Load calculations questions

[Dengineer]

Hello all,

I am working on some service load summary calculations for a mixed use building and had a few questions.

I currently have 3-phase 4 wire incoming service that will feeder a 3phase meter bank and single phase downstream residential panels.

Now when sizing a service using the service load calculations are you just sizing the incoming service conductors or do you also size the service tap conductors. Lets say my service calculations for a meter bank and house panel are 260A and the commercial space is 95A. Now I want to remove the house panel from the meter bank and do a direct taps on a wire trough to the service conductors. Is the meter bank tapped at 300A or can I downsize to 250A because I removed the house load and will do a separate service tap. Note I am running the service conductors into a wire trough and service tapping for each space with no dedicated upstream disconnect, so everything would be a service tap instead of a feeder tap.

 

[wwhitney]

I didn't unpack your description of your configuration. But maybe the following example will address your questions:

Say one set of service conductors supplies two sets of service entrance conductors (SECs). The load calculation for SEC set 1 is 50A. The load calculation for SEC set 2 is 250A. And the load calculation for the service conductors supplying them both is 290A (can be smaller than 50 + 250 due to demand factors that get smaller with larger connected load).

Then the service conductors need an ampacity of at least 290A, SEC set 1 needs an ampacity of at least 50A, and SEC set 2 needs an ampacity of at least 250A. That assumes a 50A overcurrent device protecting SEC set 1, and a 250A overcurrent device protecting SEC set 2, and no continuous loads. If for whatever reason SEC set 1 is actually protected by a 100A overcurrent device, then you'd need to bump up the ampacity of SEC set 1 to at least 91A (enough to satisfy 240.4(B)).

Cheers, Wayne

 

[Dengineer]

I think I understand. So in my situation my the service conductors are sized at 400A, which accounts for all the loads. The SECs are the taps on my main service conductors and those are sized per the load on the individual units. Meter bank at 250A, house panel at 50A and commercial panel at 125A. These taps are the SECs correct?

 

[wwhitney]

I think I understand. So in my situation my the service conductors are sized at 400A, which accounts for all the loads. The SECs are the taps on my main service conductors and those are sized per the load on the individual units. Meter bank at 250A, house panel at 50A and commercial panel at 125A. These taps are the SECs correct?

Taps in the colloquial sense, not be confused with 240.21(B) feeder taps.

But yes such taps or spurs would definitely be SECs. Some or all of the 400A conductors may be SECs, but that distinction is irrelevant to your question. What matters is the distinction between "service conductors carrying all of the load on the service" and "service conductors carrying only part of the load on the service."

Each service conductor is required to have an ampacity of at least the load it carries, and further if it is protected by a single overcurrent device, of sufficient ampacity to be effectively protected by that overcurrent device in accordance with 240.4. Service conductors not protected by a single overcurrent device are in effect only protected by the load calculation, see 230.90 including the exceptions.

Cheers, Wayne

 

[Dengineer]

Taps in the colloquial sense, not be confused with 240.21(B) feeder taps.

But yes such taps or spurs would definitely be SECs. Some or all of the 400A conductors may be SECs, but that distinction is irrelevant to your question. What matters is the distinction between "service conductors carrying all of the load on the service" and "service conductors carrying only part of the load on the service."

Each service conductor is required to have an ampacity of at least the load it carries, and further if it is protected by a single overcurrent device, of sufficient ampacity to be effectively protected by that overcurrent device in accordance with 240.4. Service conductors not protected by a single overcurrent device are in effect only protected by the load calculation, see 230.90 including the exceptions.

Cheers, Wayne

this is exactly what I was trying to understand. Thank you!