Table 310.15(B)(6) and generators

[Davis]

kwired: are you familiar with loadshedding to meet the 83 amps max of the 20kW? For example if the calculated load is 130 amps you just add contactors until you drop the load to generator capacity.

 

[kwired]

Yes, but what remains is based on using the NEC to do a load calc based on diversity. The generator is automatic and it must meet the code. And that is figured out by using, for example the optional method to calculate the load.

kwired: are you familiar with loadshedding to meet the 83 amps max of the 20kW? For example if the calculated load is 130 amps you just add contactors until you drop the load to generator capacity.

Yes I am familiar with everything you mentined.

The load calculation of a load connected to a 100 amp service or feeder may already be less than 83 amps, if so no load shedding devices are necessary.

The use of 310.15(B)(16) requires entire dwelling load to be carried by the conductor.

If you are using load shedding methods you are limiting the diversity that 310.15(B)(16) depends on for the values it states.

Backing up one 200 amp panel out of 2 - 200 amp panels serving a dwelling is load shedding.

 

[Davis]

So with all that said and points well taken I would like to sum up and add a question.

We will go at it from another angle. I?ll work down in a logical sequence.

The table in question has a certain intent. Which is key. This table is for reducing the wire size and thus ampacity of conductors that feed residential services of 120 /240 volt.

This is allowed because it has been determined that in load calcs used for a residence there is a high degree of diversification thus allowing for a reduced conductor size.

According to 702.5(B) the load for a generator shall be calculated according to Article 220. That is the same calculations used to determine the house load in the first place, when it was built. Thus originally allowing for diversification and a smaller conductor.

So upon transfer to a generator, the load should bare the same characteristics of an accurately calculated load according to Article 220, according to the NEC, thus allowing for diversification and a smaller conductor. The generator system complete with load shedding capability meets that requirement. Whatever is on the generator after any loads are shed are subject to Article 220! Diversification.

And that would meet the intent and purpose of Table 310.15(B)(6). The intent is that because of diversification a reduced wire size is allowed. Whether you have shed loads or not. Whatever is there or remaining is calculated according to Article 220, allowing for a reduced conductor size. I believe the intent is not to make sure all the loads are powered up but to allow for reduced wire size based on Article 220 which was used for the house originally and is also used for the generator.

The question:

Is the description in 310.15(B)(6) which is describing mechanical interconnection as part of the same system, so as to be clear, or does it mean it collectively all must be capable of being powered up without load shedding?

? the feeder supplies all loads that are part of or associated with the dwelling unit ?

 

[ceb58]

Realize, of course, it matters not what you, I or anyone thinks other than your AHJ.
To me, 100 scenarios and the same rule (wording) applies. "all loads associated with the dwelling unit" is what the Code states.
Not all connected loads, not "other than loads"...all loads associated with the dwelling. Unless you have that, IMO, 310.15(B)(*) does not apply.

What is your take on this wording from 310.15(B)(*)

For application of this section,
the main power feeder shall be the feeder between the
main disconnect 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.

That is another reason I stand by the table dose not apply. You can try and say if the power is out the generator is the main source but IMO it is not. What dose the generator become when the power is restored? The only way I would see a generator as being the main source was if it were truly the only source attached to the dwelling.

 

[mivey]

I noticed that George and others have submitted proposals to have this table deleted. What has the CMP said about keeping this section in the past? Do they still think the special-case diversity for residential applies? I have looked through some of the committee stuff but am not good at navigating through the information and finding their rulings/findings/opinions.

Can someone who has followed the committee's opinion through the code revisions summarize their take?

At best, it is an atypical unconservative table. FWIW, since it is so atypical, and specifically identified for a vary narrow use, I am of the opinion that the table does not apply to the standby system either.

 

[Davis]

ceb58, OK, I'll look at that problem.

That is a good point and I will look at it a step at a time. This might be a stretch here. :jawdrop:

First looking at Article 100, 225 & 230, I can't find a definition of " main disconnect " even though that is what we call the service disconnect. ( SERVICE EQUIPMENT mentions main control ) And then it is assumed all feeders proceed away from the " main ".

