Table 310.15(B)(6) and generators

[Davis]

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?

 

[LEO2854]

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?

Do you mean table 310.15(B)(16)?

Welcome to the forums....:thumbsup:

 

[ceb58]

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?

Every thing you have stated makes for a good argument. But IMOP I don't see how you can use 310.15 (B)(6) for the generator because of the wording

" that serve as the main power feeder to each dwelling"

and

" for application of this section, the main power feeder shall be the feeder between the main disconnect and the panel board"

. The feeders from the generator is not the "main" feeders unless it is the only source of power.

 

[Davis]

LEO, thanks! No, I have the right table. The whole point is that if you use the table you can get away with smaller conductors and they are easier to work with on some generators.

Hi ceb58. We'll see what the big boys say over here compared to "the other place ". As stated in my post they ARE all met! And it passes every day in WA. When the power is out the generator is the main power and the conductors are between the generator ciircuit breaker and the entire load at the house.

So the questions remains:

Can you use the table to size conductors from a generator to a residence. ( under the specified conditions )

 

[LEO2854]

LEO, thanks! No, I have the right table. The whole point is that if you use the table you can get away with smaller conductors and they are easier to work with on some generators.

Hi ceb58. We'll see what the big boys say over here compared to "the other place ". As stated in my post they ARE all met! And it passes every day in WA. When the power is out the generator is the main power and the conductors are between the generator ciircuit breaker and the entire load at the house.

So the questions remains:

Can you use the table to size conductors from a generator to a residence. ( under the specified conditions )

I say yes you can.

2011 NEC Changed the table number to Table 310.15(B)(7)

Table 310.15(B)(7) Conductor Types and Sizes for
120/240-Volt, 3-Wire, Single-Phase Dwelling Services and
Feeders. Conductor Types RHH, RHW, RHW-2, THHN,
THHW, THW, THW-2, THWN, THWN-2, XHHW,
XHHW-2, SE, USE, USE-2

 

[ceb58]

LEO, thanks! No, I have the right table. The whole point is that if you use the table you can get away with smaller conductors and they are easier to work with on some generators.

Hi ceb58. We'll see what the big boys say over here compared to "the other place ". As stated in my post they ARE all met! And it passes every day in WA. When the power is out the generator is the main power and the conductors are between the generator ciircuit breaker and the entire load at the house.

So the questions remains:

Can you use the table to size conductors from a generator to a residence. ( under the specified conditions )

We will agree to disagree. As I stated before all are NOT met. When the power is out the generator is not the main power source it is an optional standby system and is covered under Article 702. Optional being the word. Again, IMPO when 310.15 (B)(6) was written there was not the proliferation of these stand by units and I do not see the intent to include them in that code section. If your state accepts it then so be it. In the real world a 20Kw unit is only good for 83 continuous amps so a #4Cu would be good for it and the unit will hopefully not be loaded to the max.

 

[iwire]

If the generator serves 100% of the dwelling unit load than yes you can use the table, if the generator only feeds some of the dwelling unit load than no you cannot.

 

[Smart $]

If the generator serves 100% of the dwelling unit load than yes you can use the table, if the generator only feeds some of the dwelling unit load than no you cannot.

The requirement [310.15(B)(7)] mentions nothing about load or percentage thereof. It simply states "conductors that serve as the main power feeder".

While on standby power, the OP Example #1 generator feeder conductors do "serve as the main power feeder".

Edit to add: OP Example #2 does not state an ATS being used. If an MTS is used, both utility and generator could be powering the dwelling concurrently.

 

[iwire]

The requirement [310.15(B)(7)] mentions nothing about load or percentage thereof. It simply states "conductors that serve as the main power feeder".

While on standby power, the OP Example #1 generator feeder conductors do "serve as the main power feeder".

Edit to add: OP Example #2 does not state an ATS being used. If an MTS is used, both utility and generator could be powering the dwelling concurrently.

I disagree with your interpretation and IMO so does the CMP.

 

[augie47]

I would have to side with the "no can do" folks. To me, because 310/15(B)(*) uses the wording "all loads that are part or associated with the dwelling unit".
The whole idea of the reduced wire size is based on load diversity of the entire house load. Once you have selected specific loads you do not have the same diversity factor.

