450.3B

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xformer

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Does 450.3B primary protection only apply to single phase and delta to delta transformers while primary and secondary protection apply to all others as pertaining to 240.4C1????
 
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I'm not sure if this will help, but keep in mind Art 450 is only for protection of the transformer and has little to do with the conductors.
450.3(B) would apply to applicable transformers, single and three phase, delta, wye, etc.
You would decide on primary and/or secondary protection of the transformer based on that Table then select your conductors for the primary based on that protection..
Once you have the transformer protected per Art 450, then you can use 240.21(C) to assure secondary conductor protection.
As you noted, in some cases the secondary conductors are considered protected by the primary.
 
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augie47 said:
I'm not sure if this will help, but keep in mind Art 450 is only for protection of the transformer and has little to do with the conductors.
450.3(B) would apply to applicable transformers, single and three phase, delta, wye, etc.
You would decide on primary and/or secondary protection of the transformer based on that Table then select your conductors for the primary based on that protection..
Once you have the transformer protected per Art 450, then you can use 240.21(C) to assure secondary conductor protection.
As you noted, in some cases the secondary conductors are considered protected by the primary.
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Hello Augie47,

Some friends and I had a small debate about this issue yesterday. The way I understand the things is that according to 240.21C there needs to be some over current protection for the secondary conductors. An install that has a delta-wye transformer where the secondary conductors are connected directly to a M.L.O. panel is NOT allowable because of 2 reasons 1.) the Primary over current device does not protect the secondary conductors. 2.) the secondary conductors do not terminate in a OCPD.

When the secondary conductors terminate in a OCPD. the rating of that OCPD be based upon 450.3B primary and secondary protection. More along the lines of the secondary OCPD. Correct?
 
Not sure I understand... some days I'm slow..some slower :D
On a delta-wye arrangement you can not rely on the transformer primary protection to protect the secondary conductors. The secondary would need to be protected in accordance with one of the 240.21(C) rules.
IF you have proper primary protection (normally 125%) then your secondary OCP could be based in the conductor requirement only per 240.21(C).
For example, lets say you have a 75 kva 3 phase 480Y/120 arrangement. Your primary protection is say 100 amps.
You can place (3) 200 amp M/B panels on the secondary and protect the 3/0s. Obviously the secondary protection is far greater than the required transformer protection, however, the transformer is protected by the primary device.
Likewise, if you took the primary protection to 225 amps, then your secondary protection would be limited to 225 amps.

That help ?
 
xformer said:
The way I understand the things is that according to 240.21C there needs to be some over current protection for the secondary conductors. An install that has a delta-wye transformer where the secondary conductors are connected directly to a M.L.O. panel is NOT allowable because of 2 reasons 1.) the Primary over current device does not protect the secondary conductors. 2.) the secondary conductors do not terminate in a OCPD.
Click to expand...

You are correct here.

xformer said:
When the secondary conductors terminate in a OCPD. the rating of that OCPD be based upon 450.3B primary and secondary protection. More along the lines of the secondary OCPD. Correct?
Click to expand...

This is not necessarily correct. As Augie noted, the transformer protection and the conductor protection are two separate issues.

So for instance, if you had a 45kVA, 480-208/120V transformer protected on the primary by a 70A OCPD, you would meet the "primary only protection" requirements from 450.3(B). If on the secondary, the conductors terminated on a 200A OCPD (which exceeds 125% or next size up) this would be fine as long as the conductors were 3/0 or larger, to meet the requirements of 240.21(C).
 
I often marvel at how complicated this is. There is an entire article in the NEC regarding transformers yet you need to jump back and forth between Articles 450 and 240 just to properly install a simple transformer. Certainly there must be someone out there who could re-write article 450 to cover all aspects of transformer installation. Am I asking for too much? :roll:
 
infinity said:
I often marvel at how complicated this is. There is an entire article in the NEC regarding transformers yet you need to jump back and forth between Articles 450 and 240 just to properly install a simple transformer. Certainly there must be someone out there who could re-write article 450 to cover all aspects of transformer installation. Am I asking for too much? :roll:
Click to expand...
Yes, I think you are asking way too much.

Why should you look in an article covering transformers to find the requirements for over current protection of the secondary conductors?

I don't see why so many have issues with this. You look to 450 for the protection of the transformer itself and to 240 for the protection of the conductors that are connected to that transformer.
 
infinity said:
I often marvel at how complicated this is. There is an entire article in the NEC regarding transformers yet you need to jump back and forth between Articles 450 and 240 just to properly install a simple transformer. Certainly there must be someone out there who could re-write article 450 to cover all aspects of transformer installation. Am I asking for too much? :roll:
Click to expand...

Infinity, i think that would start to turn the NEC into more of a design manual than just a book of codes... I can understand don_resqcapt19's of thinking it doesn't need to be addressed in 450 since its already addressed in 240. I think, if need be ,someone should write an install manual which includes the codes. :)
 
augie47 said:
Not sure I understand... some days I'm slow..some slower :D
On a delta-wye arrangement you can not rely on the transformer primary protection to protect the secondary conductors. The secondary would need to be protected in accordance with one of the 240.21(C) rules.
IF you have proper primary protection (normally 125%) then your secondary OCP could be based in the conductor requirement only per 240.21(C).
For example, lets say you have a 75 kva 3 phase 480Y/120 arrangement. Your primary protection is say 100 amps.
You can place (3) 200 amp M/B panels on the secondary and protect the 3/0s. Obviously the secondary protection is far greater than the required transformer protection, however, the transformer is protected by the primary device.
Likewise, if you took the primary protection to 225 amps, then your secondary protection would be limited to 225 amps.

That help ?
Click to expand...

Wouldn't the 200 amp install go against 240.4B??? :)
 
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