Why doesn't it say service disconnect instead of main disconnect if that is what they meant? Does main mean chief among or service or feeder disconnect?

If it means from the service disconnect outward to the house panel ( apartment or SFR etc. ) then I am SOL.

The other way to look at it is in the event of a power outage and the generator takes over providing power and it's breaker becomes the " feeder disconnect ". Whether that is the " main " or " chief " disconnect, I am not sure. But I know in our example it is not a new service so it can't be a service disconnect.

When the power is restored the generator is still the sole provider of power until it transfers. So I am not too worried about that aspect of your question.

On the same subject but a different note: Article 225.39(C) says the smallest feeder to a SFR is 100 amps. The 20kW generator is a feeder with a 100 amp CB. How does a 12 - 17 kW generator get installed with a whole house transfer switch?

So somewhere the AHJ must overlook some of these things.

Besides, it all depends on what the definition of is is.

 

[augie47]

As I stated many posts back, what matters is what the AHJ on YOUR install thinks. You may get many opinions here but thyey are acedemic.
You have presented your situation(s) in various formats and with various arguments and, as best I can tell, LEO is the only one who agrees the Table applies.

 

[kwired]

ceb58, OK, I'll look at that problem.

That is a good point and I will look at it a step at a time. This might be a stretch here. :jawdrop:

First looking at Article 100, 225 & 230, I can't find a definition of " main disconnect " even though that is what we call the service disconnect. ( SERVICE EQUIPMENT mentions main control ) And then it is assumed all feeders proceed away from the " main ".

Why doesn't it say service disconnect instead of main disconnect if that is what they meant? Does main mean chief among or service or feeder disconnect?

If it means from the service disconnect outward to the house panel ( apartment or SFR etc. ) then I am SOL.

The other way to look at it is in the event of a power outage and the generator takes over providing power and it's breaker becomes the " feeder disconnect ". Whether that is the " main " or " chief " disconnect, I am not sure. But I know in our example it is not a new service so it can't be a service disconnect.

When the power is restored the generator is still the sole provider of power until it transfers. So I am not too worried about that aspect of your question.

On the same subject but a different note: Article 225.39(C) says the smallest feeder to a SFR is 100 amps. The 20kW generator is a feeder with a 100 amp CB. How does a 12 - 17 kW generator get installed with a whole house transfer switch?

So somewhere the AHJ must overlook some of these things.

Besides, it all depends on what the definition of is is.

I think I was mentioning wrong section in previous posts, I meant to say 310.15(B)(7). It was 310.15(B)(6) in 2008 NEC. I don't think there was any changes other than code section number(s) Here is that section:

(7) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For individual dwelling units of one-family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(7), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service-lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder between the main disconnect 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. The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.

To use this section, it applies to service conductors or feeder conductors. The condition is they must be supply all loads that are a part of a single family dwelling unit. By limiting current with load shedding methods you are no longer supplying all loads. By not supplying all loads you are changing the load diversity that there would be if you were supplying all loads.

There are sections in 225 and 230 that require a disconnecting means rated at least 100 amps for a single family dwelling. This does not necessarily mean they must be supplied with conductors with an ampacity of 100 amps.

Now if you have a load calculation that is equal or less than your generator output or if you can shed loads to be equal or less than the generator output, then you can automatically transfer those loads, but nowhere does it say you can do so with 310.15(B)(7) sized conductors, unless they are supplying all loads of the dwelling as stated in 310.15(B)(7) itself.

 

[ronaldrc]

I believe you need to use Table 310.16 .

It would still be a #4 so you wouldn't have to change it.

Table 310.15(B).6 says this table is to be use with conductors supplying
the Main power feed, that would be the Electrical Power Utility and the
conductors feeding your 400 amp. Main disconnect.

The 20 KW generator example you are using with a maximum output of 83 amps.
At 240 volts feeding only 1 of your two 200 amp. panels would not qualify as the
main power source.

