UNDERSIZED OCPD FOR A SWITCHBOARD

[ED-GEE]

I have a situation where there is an existing 800a fused switch feeding a new 1200a switchboard with 3 sets of 4#600kcmil + grd. I cant seem to find the code violation for that, any thoughts?

 

[waross]

On the face of it there is no code violation.
At the 75 degree rating of 420 Amps, properly installed multiple 600 kcmil conductors is adequately protected by the 800 Amp breaker.
If there is a code violation it may be in the loading of the 1200 Amp panel.
If the conductors are properly installed and 800 Amps is sufficient for the demand load on the 1200 Amp panel, as specified by the code there is no violation.
Put another way, if the loads may be served by an 800 Amp panel, you may use a larger capacity panel but not a lessor capacity panel.
Check the code section on demand factors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jmbelectrical]

The following is assuming that the installation is subject to the National Electrical Code. I am also assuming that the conductors in question are copper.

1. On the line side of the 800A disconnect switch, two sets of 500kcmil conductors results in an ampacity of 760A. Using the "next size up" rule in the NEC, 800A fusing is acceptable.

2. On the load side of the 800A disconnect switch, three sets of 600kcmil conductors results in an ampacity of 1,260A - much higher than 800A. There may be a legitimate reason for this. Maybe the conductors were upsized to contend with voltage drop. Maybe the load-side conductors are installed within a high ambient temperature environment and needed to be derated. Maybe it is anticipated that the 800A disconnect switch will eventually be replaced with a 1,200A disconnect switch in response to future, added loads.

3. There are no issues, from a Code perspective, with serving a 1,200A switchboard from a disconnect switch fused at 800A. As long as the rating of the switchboard's bussing equals or exceeds that of the upstream overcurrent protective device, the installation is acceptable. The inverse scenario, serving an 800A switchboard from a disconnect switch fused at 1,200A, is a Code violation.

4. Ultimately, feeder conductors need to be sized to accommodate the load. Generally, their ampacity must equal or exceed the summation of 125% of the continuous load and 100% of noncontinuous load. Demand factors, the presence of motor loads, voltage drop, ambient temperatures, and other issues play a role, as well.

 

[che12345]

Dear Mr. ED-GEE
1. I do not foresee any Code violation for an 800A feeder terminated onto a 1200A board, when the cables are assumed to be adequately protected by the OCPD, upstream of the disconnector.
2. The only constrains are:
a) that the 1200A board is NOT rated to 1200A. The incoming 1200A breaker shall be set [down/lower] to 800A or lower. The [setting shall be sealed] to prevent it from inadvertently been set to the full rating of 1200A.
b) the total load of the 1200A board shall NOT exceed 800A, even it is designed for 1200A and is installed with a 1200A incoming CB.
Che Kuan Yau (Singapore)

 

[waross]

Mr. Che
Your constraint 2. a) may not be required.
This is a call by the AHJ.
We had a similar situation where the feed to a 100 Amp panel was adequately protected at the source and.
A 400 Amp fused disconnect switch was used a a local disconnect switch.
The inspector (AHJ) ruled that switch was acceptable as a safety disconnect as overload and over current protection were provided elsewhere.
The inspector allowed the fuses to be replaced with solid jumpers, again citing that protection was provided elsewhere.
The panel must be considered as an 800 Amp panel for loading and demand calculations.
A similar but distinctly different issues sometimes arises when an available 100 Amp does not have enough breaker spaces.
A 200 Amp panel may be used to gain the additional breaker spaces.
At one time our local code specified the minimum allowable breaker spaces for various ampacity panels.
Situations would arise where a 200 Amp panel would be used in place of a 100 Amp panel (With the main breaker changed to 100 Ampa) to obtain the needed breaker spaces.
However if this is a service entrance panel the main breaker provides overload protection for the 100 Amp feeders.
In this instance the main breaker must be changed out for a 100 Amp breaker.
Note: these solutions were discussed with the AHJ in the planning stage.

Bill
--------------------
"Why not the best?"
Jimmy Carter