current, what protection is on the secondary side comes into play as well.

 

[publicgood]

Not More than 600V: Transformer Primary Conductors

Thanks, kwired. I wish the code spelled it out as well for 600V as it does for MV, regarding transformer primary feeder conductors.

In discussions with some in my office, they suggest not upsizing conductors for motors is okay because they have overloads...but insist this isn’t the case for transformers.

In 240 there are references to conductors on the transformer primary minimum size to be 1/3 ampacity of the OCPD when primary protection only is used....this speaks to wire being allowed to be smaller than OCPD. Just one example for my stance that oversizing OCPD for transformer inrush is okay and not having to increase wire size.

 

[kwired]

Thanks, kwired. I wish the code spelled it out as well for 600V as it does for MV, regarding transformer primary feeder conductors.

In discussions with some in my office, they suggest not upsizing conductors for motors is okay because they have overloads...but insist this isn’t the case for transformers.

In 240 there are references to conductors on the transformer primary minimum size to be 1/3 ampacity of the OCPD when primary protection only is used....this speaks to wire being allowed to be smaller than OCPD. Just one example for my stance that oversizing OCPD for transformer inrush is okay and not having to increase wire size.

That inrush is only very briefly when first energizing the circuit, it won't contribute to much heating in the conductors. Also the resistance of those conductors actually will limit the inrush current to some extent, smaller they are the more resistance they will have and the more limiting they can be. This can make a difference in whether the primary overcurrent device will hold long enough for the primary field to stabilize. Large conductor - may allow enough current to flow that it trips the overcurrent device. Smaller conductor (but still large enough to meet code minimums) may limit current enough the overcurrent device never sees instantaneous trip level current.

 

[electrofelon]

I don't quite understand your question. Are you asking if we can have an "oversized" OCPD relative to the conductor size like we can with motors? Remember, 450 is only about the transformer itself.

 

[infinity]

I don't quite understand your question. Are you asking if we can have an "oversized" OCPD relative to the conductor size like we can with motors? Remember, 450 is only about the transformer itself.

That was my take on what's described in the OP. That answer would be no.

 

[kwired]

I don't quite understand your question. Are you asking if we can have an "oversized" OCPD relative to the conductor size like we can with motors? Remember, 450 is only about the transformer itself.

That was my take on what's described in the OP. That answer would be no.

What is the point of having 240.21(B)(3) if the answer is no? This is likely the section OP was referring to in post 5.

 

[jap]

In 240 there are references to conductors on the transformer primary minimum size to be 1/3 ampacity of the OCPD when primary protection only is used....this speaks to wire being allowed to be smaller than OCPD. Just one example for my stance that oversizing OCPD for transformer inrush is okay and not having to increase wire size.

240.21 (1) (2) (3) (4) and (5) are rules for Feeder "Taps".

What does this have to do with sizing an OCPD for a feeder to a transformer ?


JAP>

 

[publicgood]

Okay, let's try this again with an example. 500kVA, 480:208/120V. Suppose I decide to set the primary OCPD to be 1000A.

What is the code minimum feeder size (THWN in conduit) to the primary side of the transformer?

 

[jap]

Okay, let's try this again with an example. 500kVA, 480:208/120V. Suppose I decide to set the primary OCPD to be 1000A.

What is the code minimum feeder size (THWN in conduit) to the primary side of the transformer?

Are you going to have primary and secondary protection?

JAP>

 

[publicgood]

Are you going to have primary and secondary protection?

JAP>

Yes - both primary and secondary protection. Required due to wye connection on the secondary.

 

[david luchini]

Yes - both primary and secondary protection. Required due to wye connection on the secondary.

You can protect the transformer primary only, if you wanted. Secondary conductor protection is a separate issue.

 

[david luchini]

Okay, let's try this again with an example. 500kVA, 480:208/120V. Suppose I decide to set the primary OCPD to be 1000A.

What is the code minimum feeder size (THWN in conduit) to the primary side of the transformer?

3 sets of 400 mcm.

 

[infinity]

Okay, let's try this again with an example. 500kVA, 480:208/120V. Suppose I decide to set the primary OCPD to be 1000A.

What is the code minimum feeder size (THWN in conduit) to the primary side of the transformer?

It sounds like you're asking if the 1000 amp OCPD with 1000 amp conductors and the OCPD is not large enough to compensate for the inrush current can you use a larger OCPD with the same 1000 amp conductors, is this accurate?

 

[publicgood]

It sounds like you're asking if the 1000 amp OCPD with 1000 amp conductors and the OCPD is not large enough to compensate for the inrush current can you use a larger OCPD with the same 1000 amp conductors, is this accurate?

I wanted to size the conductors at 125% all the time no matter the primary OCPD...like is done for motors. I’m hearing this can’t be done by code. Even though you only upsized the primary OCPD for inrush, the conductors also need to be upsized to stay protected.

It is too bad above 600V tells us it is okay to do this in 215.

 

[david luchini]

I wanted to size the conductors at 125% all the time no matter the primary OCPD...like is done for motors. I’m hearing this can’t be done by code. Even though you only upsized the primary OCPD for inrush, the conductors also need to be upsized to stay protected.

It is too bad above 600V tells us it is okay to do this in 215.

215 doesn't say that for above 600V. The conductor must still be protected. Of course the rules for protecting conductors above 600V are different.

 

[kwired]

240.21 (1) (2) (3) (4) and (5) are rules for Feeder "Taps".

What does this have to do with sizing an OCPD for a feeder to a transformer ?


JAP>

Well if he has overcurrent protection higher then the conductor ampacity then he must have a "feeder tap".

If not allowed from the transformer's perspective then it is pointless to even have 240.21(B)(3) because it only applies to tap conductors supplying a transformer. Overcurrent protection on secondary side of transformer is a part of this rule though.

 

[jap]

Well if he has overcurrent protection higher then the conductor ampacity then he must have a "feeder tap".

If not allowed from the transformer's perspective then it is pointless to even have 240.21(B)(3) because it only applies to tap conductors supplying a transformer. Overcurrent protection on secondary side of transformer is a part of this rule though.

Well that's true.

JAP>