At least that's the way I read it.

Ronald

 

[Smart $]

... The condition is they must be supply all loads that are a part of a single family dwelling unit. By limiting current with load shedding methods you are no longer supplying all loads. By not supplying all loads you are changing the load diversity that there would be if you were supplying all loads.

...

Now if you have a load calculation that is equal or less than your generator output or if you can shed loads to be equal or less than the generator output, then you can automatically transfer those loads, but nowhere does it say you can do so with 310.15(B)(7) sized conductors, unless they are supplying all loads of the dwelling as stated in 310.15(B)(7) itself.

Though I changed my position earlier, I still have to question the interpretation, especially knowing the following...

220.60 Noncoincident Loads. Where it is unlikely that
two or more noncoincident loads will be in use simultaneously,
it shall be permissible to use only the largest
load(s) that will be used at one time for calculating the total
load of a feeder or service.

Automatic load shedding establishes noncoincident loads

Nowhere in 310.15(B)(7) does it say all of the dwelling loads per service calculation, or all such loads at all times.

Edit to add: ...and would it make any difference if the generator is set up as an SDS...???

 

[ronaldrc]

10.15(B)(7)

Service-entrance conductors for 120/240-volt,3-wire, single-phase dwelling
services are permitted to be sized from Table 310.15(b)(7) instead of from Table
310.15(B)(16). Feeders that carry the whole load of a dwelling unit can also be
sized from Table 310.15(B)(7).[p]

If feeders to a dwelling unit do not carry the whole load for the dwelling,then
310.15(B)(7) cannot be used.

Ronald

 

[kwired]

10.15(B)(7)

Service-entrance conductors for 120/240-volt,3-wire, single-phase dwelling
services are permitted to be sized from Table 310.15(b)(7) instead of from Table
310.15(B)(16). Feeders that carry the whole load of a dwelling unit can also be
sized from Table 310.15(B)(7).[p]

If feeders to a dwelling unit do not carry the whole load for the dwelling,then
310.15(B)(7) cannot be used.

Ronald

I have stated that twice in this thread, and don't plan to change my position.

Different scenario but yet a situation where 310.15(B)(7) can not be used: A load center mounted on exterior of a dwelling that has a main breaker serving as the service disconnecting means. Only other breakers in the load center are the air conditioner and a feeder to another panel inside the dwelling which happens to serve the remainder of the dwelling. The feeder can not use 310.15(B)(7) because the air conditioner is not supplied by this feeder and it is part of the dwelling load. The service conductors however could use 310.15(B)(7). If this were 100 amp service and 100 amp feeder you could use 4 AWG copper per 310.15(B)(7) for the service conductors, but the feeder that carries same load less the AC unit, has to be at least 3 AWG copper. Might not seem to make common sense but that is what the rules are. I don't see this generator issue to be any different. If a conductor is not carrying ALL the dwelling unit load, it can not be sized with 310.15(B)(7). I am not saying you can not size a conductor supplied by a generator with 310.15(B)(7), if it feeds ALL of a dwelling unit load then you can, use of shedding methods means you are not supplying all the load though.

 

[elohr46]

I am wondering how others see this situation.

Table 310.15(B)(6) list conductor types and sizes specifically for 120/240-volt, 3 wire, single phase dwelling services and feeders that carry the entire load of the dwelling unit.

In our state the circuit breaker on the generator is the required disconnect even if it is not specifically service rated. If the generator is readily accessable and located within site of the building the circuit breaker on the unit will suffice.

So the conductors that supply the load at the house are between the main disconnect on the generator and the panelboard which have the loads.

Home generators are classified Optional Standby Systems and as we know they are a feeder.

Also load shedding would have no bearing on the situation. If a generator fed an entire home regardless that some loads could possibly be shed makes no difference. It still is connected to the entire house load.

Example #1: The house has a 200 amp service, 1 200 amp panel.

20kW run to a 200 amp SE ( service enterance ) load shed transfer switch. Some loads are shed as necessary to meet Code.
The generator has a 100 amp CB. According to the table, #4 cu or #2 al is ok. All the conditions of Article 310.15B)(6) are met.

