Subpanel

[Cleveland Apprentice]

Hello,

I plan on installing a subpanel off of a 200 amp 240/120 volt service. I plan on running 100 amps to a subpanel with feed thru lugs. Off of the feed thru lugs of the subpanel, I plan on running a feeder to a 2nd subpanel in a different location on 2nd floor. The feeder will be sized #4 awg (for residential) My question is: Does the code prohibit linking to subpanels together using feed thru lugs? Thank you.

 

[chris kennedy]

Hello,

I plan on installing a subpanel off of a 200 amp 240/120 volt service. I plan on running 100 amps to a subpanel with feed thru lugs. Off of the feed thru lugs of the subpanel, I plan on running a feeder to a 2nd subpanel in a different location on 2nd floor. The feeder will be sized #4 awg (for residential) My question is: Does the code prohibit linking to subpanels together using feed thru lugs? Thank you.

Welcome to the Forum. All sounds fine except for the #4's. You got a permit for this project?

 

[kwired]

The feed to the second subpanel is technically still the same feeder as what is feeding the first panel, nothing wrong with this. Protecting the 4AWG (assuming copper) is improper. 310.15(B)(6) allows a 4 AWG to be protected at 100 amps but that table can only be used for conductors supplying the entire load of a dwelling unit. You indicate you are only feeding a portion of a dwelling unit so the general rules apply and conductors must be sized according to table 310.16. If you were going to use a SE type cable as the feeder you would have to use the 60? column of T310.16 and would have to use a #2 copper(if your load calculation is 95 amps or less) or 1/0 aluminum.

I would do a load calculation and find out what size of feeder is actually needed. Unless you are feeding electric heating, water heating or other major loads like that I'm guessing you can easily get by with a smaller feeder.

 

[Dennis Alwon]

The feed to the second subpanel is technically still the same feeder as what is feeding the first panel, nothing wrong with this. Protecting the 4AWG (assuming copper) is improper. 310.15(B)(6) allows a 4 AWG to be protected at 100 amps but that table can only be used for conductors supplying the entire load of a dwelling unit. You indicate you are only feeding a portion of a dwelling unit so the general rules apply and conductors must be sized according to table 310.16. If you were going to use a SE type cable as the feeder you would have to use the 60? column of T310.16 and would have to use a #2 copper(if your load calculation is 95 amps or less) or 1/0 aluminum.

I would do a load calculation and find out what size of feeder is actually needed. Unless you are feeding electric heating, water heating or other major loads like that I'm guessing you can easily get by with a smaller feeder.

I agree but some would argue 215.2(A)(4)

 

[kwired]

I agree but some would argue 215.2(A)(4)

I am assuming that is something new in 2011 NEC. There is no (A)(4) in my 2008.

 

[Dennis Alwon]

I am assuming that is something new in 2011 NEC. There is no (A)(4) in my 2008.

A (3) in the 2008

 

[kwired]

A (3) in the 2008

As far as 2008 NEC goes 215.2(A)(3) does nothing but point you to 310.15(B)(6) for feeders that supply a dwelling unit. 215.2(A)(3) could be deleted and you really would not have any change in requirements for these installations, as it is already covered by 310.15(B)(6).

 

[Dennis Alwon]

As far as 2008 NEC goes 215.2(A)(3) does nothing but point you to 310.15(B)(6) for feeders that supply a dwelling unit. 215.2(A)(3) could be deleted and you really would not have any change in requirements for these installations, as it is already covered by 310.15(B)(6).

It states that feeders don't need to be larger than service conductors. If the service conductor is #4 then some would say the feeder to a sub panel that doesn't carry the load of the house only needs to be #4

 

[kwired]

It states that feeders don't need to be larger than service conductors. If the service conductor is #4 then some would say the feeder to a sub panel that doesn't carry the load of the house only needs to be #4

I have to disagree.

Feeder conductors for individual dwelling units or mobile homes need not be larger than service conductors.

If they remove the word "individual" then I would agree.

It also says:

Paragraph 310.15(B)(6) shall be permitted to be used for conductor size.

That paragraph contains the wording:

..all loads that are part or associated with the dwelling unit.

 

[curt swartz]

I'm confused here. The OP is feeding the 2 new 100 amp panels off of a 200 amp panel. How could they possible use 310.15(B)(6) to size these 100 amp feeder conductors?

 

[kwired]

I'm confused here. The OP is feeding the 2 new 100 amp panels off of a 200 amp panel. How could possible use 310.15(B)(6) to size these 100 amp feeder conductors?

