Sub Panel Load Calcualtions

[mdion69]

I'm sure this question has been asked before.
Basic dwelling unit that has a 200 amp Service basic load center.
When installing a 100 amp 16 circuit load center ATS.

Are we allowed to use Optional Load Calculation Method? in this instance? Is this really considered a sub panel as well?

 

[Dennis Alwon]

I think it is fine if you have a 100 amp feeder

220.82 Dwelling Unit.
(A) Feeder and Service Load. This section applies to a
dwelling unit having the total connected load served by a
single 120/240-volt or 208Y/120-volt set of 3-wire service
or feeder conductors with an ampacity of 100 or greater. It
shall be permissible to calculate the feeder and service
loads in accordance with this section instead of the method
specified in Part III of this article. The calculated load shall
be the result of adding the loads from 220.82(B) and (C).
Feeder and service-entrance conductors whose calculated
load is determined by this optional calculation shall be permitted
to have the neutral load determined by 220.61.

 

[kwired]

Maybe a little more clarification is needed. I am guessing you are talking about "a 100 amp 16 circuit load center ATS", I assume you are only powering a portion of the total dwelling with that load center and are able to power it from normal or a standby supply.

The code Dennis quoted is for and says right in the quote "This section applies to a dwelling unit having the total connected load served by..."

IMO you need to calculate the load on the circuits that are served by this panel, demand factors that can be applied to the entire dwelling load can not be used on just a portion of the entire load.

 

[Smart $]

...The code Dennis quoted is for and says right in the quote "This section applies to a dwelling unit having the total connected load served by..."

Sure, the dwelling unit must first qualify for calculation under 220.82, but once qualified, any feeder supplied therein can be calculated under this section, even if it is under 100A.

 

[GoldDigger]

Sure, the dwelling unit must first qualify for calculation under 220.82, but once qualified, any feeder supplied therein can be calculated under this section, even if it is under 100A.

I think you missed the " , I assume you are only powering a portion of the total dwelling with that load center..." part of kwired:s post. That load center could be an essential load panel off the ATS rather than connected to a whole house generator.

 

[Smart $]

I think you missed the " , I assume you are only powering a portion of the total dwelling with that load center..." part of kwired:s post. That load center could be an essential load panel off the ATS rather than connected to a whole house generator.

I don't believe I missed anything... but it wouldn't be the first time if I did. :angel:

I believe the OP has a combo ATS-panelboard powered off main panelboard and to serve as essential loads. The point I was trying to make was once it is established the dwelling unit is served by 3-wire 100A service or feeder, any sub feeder therein can also be calculated under 220.82.

I think kwired's point is that you can't just take the load as a percentage of the calculated [total] service or feeder load. Rather the subpanel load has to go through the entire paradigm separately, and qualifying for demand based solely on its connected load.

 

[GoldDigger]

I don't believe I missed anything... but it wouldn't be the first time if I did. :angel:

I believe the OP has a combo ATS-panelboard powered off main panelboard and to serve as essential loads. The point I was trying to make was once it is established the dwelling unit is served by 3-wire 100A service or feeder, any sub feeder therein can also be calculated under 220.82.

I think kwired's point is that you can't just take the load as a percentage of the calculated [total] service or feeder load. Rather the subpanel load has to go through the entire paradigm separately, and qualifying for demand based solely on its connected load.

There is where we differ. The code language does not appear to apply directly to sub feeders. If you are arguing that a sub feeder need not be any larger than the service conductors, please give me a code section.
I definitely do not accept that we can directly apply the same downsizing factor to any subfeeder calculations that do not, themselves, carry the entire dwelling load.

 

[Smart $]

There is where we differ. The code language does not appear to apply directly to sub feeders. If you are arguing that a sub feeder need not be any larger than the service conductors, please give me a code section.
I definitely do not accept that we can directly apply the same downsizing factor to any subfeeder calculations that do not, themselves, carry the entire dwelling load.

So are you saying 220.82 only applies to service or feeder conductors which supply the entire load of a dwelling unit and no feeder thereunder?

Say for example you have an apartment dwelling unit supplied with a 400A, 3-wire feeder (yes, unlikely, but use your imagination) and it is split into two 200A subpanels, then a 100A feeder from one 200A panelboard to a 100A panelboard. Are you saying 220.82 does not apply to this 100A panelboard feeder?

If yes, that is absolutely absurd. :blink::blink::blink:

 

[iwire]

I definitely do not accept that we can directly apply the same downsizing factor to any subfeeder calculations that do not, themselves, carry the entire dwelling load.

Where are the words in the code to support that thought?

 

[kwired]

220.82 Dwelling Unit.
(A) Feeder and Service Load.
This section applies to a dwelling unit having the total connected load served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors with an ampacity of 100 or greater.

I maybe am second guessing what I mentioned earlier - all depends on whether this quote section is referring to the service or feeder conductors supplying the entire dwelling load or if it can be any portion of the dwelling load. I still am leaning toward it only applies to the main supply that carries entire dwelling load, not a sub feeder that only carries a portion of the dwelling load.

 

[GoldDigger]

I maybe am second guessing what I mentioned earlier - all depends on whether this quote section is referring to the service or feeder conductors supplying the entire dwelling load or if it can be any portion of the dwelling load. I still am leaning toward it only applies to the main supply that carries entire dwelling load, not a sub feeder that only carries a portion of the dwelling load.

That has been the consensus of the Forum in the past.
Notably leading to the apparent absurdity that if you have an existing feeder sized under the rule and decide to tap off some of the building load upstream you must now make the feeder larger to carry that reduced load.
Whether we like it or not, that is what the Code says.