Example #2: the house has a 400 amp ( actually a 320 amp ) service, 2 - 200 amp panels.

20kW run to 1 - 200 amp SE transfer switch with only one panel connected at the house. The other panel is not backed up.
All the conditions are now NOT met and using the table would not be allowed. The generator is NOT now powering up the entire house.

How do others see this?

If this helps you a 20kw(82 amps @ 240 v) Generac generator comes prewired from the factory with #4 cu(85 amps @ 75 deg) for a feeder. The generator itself does not see any load diversity, only the load that is connected to it, therefore the conductors must be able to carry the maximun load that the genset put out. FWIW generator manufacturers highly recommend that you load them to 80% or less of their capacity for a longer life span. I don't see how table 310.15(B)(6) even applies to generators at all.

 

[kwired]

If this helps you a 20kw(82 amps @ 240 v) Generac generator comes prewired from the factory with #4 cu(85 amps @ 75 deg) for a feeder. The generator itself does not see any load diversity, only the load that is connected to it, therefore the conductors must be able to carry the maximun load that the genset put out. FWIW generator manufacturers highly recommend that you load them to 80% or less of their capacity for a longer life span. I don't see how table 310.15(B)(6) even applies to generators at all.

Lots of listed equipment has conductors smaller than NEC would allow, but they are tested and listed with those conductors and NEC does not apply to them if they are a part of listed equipment.

I don't get what you are trying to say with "The generator itself does not see any load diversity, only the load that is connected to it".

But at same time we are not really talking about the generator, we are talking about conductors between a generator and a load.

 

[Davis]

Thanks for all the replies and ideas. Noncoincidental Loads idea is the best so far. But ceb's question is still very good!
However I want to respect this forum and I think that at post #27 augie47, a moderator, :thumbsup: tried in a polite way to say that to continue on further would be like beating a dead horse to death. Check with AHJ is ultimately the answer.

Thanks all.

 

[augie47]

thanks

 

[ceb58]

If this helps you a 20kw(82 amps @ 240 v) Generac generator comes prewired from the factory with #4 cu(85 amps @ 75 deg) for a feeder. The generator itself does not see any load diversity, only the load that is connected to it, therefore the conductors must be able to carry the maximun load that the genset put out. FWIW generator manufacturers highly recommend that you load them to 80% or less of their capacity for a longer life span. I don't see how table 310.15(B)(6) even applies to generators at all.

And that is another can of worms. So we say the argument about 310.15 (B)(6) is moot. If we were to wire the feeder to the gen. We would have to follow 240.4 and use #3 on the 100amp breaker.

 

[elohr46]

Lots of listed equipment has conductors smaller than NEC would allow, but they are tested and listed with those conductors and NEC does not apply to them if they are a part of listed equipment.

I don't get what you are trying to say with "The generator itself does not see any load diversity, only the load that is connected to it".

But at same time we are not really talking about the generator, we are talking about conductors between a generator and a load.

My point is that the conductors between the gen. and the load should be large enough to carry the entire capacity of the generator's output, load diversity has nothing to do with that at all. Example, if you have a maximum load of 50 amps connected to a gen. that can supply a maximum of 80 amps what size conductor would you install? The answer should be conductors that can carry 80 amps.

 

[kwired]

My point is that the conductors between the gen. and the load should be large enough to carry the entire capacity of the generator's output, load diversity has nothing to do with that at all. Example, if you have a maximum load of 50 amps connected to a gen. that can supply a maximum of 80 amps what size conductor would you install? The answer should be conductors that can carry 80 amps.

You only need 50 amp conductors, but you will need to provide overcurrent protection of 50 amps also.

 

[elohr46]

You only need 50 amp conductors, but you will need to provide overcurrent protection of 50 amps also.

Agreed, or you could opt to install conductors capable of handling the full output of the generator for future expansion. Getting back to the OP, I'm not seeing where table 310.15(B)(6) or load diversity enters into the equation.