If the feeder is feeding a dwelling unit (the entire unit) as defined in Art 100, it can if the total load calc of the dwelling is 100 amps or less. I understand the OP to be feeding additional circuits within a dwelling with the feeder he is asking about and therefore he can not use 310.15(B)(6).

 

[Cleveland Apprentice]

Thanks for replies. I realize now that I can't use #4 cu and will upsize. Also, a permit will be pulled. I'm in planning stage now.

 

[120/240]

so can you use a #4awg to feed a subpanel if the main is rated at 100I ...and please explain again why he cannot use 4awg to feed a subpanel when its 200I service thank you for your time

 

[Dennis Alwon]

so can you use a #4awg to feed a subpanel if the main is rated at 100I ...and please explain again why he cannot use 4awg to feed a subpanel when its 200I service thank you for your time

The code states that #4 is good for services of 100 amps residential. It also state in 310.15 that if the feeder-- wire going to the sub panel-- carries the entire load of the service then we could use T. 310.15(B)(6) which allows #4.

So meter to main disconnect only panel -- #4 is compliant. Main disconnect to MLO panel #4 is still compliant.

Now meter to main disconnect panel with other circuits-- #4 is compliant
From main panel to sub panel #4 is no longer good for 100 amps and you must use T310.16

 

[kwired]

so can you use a #4awg to feed a subpanel if the main is rated at 100I ...and please explain again why he cannot use 4awg to feed a subpanel when its 200I service thank you for your time

The feeder in the OP is not feeding a "dwelling unit" it is feeding loads in a dwelling unit but not the entire unit. Size of service has nothing to do with it what the feeder is supplying does.

T310.15(B)(6) has taken into account that on the main feed to a dwelling that not all load in the dwelling will be used at the same time. They have come up with the values in the table as being acceptable for this situation. If the feeder is not supplying the entire load of a dwelling it is harder to determine the load diversity in generic terms and create such a table. If the feeder happens to feed only HVAC or something like that it may very well be loaded 100 percent quite frequently.

There are cases where you can run conductors smaller than required by 310.16 but they require engineering supervision to do so. 310.15(B)(6) has done the engineering supervision part to a certain extent for you but only for a feeder or service conductor that supplies the entire load to a dwelling unit.

 

[charlie b]

. . . Refering to 215.2(A)(4). . . It states that feeders don't need to be larger than service conductors. If the service conductor is #4 then some would say the feeder to a sub panel that doesn't carry the load of the house only needs to be #4

Those who would say that would have to get around the point where it states that it applies to "feeder conductors for individual dwelling units." That phrase tells me that a feeder "to" a dwelling unit can use the table, but a feeder "within" a dwelling unit cannot.

 

[Dennis Alwon]

Those who would say that would have to get around the point where it states that it applies to "feeder conductors for individual dwelling units." That phrase tells me that a feeder "to" a dwelling unit can use the table, but a feeder "within" a dwelling unit cannot.

I could read that as a feeder to an individual unit as opposed to two dwelling units. Mind you I am not sure how this is meant to be.

 

[stew]

Just to add my warped logic to this. The table that allows smaller than normal wire for dwelling units was developed with the diversity of loads that are attached to it correct? That being the case the case which i believe it is then a panel that includes a portion of those diverse loads ought to be able to be fed with the smaller wire as well eh? loads are just as diverse on the sub as they are in the main IMHO.

 

[kwired]

Just to add my warped logic to this. The table that allows smaller than normal wire for dwelling units was developed with the diversity of loads that are attached to it correct? That being the case the case which i believe it is then a panel that includes a portion of those diverse loads ought to be able to be fed with the smaller wire as well eh? loads are just as diverse on the sub as they are in the main IMHO.

Not necessarily. As I mentioned a couple posts back, what if the feeder was supplying primarily HVAC? Maybe there is electric heat involved in this load. Your service calculations will include the heating or cooling load which ever is greater plus whatever else is required in all of the dwelling. Your feeder to this HVAC panel may only be sized for the load connected to it. If your calculated feeder size comes up to 95 amps you have to supply it with #3 copper or #1 aluminum minimum there is no load diversity factored into the somewhat limited load connected to this feeder. On the coldest or warmest of days this load could run fairly continuously.

If you came up with a load calculation of 95 amps for the entire dwelling, the likelihood of seeing it loaded to 95 amps (especially for anymore than a short duration) is not as likely as it is for the HVAC panel described in the last paragraph

 

[stew]

I think you are adding too much if factor to this. A small home would not require a very big air condtioner any any case so that load would be diversive also. You certainly arent going to have more than one ac unit on the panel anyway. I know what the code says but I dont agree that is necessarily logical when it comes to this specific area. Anyhow just use 2 al and a 90 amp breaker and you are good to go.