Homework question about sizing fuses for particular scenarios

[JasonCo]

I have a few homework questions that ask what size fuses I'll need for the scenarios it gives. I'll just spit out one example, and then will try and solve it, am just hoping to get feedback on if I'm doing it right, or how to do it if I'm not!

Scenario: A fused safety switch feeds a 480 to 208Y/120-v, 3phase, 30 KVA transformer. The transformer supplies power to a 3phase 100-amp main breaker panelboard. According to NEC 450.3, the fuses in the safety switch are permitted to be not larger than ___ amps so that the transformer is adequately protected against overload.

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Okay, so I assume I need to find amps. Amps for 3 phase is Power (30,000) / (208 x 1.732) = 83.27 amps

I assume the answer is 90 amp fuse..

Also, looking at table 450.3(B), does this table apply to this question? It is under 1000v and the amps is 83 amps so its over 9 amps (shown in the table), which says 125%... Idk if any of that even applies to this question, just curious! Really appreciate your time and help on this question!

 

[Smart $]

...I assume the answer is 90 amp fuse..

Also, looking at table 450.3(B), does this table apply to this question? It is under 1000v and the amps is 83 amps so its over 9 amps (shown in the table), which says 125%... Idk if any of that even applies to this question, just curious! Really appreciate your time and help on this question!

Does it?

Does it say that is the only size, only option?

 

[don_resqcapt19]

You need to do the primary and secondary transformer current calculations and apply the rules from Table 450.3(B). It appears that they are asking for the maximum permitted primary OCPD rating. It also appears that you have secondary protection.

 

[JasonCo]

This is what I need help on, I don't even know where to get started... :/ I'm pretty lost

 

[Smart $]

This is what I need help on, I don't even know where to get started... :/ I'm pretty lost

OK... let's start with what 450.1 says. That'll tell you whether 450.3(B) applies and subsequently Table 450.3(B) itself.


Now if you are required to use Table 450.3(B), how many protection methods are available?


What is the difference in protection methods?


Is the method used rigid or optional?


Note the title says percentage of transformer-rated current. How do you do that when using Secondary Protection?

 

[Ingenieur]

Calculate the primary and secondary rated currents
(they want you to determine prim protection and you calc'ed the sec current)
determine if you have prim and sec protection
are you required to have both?
does the sec comply with the table?
if so what does it allow on the primary?

 

[Smart $]

JasonCo

So where are you at on this? We are trying to help you understand without actually giving you the answer. It is hard to do otherwise when you don't provide feedback. We can help you understand better when you ask pertinent questions (other than "What is the answer?")