Feeding Sub Panel in Same Building

[Design_tech]

I am designing a building with a 400A main service with a 100A subpanel. The subpanel will be located about 50' from the main panel. The main service is a 4-wire 3 phase. How do I show the feed to the subpanel on the riser diagram? I'm thinking the wire size would be #2 AL, but I didn't know what type of cable and how many conductors. I thought it only needed to be 3-wire since it's inside the same building. Any answers?

 

[iwire]

May I suggest having an engineer do it?

 

[wbalsam1]

I am designing a building with a 400A main service with a 100A subpanel. The subpanel will be located about 50' from the main panel. The main service is a 4-wire 3 phase. How do I show the feed to the subpanel on the riser diagram? I'm thinking the wire size would be #2 AL, but I didn't know what type of cable and how many conductors. I thought it only needed to be 3-wire since it's inside the same building. Any answers?

My answer will agree with iwire's. There is an apparent lack of understanding of the fundamental functions behind the equipment grounding conductor and the grounded conductor and their need to be isolated from one another downstream of the service disconnecting means that declares itself loudly in your question. It's only for this reason that I respond in this way. Please refer to an electrical design professional. Thanks. And welcome to the forum. :smile:

 

[LarryFine]

I thought it only needed to be 3-wire since it's inside the same building. Any answers?

Actually, being inside the same building is specifically not within the exception that used to permit omission of a separate EGC; it was for detached buildings.

As for 3-wire vs 4-wire, that debate is for 1-ph feeders; yours would be 4-wire vs. 5-wire if your sub-panel is 3-ph, unless your sub-panel has no neutral load.

In any case, #2 al is not large enough for a sub-panel feeder; see T 310.16.