 

[Smart $]

I disagree with your interpretation and IMO so does the CMP.

I would have to side with the "no can do" folks. To me, because 310/15(B)(*) uses the wording "all loads that are part or associated with the dwelling unit".
The whole idea of the reduced wire size is based on load diversity of the entire house load. Once you have selected specific loads you do not have the same diversity factor.

With you guys' prodding, coupled with the "black and white", I'm abandoning my former position.

 

[iwire]

With you guys' prodding, coupled with the "black and white", I'm abandoning my former position.

If I was standing I would have fallen down, that was so far from what I was expecting when I clicked on this I am totally taken aback. :jawdrop:

 

[Smart $]

If I was standing I would have fallen down, that was so far from what I was expecting when I clicked on this I am totally taken aback. :jawdrop:

I'm glad you weren't injured

 

[Davis]

Thanks all. The reason for the question is that right now generator installation is just booming. Hurricane Sandy has increased awareness and caused a huge demand. 20kw's are used as an example because that size is the most common where this table would be used.

I suppose another example would be a 30kW ( or whatever liquid cooled ) installed at a home with a 320 amp service and 2 - 200 amp panels with 2 automatic transfer switches. The entire house is now backed up but on a larger scale than first example. Load shedding used also but I contend load shedding does not matter or stop you from using the table to reduce conductor size. The generator is powering the entire house.

Smart$: Both examples have the same components. I only shortened it to not have redundancy. The idea is that in the second example the table can not be used because the criteria does not meet the code.

augie47: As long as the generator is the sole source of power to the home I believe the table applies.

I will add one more idea to the mix. I hope this makes the concept super clear.

Example #3:

Same criteria as before in example one. Only this time use contacts on the transfer mechanism to always lock out permanently some really big load you actually don't care about. You don't want to load shed it IF the generator starts to exceed it's capacity, you just want it to go away as soon as generator power comes on. The rest of the house is on generator power.

Now some believe the generator must literally Power! the entire house while I say the entire house loads are now controlled in some manner and fed by the generator and therefore can fall under the table for reduced wire size.

Holy cow it's too early for this much thought! Somebody get me coffee.

 

[augie47]

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.

 

[Smart $]

...

Smart$: Both examples have the same components. I only shortened it to not have redundancy. The idea is that in the second example the table can not be used because the criteria does not meet the code.

...

Having changed my position on the matter makes the component issue you speak of moot.

 

[iwire]

As long as the generator is the sole source of power to the home I believe the table applies.

And I maintain you are mistaken.

Now some believe the generator must literally Power! the entire house while I say the entire house loads are now controlled in some manner and fed by the generator and therefore can fall under the table for reduced wire size.

Unless the generator powers 'either by branch circuits or by feeders, or both, all loads that are part or associated with the dwelling unit.' the table does not apply.

Notice it does not say the loads connected to the feeder or branch circuit is it all loads that are part or associated with the dwelling unit.

The loads that are part or associated with the dwelling unit remain part or associated with the dwelling regardless of them being off, load-sheded, locked out etc.

The reason for this is load diversity and depending on what part load is chosen may not have diversity.

As a matter of fact if they are saving money by choosing only a certain number of circuits to be backed up I think we can assume almost no diversity. There is a good change all the circuits they choose to back up will be in use at the same time while on the genset.

 

[iwire]

Realize, of course, it matters not what you, I or anyone thinks other than your AHJ.

:rant::rant::rant:






:lol:

Excellent point.

 

[kwired]

In the OP, 310.15(B)(16) doesn't even apply to the service supply conductors of this install as they are not carrying the entire dwelling load, then we add a standby generator and supply even less load because of a reduction in capacity of the generator over what the service is capable of supplying.

I will add that a 20 kW @ 240 volts and 100% PF is only 83.3 amps and 30 kW is only 125 amps.

20 kW is common to back up 100 amps services or feeders but to do so with an ATS would still mean calculated load needs to be 83 amps or less.

 

[Davis]

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.