Sub-Panel Conductor Sizing

[mwh1023]

A quick question on the size of the conductors that feed a sub-panel. I understand the sizing of the conductors is based on the calculated load. The protection of the conductors is per the OCPD that the condcutors attach to and I also understand that the rating of the panel cannot be less than the OCPD. With all of this, does 240.4B (next breaker up) apply to these conductors when you dont know the actual load that the panel serves?

Example: I have a 200 amp panel in a strip mall. No idea what this panel may some day serve since the retail space is empty. I size the conductors off of the 200 amp OPCD at the meter, so 250 kcmil XHHW aluminum is installed, rated at 205 amps since the terminations of the breaker is 75 degrees. My comepetition at another suite installs 4/0 aluminum rated at 180 amps, applies 240.4B, next breaker size up is a 200 amp OPCD.

This seems logical since the panel shouldn't be loaded past 80%, but who really checks the panel loading when there could be a number of EC's (and possibly non-electricians) adding loads to this panel?

Should I assume that you can pretty much always install "the next size wire under" the breaker (commercial applications) except for the exceptions listed in 240.4B?

Mike

 

[charlie b]

Your competitor is cheating. The "next size up" rule does not trump the "ampacity must exceed load" rule. The panel can have 200 amps of load, not just 160. The 200 amps cannot be continuously running, or you would have to calculate it at 125%. But a non-continuous load of 200 amps can legally be operating from the panel. If you do not, at the start of the project, know whether that will actually ever happen, I believe you are obliged to size everything (i.e., conductors and OCPD) to allow for a 200 amp load.

 

[Dennis Alwon]

If the calculated load is less than 180 amps then it is perfectly legit. I don't know how you can install a 200 amp panel and not know what the calculated load is.

I did a retail store and I gave the customer the option. The calculated load was 105 amps so she opted for 4/0.

 

[Npstewart]

If it is for a FUTURE retail space though, most likely the calculated load is just a few lights, emergency lighting, and a few general receptacles.

In my opinion, spaces built out for future offices/retail should be designed for the full capacity across the board.

---

Many times our company does site visits for empty spaces. If we go to the meter center and there is a 200A breaker, and we go to the panel and we see it is rated for 200A, it is assumed that the wire in between is a 3/0 (if it is copper).

You have to look at it this way, the demand on FUTURE office/retail spaces is the absolute MINIMUM that the space will ever see. So sizing the conductors at the calculated load is bad practice. All they have to do is add one copy machine and a few computers and they are over the calculated load, it would be impractical to pull a new feeder and at that point you may even have to replace the conduit.

If your going to only install conductors rated for the calculated load in a future retail/office space and not sized per the OCPD, you might as well just install the conduit only w/ a pull string and let them pull their own feeder.

 

[Dennis Alwon]

I use the next size up rule but I am not sure I like it. It is convenient but most EC don't do load calculations-- esp. in residential work. I probably have done less then a dozen.

 

[infinity]

If the calculated load is less than 180 amps then it is perfectly legit.

I agree, 180 amps or less, that's all that's required.