Transformer Conductor Size

[blues]

As allowed per Nec 450.3(B) tranformer primary OCPD may be set at 250% of primary current rating provided the secondary is not set no more that 125% of FLC. My question is do the primary conductors need to be sized per the OCPD (250%) and not the FLC of transformer primary plus 125% as I would normally do?

Thanks,
Dan

 

[charlie b]

Table 450.3(B) is all about protecting the transformer itself. Protection of the conductors leading to, and leading away, from the transformer is a different thing entirely. Whatever OCPD you select to protect the transformer's primary windings must also protect the primary feeder conductors. So you do select a primary feeder that is large enough to be protected by the primary (i.e, up to 250%) OCPD.

 

[mull982]

Table 450.3(B) is all about protecting the transformer itself. Protection of the conductors leading to, and leading away, from the transformer is a different thing entirely. Whatever OCPD you select to protect the transformer's primary windings must also protect the primary feeder conductors. So you do select a primary feeder that is large enough to be protected by the primary (i.e, up to 250%) OCPD.

Can we use the "Next Standard Size Up" rule here for the breaker size in relation to the cable ampacity as long as we are under 800A?

 

[david luchini]

Can we use the "Next Standard Size Up" rule here for the breaker size in relation to the cable ampacity as long as we are under 800A?

Yes, the "next standard size up" rule for the breaker size in relation to the primary cable ampacity for 800A or under.

However, the "next standard size up" rule does not apply to the secondary conductors.

 

[mull982]

However, the "next standard size up" rule does not apply to the secondary conductors.

Really? Why is that? Is that becasue they are considered tap conductors in some cases?

 

[jumper]

Wait a minute, I am confused. For this situation.
T450.3(B) says primary cannot use the "next size rule", only the secondary can.

 

[don_resqcapt19]

Wait a minute, I am confused. For this situation.
T450.3(B) says primary cannot use the "next size rule", only the secondary can.

The rules in Article 450 are for the protection of the transformer itself. The question in this thread is about the protection of the primary conductors and those rules are found in Article 240. Both sets of rules have to be complied with.

 

[david luchini]

Really? Why is that? Is that becasue they are considered tap conductors in some cases?

Its not because they are considered tap conductors, its because they are "transformer secondary conductors" and must be protected in accordance with 240.21(C). 240.21(C) generally requires that the ampacity of the secondary conductors is "not less than the rating of the OCPD" at their termination, or that they terminate at on OCPD that "limits the load to the ampacity of the conductors."