ECN Forum Forums Code Related Discussion NEC & other Code issues The sum of the ratings of the circuit breakers shall be permitted to exceed the ampac

VII. Service Equipment — Overcurrent Protection

230.90 Where Required.



Can someone explain the wisdom of this?

If this is applied you are leaving it up to the next person to do load calculations for each and every addition to the service.

For all we talk about taps and proper OCP this seems to go against the general requirements.

Bob

I don't think most contractors are aware of this exception. Those that are, when they have a set of Service Entrance conductors that are being tapped they will either do the calculation or call me and ask if they can do it. My answer-Do the Calculation submit the plans. If they can't do the calculation they shouldn't be doing the job. I'm mean. Residential is not a scary as industrial or commercial because of the connected load v. demand load. Residential load is not continuous either. Sometimes your actual load on the panel is very small compared to the number of separate branch circuits connected.

Diversity of loads allows this with little risk of OC damage. As far as the next guy overloading the system, the next guy is required to pull a permit in most states. He would then also have to prove to the AHJ (and the power company) that the service would not be overloaded.

Earl

Bob ;,,

I did heard that one pretty often and most commercal and industrail building do use the load demand caluation, but unforealy this is very common pratice not only in usa,, this been done in europe too. the French electrical codes do allow 4 ocpd on main devices very simuair what the NEC 230.90 is saying there and the French electrical code did say real clear to make sure you get the load demand for any upgrading .

merci, marc

[This message has been edited by frenchelectrican (edited 05-30-2004).]

Bob
Because of this exception, we see the 'ampacity' number from the individual breakers add up to more than the rating of the conductor ampacity that usually feed the panelboard they are installed on.
Again it is the actual or calculated load that is important.
In some residential 200 amp panels, you may see 60 or seventy breakers that add up to 700 amps or more. The actual load may only be 100 or so amps.

And yes you mention that additional installations are added all of the time without regard for the load. In my experience the average 'installer' does not know how to figure this out and will either ignore it or tell people to increase the service size, even if it is unnecessary.
The typical statement "I do this all the time".

Pierre

That is my problem with this, I do not know many electricians that do, or would do the calculations.


Of course, but there is a main in the panel that will protect the service conductors if to to much load is added to this panel.

Don brought up this exception when I asked about a commercial service that was using 6 breakers that added up to about 3000 amps as the service disconnect.

My problem with this was the MLO switch gear these 6 breakers where installed in was only rated 1600 amps. The next OCPD up stream would be the high voltage cut outs feeding the transformer suppling this board.

I just find this odd.

Bob

Surely seems odd but we do it all the time. On a three family dwelling with a 200 amp service I would put three 70 amp panels and one 30 amp landlord panel. This would allow you to draw 240 amps at full load. Will this ever happen? No. Does any one do load calculations for this. Nobody I know. My load calculation for this would be to ask the owner if the main on the existing service (usally 60 or 100 amp) ever blows or trips out.

Anytime I see this exception used in the feild, I immediatley ask for calculations from the installer. If the calculations jibe, there is nothing I can do, but if they don't, well, they get a main breaker.

I write this up so often that I have the code number, right down to exception 3, memorized.

You may be interested to see what I have written to train our electric utility power engineers. This is not for use outside of my industry!

"In general, any way you use to estimate load is fine if it has basis in fact. For commercial buildings, a reasonable rule is to use a demand factor of 50% for all connected load. Another rule, although somewhat shaky, is to take 80% of the switch size and multiply the resultant by 50%."

Generally speaking, the NEC is very conservative when it comes to sizing service equipment, service entrance conductors, and feeders. I do not have a problem at all with that section. However, you still have to do the calculations. It's sorta like cleaning the ice off the windshield before driving. If you clean a little hole, you can see a little bit and guess whether you are right or not. If you clean the whole thing, you can be sure. UH OH - sorry, the guys from the South won't understand the analogy

Well thanks for the response everyone, I have a follow up question.

I go to a house that has a 100 amp service that is in excellent shape, the load is OK but I do not have enough spaces for all the circuits.

Example 1) Why can't I just change the panel to a 200 amp 42 circuit panel leaving the 100 amp service conductors?


Example 2) By using 230.90(A)(1)Exception No. 3 I could leave the existing 100 amp panel and add one right beside it.

I do not understand the logic here?

How is example 2 any safer than example 1

Thanks, Bob