metering 6 gang w/200 amp main breaker each

[kentaylor04]

have a 6 gang meter socket total calculated demand on 6 apt.s 238 amps. feeder sized at 500mcm copper. this sound right?

 

[bphgravity]

Re: metering 6 gang w/200 amp main breaker each

500 kcmil is good for 380-amperes at 75? C. So, at 238-amperes of load, you should be just fine.

 

[jtester]

Re: metering 6 gang w/200 amp main breaker each

Remember, you can probably use Table 310.15(B)(6) to size service conductors and feeders if you choose to reduce your service size. Per that table you could use 4/0 copper.
Jim T

 

[iwire]

Re: metering 6 gang w/200 amp main breaker each

Originally posted by jtester:
Remember, you can probably use Table 310.15(B)(6) to size service conductors and feeders if you choose to reduce your service size. Per that table you could use 4/0 copper.
Jim T

Jim how do we know that, there is no 250 amp OCP?

This is a real question, I am not busting your chops.

This is exactly why I started a thread called Table 310.15(B)(6)

I would say we would have to use 250 Kcmil copper per 310.16 with it's rating of 255 amps to cover the 238 amps of connected load along with Exception 3 of 230.90.

Still much cheaper than 500 kcmil copper.

I am left with more questions about Table 310.15(B)(6) than I started with.

Bob

 

[steve66]

Re: metering 6 gang w/200 amp main breaker each

Iwire:

I don't know if you have the handbook, but it has almost an entire page devoted to this table. But most of it seems to deal with reductions in the neutral conductor size. So the handbook may just leave you with more questions than answers.

It does say

Section 230.42 requires service conductors to be of sufficient size to carry the load calculated in accordance with article 220.

But I'm still not sure if that answers your question. I think the reason this reduction in size is allowed is because the POCO will size the service conductors even smaller.

Steve

 

[jtester]

Re: metering 6 gang w/200 amp main breaker each

quote by iwire
Jim how do we know that, there is no 250 amp OCP?
This is a real question, I am not busting your chops.
Bob I read your other post a couple of times, and am still unsure exactly what you mean. I read Article 310.15 as Ampacities of Conductors. This determines the allowable amount of current that can be passed thru a conductor. Other sections cover OCPD's.

I believe that Table 310.15(B)(6) increases the permissible capacity of the wire determined under certain circumstances. Table 310.16 provides ampacities determined under more general circumstances.
Since Table 310.15(B)(6) allows 250 amps on 4/0, I believe I can use it with the six disconnects, because the 4/0 is installed per 230.90 Exception 3. Clearly it isn't adequately protected, but perhaps the calculations leading up to this point have built in sufficient cushion to avoid a problem.

I'm interested in your thoughts.
Jim T

 

[iwire]

Re: metering 6 gang w/200 amp main breaker each

Jim I appreciate your time here.

Originally posted by jtester:
I believe that Table 310.15(B)(6) increases the permissible capacity of the wire determined under certain circumstances.

That is the heart of my question, a conductor is a conductor in a house or a office building.

Obviously the conductor properties do not change so how can the allowable ampacity change?

What I see is that the NEC is counting on the actual load to be less than the 310.16 capacity of the conductors by virtue of the conservative calculations of Article 220.

So in essence they have not changed the ampacity of the conductors they just admitted the calculated load per 220 is unnecessarily high.

So my simplified question is this.

With a total calculated load of 400 amps should I be able to use 400 Kcmil copper?

I fully understand 'it works' however this is often what is said in field by electricians who are ignoring derating or other conductor rating requirements

IMO Table 310.15(B)(6) gives these violators some 'back up' to justify their violation.

IMO a better way to handle this issue would be revisions to Article 220 that bring the calculated load closer to the actual load. Then get ride of Table 310.15(B)(6) altogether.

Thanks all, Bob

 

[bphgravity]

Re: metering 6 gang w/200 amp main breaker each

Originally posted by iwire:

IMO a better way to handle this issue would be revisions to Article 220 that bring the calculated load closer to the actual load. Then get ride of Table 310.15(B)(6) altogether.

Thanks all, Bob

I completely agree, hence my proposal to eliminate standard and optional calculations for dwellings, replacing them with a simplified minimum calculation that is more accurate and more comparable to actual loads experienced in typical dwellings.

 

[jtester]

Re: metering 6 gang w/200 amp main breaker each

quote by Bob
That is the heart of my question, a conductor is a conductor in a house or a office building.
Obviously the conductor properties do not change so how can the allowable ampacity change?

The ampacity as determined by some set of standard calculations changed. The physical current carrying capacity didn't. Most recognize that an apartment will not draw the power that the calculations in Article 220 require you to provide for. So there is nothing wrong with using a different set of conductor ampacities for apartment services than those required by a different set of calculations.
The more I type, the more convoluted it sounds, but since we're not dealing with real amps, we're dealing with calculated ones, what is wrong with dealing with adjusted ampacities.
Jim T

 

[iwire]

Re: metering 6 gang w/200 amp main breaker each

Originally posted by jtester:
The more I type, the more convoluted it sounds,

Thanks for your candor.

That is kind of my point.

Why not make 220 more realistic and forget about the table. It seems the NEC took a long and winding road that could be shortened by some 220 changes.

Thanks Again, Bob