Table 310.15(B)(6) and generators

[mivey]

It is my opinion they chose the words associated with the dwelling unit to make clear it did not mean only the electrically connected loads.

You don't see the potential for zero load diversity when a generator is only set up to supply a cherry picked group of loads?

Exactly. As an illustration, just use the case where the generator serves only one load in the house: where is the diversity then?

 

[iwire]

Exactly. As an illustration, just use the case where the generator serves only one load in the house: where is the diversity then?

None and in that case full size conductors are required.

But you lost me as you say 'exactly' like it proves your point when I see it as proving my point.:?

 

[mivey]

None and in that case full size conductors are required.

But you lost me as you say 'exactly' like it proves your point when I see it as proving my point.:?

That's because I was agreeing with you. Are you on the defensive today?

 

[Smart $]

Exactly. As an illustration, just use the case where the generator serves only one load in the house: where is the diversity then?

Diversity applies to any and all feeders, not just services. And 310.15(B)(7) is not limited to just service conductors. To determine whether the requirement of 702.4(B)(2)(b) is met, you must do a load calculation of the unshed loads per Article 220.

 

[mivey]

Diversity applies to any and all feeders, not just services. And 310.15(B)(7) is not limited to just service conductors. To determine whether the requirement of 702.4(B)(2)(b) is met, you must do a load calculation of the unshed loads per Article 220.

How does that validate using 310.15(B)(7)? Apply that to two 10 ton AC units on the generator as that should be right around 100 amps running load.

 

[Smart $]

How does that validate using 310.15(B)(7)? Apply that to two 10 ton AC units on the generator as that should be right around 100 amps running load.

I've already explained all that. And just because I've been assuming the role of devil's advocate here doesn't mean I wish to keep repeating myself. :happysad:

 

[jazer]

I am late to the party/argument/debate on this one, but I think the key to this dilemna is "the main power feeder shall be the feeder between the main disco and the panelboard that supplies, either by branch circuits or by feeders, or both, ALL LOADS that are part or associated with the dwelling unit." Hence, if you are installing a transfer switch before the main (meter feeds normal side and gen feeds emergency side), then 310.15(B)(7) is acceptable due to the diversity of the dwelling. However, if you install a transfer switch after the main, fed by a feeder from the utility on the normal and the generator on the emergency, to pick up critical loads/equipment, then 310.15(B)(16) is to be used due to the LACK of diversity.

 

[mivey]

I've already explained all that. And just because I've been assuming the role of devil's advocate here doesn't mean I wish to keep repeating myself. :happysad:

My example should have made it obvious that the extra diversity that the code panel allowed for all associated house loads combined is not going to work for a few select loads and that does not hinge on Article 220.

 

[Smart $]

My example should have made it obvious that the extra diversity that the code panel allowed for all associated house loads combined is not going to work for a few select loads and that does not hinge on Article 220.

Diversity applies to all feeders the same as service conductors, does it not?

Is there a difference in allowed diversity when 310.15(B)(7) applies and not?

Are the loads typically shed those to which the greatest diversity is applied?

What diversity is permitted for two 10 ton AC units? ...and when would these likely be the only "unshed" loads for a dwelling unit without utility power?

The only difference between 310.15(B)(7) applying and not is the load being all associated dwelling unit loads and not. When a whole house, service ATS is used, all associated dwelling unit loads at the time of generator use are required to be sufficiently powered. Though not specifically called out in the requirement, that determination would be by way of an Article 220 calculation of those loads. So where is the diversity any different?

 

[mivey]

Diversity applies to all feeders the same as service conductors, does it not?

It does but series diversity is different. The table recognizes that there is an additional diversity that will allow us to use a smaller conductor.

Is there a difference in allowed diversity when 310.15(B)(7) applies and not?

Well of course. The table itself has an additional diversity allowance built in.

Are the loads typically shed those to which the greatest diversity is applied?

Could be.

What diversity is permitted for two 10 ton AC units?

I'm pretty sure it is zero.

...and when would these likely be the only "unshed" loads for a dwelling unit without utility power?

Very likely if there is a backup unit just for the ACs

The only difference between 310.15(B)(7) applying and not is the load being all associated dwelling unit loads and not. When a whole house, service ATS is used, all associated dwelling unit loads at the time of generator use are required to be sufficiently powered. Though not specifically called out in the requirement, that determination would be by way of an Article 220 calculation of those loads. So where is the diversity any different?

The diversity is different because the CMP evidently recognizes that a residential home and all of its associated loads together have a differnent diversity than a cherry-picked sub-set of loads from that home.

 

[Smart $]

It does but series diversity is different. The table recognizes that there is an additional diversity that will allow us to use a smaller conductor.

Well of course. The table itself has an additional diversity allowance built in.

Could be.

I'm pretty sure it is zero.

Very likely if there is a backup unit just for the ACs

The diversity is different because the CMP evidently recognizes that a residential home and all of its associated loads together have a differnent diversity than a cherry-picked sub-set of loads from that home.

I generally respect your posts, but as far as I'm concerned you are pulling this one out of your a$$.

 

[AllCircuitsElectrical]

are you all talking about 2008 >>>> 702.5 not 702.4 ??? ....LOL

and so 702.5 (B) (2) (b) covers everything ....IMHO .

and in the ATS the "disconnnect" is just that a disconnect NOT over current protection ??



so based on that the senerio with 400 amp service and 2 200 amp panel being feed individually from 2 seperate service cables having the 1 200 amp panel connected to the Nexus switch IMHO opinion would be perfectly legal.

 

[AllCircuitsElectrical]

445.13 for the the wire ...

