Generators & Tap rules

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makalroy

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Does the tap rule apply to Generators?
Heres what I got.
One gen. with a name plate rating of 460 amps
the gen has a 630 amp ocpd.
I am feeding a single 315amp MDP.
What size does my feeder have to be?
all this is outside.
 
makalroy said:
Does the tap rule apply to Generators?
Heres what I got.
One gen. with a name plate rating of 460 amps
the gen has a 630 amp ocpd.
I am feeding a single 315amp MDP.
What size does my feeder have to be?
all this is outside.
Click to expand...
mak -
What are you looking to do here? It sounds like this is a money issue. You want to put in the absolute minimum sized conductors allowed, between the gen 630A CB and the 315A MDP with no consideration given to gen capacity.

If so, here are some comments:

There is no issue with the tap rules applying to a gen. Any connection to the system is after the first CB. The system is treated as any other distribution system. (Exception: You may wish to pay attention to the downstream coordination - generators have notoriously low short circuit currents)

The setting of the gen CB is a design issue - not a code issue. 445.12.A does not specify the size of the Gen CB. Just a guess on my part, 630A is to high. Without additional information, I probably would not go above 125% of the gen FLA, 575A.

The installation is outside, so 240.21.B.5 applies. The feeder to the MPD can be sized for the CB at the MPD end, next size up CB is not allowed.

I've never seen a MPD rated for 315A. Is this the load calculated on the MDP? Does the MDP main CB have an adjustable trip?

Say the MDP main CB is a 400A molded case. And the MDP calculated load is 400A or less. Then you could size the feeder ampacity to 400A or more - say parallel 250kcmil in one raceway

Another scenerio:
The calculated MDP load is 315A. The MDP CB is adjustable and set for 315A. Now you could install parallel 3/0.

This installation is in an area that requires understanding the system and actually doing a design. There is no decent NEC cookbook design available.

Let us know how it goes.

cf
 
Thanks.
Its not a money issue, its limited space issue. There is no way I could get the cables into my enclosures if I had to size them to the 630 amp OCPD.
The actual load is only around 100 amp but I?m working in Iraq and we have to use what?s available. Thanks again
 
makalroy said:
Thanks.
Its not a money issue, its limited space issue. There is no way I could get the cables into my enclosures if I had to size them to the 630 amp OCPD. ...
Click to expand...
That happens. I think you have a handle on it.

makalroy said:
...The actual load is only around 100 amp...
Click to expand...
It the MPD CB is adjustable, turn it down to your calculated load and size the feeder accordingly.

makalroy said:
...but I?m working in Iraq and we have to use what?s available. ...
Click to expand...
Yeah, that happens.

makalroy said:
... Thanks again
Click to expand...
You're welcome

cf
 
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