 

[kwired]

That has been the consensus of the Forum in the past.
Notably leading to the apparent absurdity that if you have an existing feeder sized under the rule and decide to tap off some of the building load upstream you must now make the feeder larger to carry that reduced load.
Whether we like it or not, that is what the Code says.

Not quite what I am trying to get at. Lets go back to what I think the OP has. Say a 400 amp service or feeder supplying the entire dwelling. I have no issue with doing the load calculation on that per 220.82. Now we decide we are going to put in an optional standby generator and supply selected circuits via a subpanel from that standby generator or the normal supply depending on position of a transfer switch. Can you use 220.82 to determine the load calculation for that feeder or does it only apply to the service or feeder carrying the entire dwelling load?

 

[Smart $]

... Can you use 220.82 to determine the load calculation for that feeder or does it only apply to the service or feeder carrying the entire dwelling load?

You can is what I've been saying, once the criteria of the first sentence is satisfied.

 

[GoldDigger]

You can is what I've been saying, once the criteria of the first sentence is satisfied.

So you are saying that if such a service or feeder exists it allows the ampacity reduction factor to apply to every feeder in the dwelling unit? If that had been intended, the wording would have been different.
The condition clearly (to me anyway) applies to the service or feeder wire whose ampacity you are calculating. Simple English grammar.
Under your interpretation the factor would apply to every feeder in almost every dwelling and most of our accepted test answers would be wrong.

 

[Smart $]

So you are saying that if such a service or feeder exists it allows the ampacity reduction factor to apply to every feeder in the dwelling unit? If that had been intended, the wording would have been different.

Perhaps. I cannot speak to the intent with any certainty.

If the main is 100A, the likelihood of having a sub feeder is greatly reduced. Additionally, demand reduction for general loads doesn't even kick in until the load goes over 10kVA (42A at 240V). Where the general load is less than the main, the demand as a percentage of the before-reduction load would be greater.

The only place I see a potential problem is under (C)(5) where four or more space heater loads are permitted a 40%-of-nameplate demand.

The condition clearly (to me anyway) applies to the service or feeder wire whose ampacity you are calculating. Simple English grammar.

Exactly. Including any feeder that does not supply the total load. Simple English grammar does not exclude a sub feeder. The context of the first sentence of (A) establishes the dwelling unit must be supplied by conductors, service or feeder, of 100A or greater. Dwelling unit is the subject of the first sentence. There is no conditional clause regarding the feeder mentioned in subsequent sentences. One could say the feeder mentioned in subsequent sentences must be the same feeder mentioned in the first sentence, which is your position. I do not see enough context in the subsequent sentences to support that interpretation.

Under your interpretation the factor would apply to every feeder in almost every dwelling and most of our accepted test answers would be wrong.

The scope of that statement is quite broad. Which feeders? What answers? You'll have to be more specific.

 

[kwired]

Perhaps. I cannot speak to the intent with any certainty.

If the main is 100A, the likelihood of having a sub feeder is greatly reduced. Additionally, demand reduction for general loads doesn't even kick in until the load goes over 10kVA (42A at 240V). Where the general load is less than the main, the demand as a percentage of the before-reduction load would be greater.

The only place I see a potential problem is under (C)(5) where four or more space heater loads are permitted a 40%-of-nameplate demand.


Exactly. Including any feeder that does not supply the total load. Simple English grammar does not exclude a sub feeder. The context of the first sentence of (A) establishes the dwelling unit must be supplied by conductors, service or feeder, of 100A or greater. Dwelling unit is the subject of the first sentence. There is no conditional clause regarding the feeder mentioned in subsequent sentences. One could say the feeder mentioned in subsequent sentences must be the same feeder mentioned in the first sentence, which is your position. I do not see enough context in the subsequent sentences to support that interpretation.


The scope of that statement is quite broad. Which feeders? What answers? You'll have to be more specific.

Regarding the (C)(5) - you have to use the largest value of (C)(1) through (C)(6). Most of the time one of the other five values will be larger then what (C)(4) gives you.

I am starting to see this as you can use 220.82 for the situation the OP describes as long as the feeder in question is at least 100 amps capacity. Demand factors I was concerned about earlier won't be much of a factor unless you do have quite a bit of load involved with the feeder in question. And as you mentioned one of the first ones to kick in in many instances will require at least 10kVA of general load before you can even use that demand factor.

 

[kwired]

Another reality that goes along with a feeder for a "standby" panel is the fact that you likely are only loading the panel with items the generator can actually run, even though the utility supply will be able to handle more. Many homes only have 10 - 16 kW generators max, quite a few of those are even less then that.

 

[mdion69]

SubPanel Load calculations

SubPanel Load calculations

This was the comment form a Plans Processor:

4. NEC 220.82 CAN BE USED ONLY WHEN THE TOTAL LOAD IS FED FROM A SINGLE SET

OF CONDUCTORS FOR DWELLING UNITS - THIS HOUSE=SERVICE ONLY - PROVIDE

STANDARD LOAD CALCS FOR PANEL 'E'

Any thoughts?

 

[Smart $]

This was the comment form a Plans Processor:

4. NEC 220.82 CAN BE USED ONLY WHEN THE TOTAL LOAD IS FED FROM A SINGLE SET

OF CONDUCTORS FOR DWELLING UNITS - THIS HOUSE=SERVICE ONLY - PROVIDE

STANDARD LOAD CALCS FOR PANEL 'E'

Any thoughts?

Do a standard load calculation. What is the demand kVA?