Power Distribution Design

[PhaseShift]

I'm guessing you are talking about the transformer primary cb and primary conductors. If not, none of the applies.

One usually sizes the CB feeding a transformer at least 125% nameplate current so the CB doesn't trip on inrush. Look at T450.3.B. Keep in mind that 450 addresses transformer protection not conductor protection.

For sizing the primary conductors, look at 240.4 and 240.4.B. They have to be protected by the CB you picked in 450. So the conductor ampacity is going to be 125% nameplate current. You can oversize the conductors all you want

Nothing stopping you from sizing the primary CB less than 125%, but it looks pretty bad if the CB trips on inrush

cf

CF

Thanks for the information. Like I mentioned this transfromer will be located aprox 1000ft away from the MCC which it is fed, and the MCC itsef will have a breaker which the feeders come of of.

Is the breaker located on the MCC adequate for providing the primary protection of the transformer, if it is sized at 125% of transformer primary rated current? In other words, is there a distance limitation to how far the transformer primary breaker can be from the transformer.

I have seen many applications where there was a feeder breaker on the MCC and then another breaker used out at the transformer primary location which was rated for the transformer primary protection. In this case, the breaker at the MCC would just be for feeder protection i imagine? Are there any distance limitations to where the breaker can be in relation to the transformer primary? Is it custom to have a feeder breaker and then another primary breaker at the traansformer when dealing with long distances?

This is why I asked my question regarding voltage drop and breaker sizing, in the event that the primary breaekr was located a far distance from the transformer primary.

 

[Cold Fusion]

You are kind of all over the place with this - which isn't necessarily bad, just have to keep them separate.

Separate the code issues from design issues. There five issues that come to mind:
Transformer inrush - design issue
xfm ocpd - code issue
xfm primary feeded protection - code issue.
Voltage drop - design issue
Outside feeder - code issue

also, a couple of things you have not asked about - but has been mentioned by others: xfm secondary ocp, xfm secondary conductor ocp.

--- Is the breaker located on the MCC adequate for providing the primary protection of the transformer, if it is sized at 125% of transformer primary rated current?

Transformer inrush: At least 125% to prevent inrush tripping - design issue. Can not exceed 250% - code issue. Yes, the MCC CB will be a fine xfm primary ocpd.

Conductor protection: The first ocpd has to protect the conductors. - code issue.

There are also code issues about having a disconnect at a separate structure. (225)

---In other words, is there a distance limitation to how far the transformer primary breaker can be from the transformer.---

None I know of

---I have seen many applications where there was a feeder breaker on the MCC and then another breaker used out at the transformer primary location which was rated for the transformer primary protection. In this case, the breaker at the MCC would just be for feeder protection i imagine? ---

---Is it custom to have a feeder breaker and then another primary breaker at the traansformer when dealing with long distances?---

Assuming only one circuit out at the end, there isn't any thing the second CB is doing for the transformer that the first one isn't already doing - code issue.

You may want a second disconnect out at the xfm for convenience of maintenance purposes - design issue.

You may have to have a disconnect at the outside structure - code issue.

Xfm seconday ocp: Depending on what you pick for the primary ocp you may (or may not) have to have a seconday ocp.

Xfm secondary conductor opc. No matter how you pick the xfm primary ocp you will still need secondary conductor ocp.

My response sounds really disjointed. And I likely missed something or somethings. However, there are lots here that will hopefullly chime in on what I missed.

Iteration: About the loads and xfm sizing - go look at the nameplates, talk to the users - don't guess.

cf