83 x 115% = 95.45

#3 copper THWN

 

[mivey]

On an ATS system, a load calculation would have to be performed using just the cherry picked group. Load diversity would apply only to those loads.

The problem is that when you state that and think that the diversity is being calculated, you ignore the fact that the NEC 2011 Table 310.15(B)(7) is an additional diversity allowance for the single-phase dwelling when all associated loads are taken together.

As I tried to illustrate, the group of loads on the generator do necessarily have things like general lighting loads, small appliance loads, misc other loads, etc. that give enough slop in the numbers that would make it ok to have an extra diversity factor as given by the table being discussed.

I generally respect your posts, but ...

Perhaps you should try pulling some numbers of your own and you would see that the table is just not applicable to a cherry-picked set of loads. You can very easily get a load sub-set having currents that exceed the conductor allowable ampacity. It will be easier if you don't let your head keep getting in the way.

 

[Smart $]

The problem is that when you state that and think that the diversity is being calculated, you ignore the fact that the NEC 2011 Table 310.15(B)(7) is an additional diversity allowance for the single-phase dwelling when all associated loads are taken together.

Yes, but the Table considers only total load, and not what those loads are. I'll agree it likely assumes at the very least the required loads, such as general lighting, but beyond that, the Table cannot take into account whether there is say a laundry circuit, or ten freezers.

As I tried to illustrate, the group of loads on the generator do [not?] necessarily have things like general lighting loads, small appliance loads, misc other loads, etc. that give enough slop in the numbers that would make it ok to have an extra diversity factor as given by the table being discussed.

What you stated seems to agree with what I'm saying, unless you inadvertently forgot the "not". More often than not (IMO), loads that will be shed will be those not required and necessary... the remainder are ones mostly considered in the application of diversity.

Perhaps you should try pulling some numbers of your own and you would see that the table is just not applicable to a cherry-picked set of loads. You can very easily get a load sub-set having currents that exceed the conductor allowable ampacity. It will be easier if you don't let your head keep getting in the way.

You can do this for many dwelling unit loads in their entirety. The load calculation will exceed the allowable ampacities of Table 310.15(B)(16) yet not exceed the values permitted under the Table 310.15(B)(7) for the same size wire. That doesn't prove anything.

 

[mivey]

Yes, but the Table considers only total load, and not what those loads are. I'll agree it likely assumes at the very least the required loads, such as general lighting, but beyond that, the Table cannot take into account whether there is say a laundry circuit, or ten freezers.

True enough. The table just assumes a minimal slop for a single-phase dwelling.

What you stated seems to agree with what I'm saying, unless you inadvertently forgot the "not".

I did. Thanks for catching the typo.

More often than not (IMO), loads that will be shed will be those not required and necessary... the remainder are ones mostly considered in the application of diversity.

Outside of freezers/refrigerators, wouldn't you think people would tend to put loads on the generator that would be in use more than the average load of the residence as a whole?

You can do this for many dwelling unit loads in their entirety. The load calculation will exceed the allowable ampacities of Table 310.15(B)(16) yet not exceed the values permitted under the Table 310.15(B)(7) for the same size wire. That doesn't prove anything.

It proves that the code panels think there is enough slop in the calculations as a whole to allow for more diversity. It should be pretty clear that the calculations for a residence as a whole tend to have more slop than the calculations for a select sub-set of loads.

 

[Smart $]

True enough. The table just assumes a minimal slop for a single-phase dwelling.

I agree.

Outside of freezers/refrigerators, wouldn't you think people would tend to put loads on the generator that would be in use more than the average load of the residence as a whole?

Use by necessity, yes. Used more than the average load, no. Loads likely to be shed first would be the more extraneous.

It proves that the code panels think there is enough slop in the calculations as a whole to allow for more diversity. It should be pretty clear that the calculations for a residence as a whole tend to have more slop than the calculations for a select sub-set of loads.

Agree with former, not with the latter sentence, the latter being a major leap in the assumption category.

 

[kwired]

Outside of freezers/refrigerators, wouldn't you think people would tend to put loads on the generator that would be in use more than the average load of the residence as a whole?

That is kind of my thoughts.

I do not have an ATS, I do have a MTS and a portable generator rated 8000 W.

When I have needed to run generator, I of course must shed certain loads because the generator is not large enough to run them all. This is even more true if the power has been off a while before connecting the generator. Things like heating, water heating, refrigerators and freezers all are going to be in immediate demand if power has been off. If transfer would take place immediately after power loss the diversity of some of those loads would likely be a lower demand. We do find ourselves turning off things that are not necessary, even if they are not really that heavy of a load, they do make a difference when the source has more limitation. I do have a hot tub that has a diversity on the system. It doesn't run a heat maintenance cycle all that often, but when running the generator, I don't want that additional heating load taking away from more essential loads so by shedding it, don't I change the total diversity of what I otherwise had?

 

[Smart $]

... I do have a hot tub that has a diversity on the system. It doesn't run a heat maintenance cycle all that often, but when running the generator, I don't want that additional heating load taking away from more essential loads so by shedding it, don't I change the total diversity of what I otherwise had?

Just to use your hot tub as an example, how much does it add to the total calculated load? If you did a load calculation with just the hot tub shed from the total load, how much less would the unshed load calculation be?

 

[kwired]

Just to use your hot tub as an example, how much does it add to the total calculated load? If you did a load calculation with just the hot tub shed from the total load, how much less would the unshed load calculation be?

Mine not much difference, the thing only has a 1000W heater. I have seen others that could possibly be near 50% of the total load on a service.