310.15(B)(6) Flaw

[A/A Fuel GTX]

Ok, I have a dwelling that has meter and a 200A, 8 space feed through load center on the outside. From the load center, a 100A main power feeder ( # 4's ) is going to a panel on the inside of the dwelling. All is well in complying with 310.15(B)(6). Now, the customer wants to add another sub panel from the exterior loadcenter to feed some loads in the attached garage. Will the additional subpanel put the original main power panel in violation as far as wire size for the feeder even though the feeders for the new panel be sized from 310.16?

 

[jbelectric777]

Will the additional subpanel put the original main power panel in violation as far as wire size for the feeder even though the feeders for the new panel be sized from 310.16?
Why would the original be in violation ? Unless your adding load ? If it was existing then the service was sized already, now if your adding load do a load calculation.

 

[raider1]

If you are feeding 2 panels with 2 separate feeders from the service equipment, then by the definition of "main power feeder" from the 2008 NEC both feeders would need to be sized based on 310.16 and not 310.15(B)(6).

Chris

 

[Dennis Alwon]

I agree with Chris. The #4 is no longer a valid size however if the load allows you may drop the breaker to the appropriate size. If it is RX then you need a 70 amp breaker. If you have conduit then you can use 75C or 90C wire and use a 90 amp breaker. The 90 can only be used if the calculated load of the panel is 85 amps or less.

 

[acrwc10]

Ok, I have a dwelling that has meter and a 200A, 8 space feed through load center on the outside. From the load center, a 100A main power feeder ( # 4's ) is going to a panel on the inside of the dwelling. All is well in complying with 310.15(B)(6). Now, the customer wants to add another sub panel from the exterior loadcenter to feed some loads in the attached garage. Will the additional subpanel put the original main power panel in violation as far as wire size for the feeder even though the feeders for the new panel be sized from 310.16?

You are right on with how you have read it. Like Dennis said, do a load calc. Size the breaker to fit the conductor or size the conductor to fit the breaker. Most customers are going to want the full 100 amps if that is what they asked for, load calc. or not, because they will feel that is what they are paying for. I have done many jobs where we put a 100 amp or more feeder and distribution panel and the load calcs would have allowed 35 amp feeders, but the plan says 100 amp.

 

[A/A Fuel GTX]

I agree with you guys but as always, I'm trying to understand the logic as to why adding a new feeder makes the original feeder suddenly invalid without modification:-? I know the section is written that way but I just don't see the reason.

 

[iwire]

In your example I am sure it would work fine but the code is written to cover the worst case situation. 310.15(B)(6) counts on the load diversity of an entire home to ensure the conductors are not overloaded. Once you start removing that diversity you could have an issue.

As an extreme example imagine a 400 amp service supplying two 200 amp panels and the electrician loads one panel up completely with electric heating loads and the other panel has all the lighting and receptacle circuits in it.

There would be no diversity on the feeder supplying the electric heat panel.

 

[Dennis Alwon]

There would be no diversity on the feeder supplying the electric heat panel.

This is exactly what T. 310.15(B)(6) is all about-- diversity. The table takes into account the entire load of the dwelling on the one main feeder. As soon as you change that then the diversity that the Table is based on is changed and everything goes out the window.

Unfortunately there are many cases where the diversity is really not affected, as in the case where an a/c unit is taken from the main panel and then a feeder goes to the interior sub panel. There is no reason that the sub panel would not work fine based on the diversity with a feeder from Table 310.15(B)(6). The problem is trying to write a rule that would take in all the situations.