500 kva secondary protection (240.4?)

[xguard]

I have a 500 kva 3 phase transformer, 480:208/120. I know I can't use the "next size up rule" for the secondary conductors but can I for secondary protection? It looks like 240.4(B) is specifically about over current protection for conductors, not transformers.

(500,000/(sqrt3*208))*1.25 = 1,735 amps making the next standard size up 1,800 amps. I'm currently calling for one set of secondary conductors to a 1200 amp fused switch and another to a 600 amp fused switch. I'd like to keep it this way if I can.

Additional information:
1200 amp secondary conductors are 4-400 kcmil per phase
600 amp secondary conductors are 2-400 kcmil per phase.

 

[jim dungar]

Conductor protection is determined regardless if they are connected to a transformer.
The location of the protective device is determined by 240.21(C) if they are transformer secondary conductors.
Transformer primary protection may be increased, per 450.3, if the secondary conductor protection is not too large.

Effectively, start your design at the load then work towards the source.

 

[electrofelon]

I have a 500 kva 3 phase transformer, 480:208/120. I know I can't use the "next size up rule" for the secondary conductors but can I for secondary protection?

Yes, see note #1 and #2 to table 450.3(B)

 

[augie47]

Tying this in with you earlier thread on transformer primary protection, you have primary protection selected at 125% so, as far as tgrabnsformer is concerned, the only limit on secondary protetcion is a limit of six (see electrofelon's post) of any size desired. Once selected, size the conductors per 240.21(C) {Your choice is fine}

 

[xguard]

Tying this in with you earlier thread on transformer primary protection, you have primary protection selected at 125% so, as far as tgrabnsformer is concerned, the only limit on secondary protetcion is a limit of six (see electrofelon's post) of any size desired. Once selected, size the conductors per 240.21(C) {Your choice is fine}

I went with a 1200 amp switch on the primary due to potential in-rush. Several sources used the 12 x primary current at 0.1s and 25 at 0.01s.

 

[augie47]

Then, according to 450.3 & Note 1 & Note 2, you are still good at a total of 1800 amps.

 

[jim dungar]

Several sources used the 12 x primary current at 0.1s and 25 at 0.01s.

Those are typical points for choosing protection for transformers with greater than 1000V primaries.
I don't think I ever used them for 480V applications. Although I did usually show the .01s point on my TCCs, I included a disclaimer if it appeared to be a problem.

 

[xguard]

Those are typical points for choosing protection for transformers with greater than 1000V primaries.
I don't think I ever used them for 480V applications. Although I did usually show the .01s point on my TCCs, I included a disclaimer if it appeared to be a problem.

ETAP's default is 8x's primary current at 0.1s. I'm not sure what they base that on. Does that sound familiar?

 

[jim dungar]

ETAP's default is 8x's primary current at 0.1s. I'm not sure what they base that on. Does that sound familiar?

Yes.
But as I recall it is shown primarily because 'grandpa did it that way'.
Many boilerplate specs require this reference point, but never tell you what to do with it.