Panel sizing for a single family home.

wrobotronic

Senior Member
Location
Colorado
Throughout my career I have always just done what the print has said. :p Thus never took the time to really understand sizing a residential panel. So, I had a panel today, a pretty standard 200A using 4/0 Al feeders.
It's very full with sev image.jpg eral half sized breakers and only 2 legit spares. The homeowner wants to add TWO 100A subs for an addition. My gut tells me this is not correct, but calculation wise, I'm not sure why. Here is pic of the panel. I'm very sorry, but I cannot seem to attach it in a bigger size. And I'm very upset with myself that the pic just randomly appears wherever it wants. I'm sure more info is required also however I'm not sure what. I hope I was able to describe my situation adequately. Thank you all.
 

John120/240

Senior Member
Location
Olathe, Kansas
Have you done a load calculation per NEC 220 ? Electric or Natural Gas for the Range, Water Heater, Furnace, Clothes Dryer ? What loads are anticipated for these new sub panels ? Spa/Hot Tub, Swimming pool, or just general lighting & receptacles ?
Your 4th wire down from the top right side shows too much bare conductor in my opinion.
 

jeremy.zinkofsky

Senior Member
Location
nj
You need to account for every single new load that will be Fed from this panel. You also need to take amp readings of every existing load with everything turned on. Once you have that data you need to make sure that it does not exceed maximum NEC requirements for feeder cable and the panel rating. If the existing panel cannot handle the additional loads then you will need to replace it with a higher rated one. You will probably have to upsize the aluminum feeders as well.
 

iwire

Moderator
Staff member
Location
Massachusetts
Basically you either have to do an NEC load calculation or this method below.


2011 NEC

220.87 Determining Existing Loads. The calculation of a feeder or service load for existing installations shall be permitted to use actual maximum demand to determine the existing load under all of the following conditions:

(1) The maximum demand data is available for a 1-year period.

Exception: If the maximum demand data for a 1-year pe-riod is not available, the calculated load shall be permitted to be based on the maximum demand (measure of average power demand over a 15-minute period) continuously re-corded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording. The recording shall reflect the maximum demand of the feeder or service by being taken when the building or space is occupied and shall include by measurement or calculation the larger of the heating or cooling equipment load, and other loads that may be peri-odic in nature due to seasonal or similar conditions.


(2) The maximum demand at 125 percent plus the new load does not exceed the ampacity of the feeder or rating of the service.

(3) The feeder has overcurrent protection in accordance with 240.4, and the service has overload protection in accordance with 230.90.
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
Throughout my career I have always just done what the print has said. :p Thus never took the time to really understand sizing a residential panel. So, I had a panel today, a pretty standard 200A using 4/0 Al feeders. It's very full with several half sized breakers and only 2 legit spares.

The homeowner wants to add TWO 100A subs for an addition. My gut tells me this is not correct, but calculation wise, I'm not sure why. Here is pic of the panel. I'm very sorry, but I cannot seem to attach it in a bigger size. And I'm very upset with myself that the pic just randomly appears wherever it wants. I'm sure more info is required also however I'm not sure what. I hope I was able to describe my situation adequately. Thank you all.
Don't worry, your picture is plenty big.

Do a load calc. The annex in the back of your code book is very helpful. I'll bet a steak dinner you have enough capacity to add the sub panels unless there is gobs of electric heat or some growing going on.
 

iwire

Moderator
Staff member
Location
Massachusetts
Do a load calc. The annex in the back of your code book is very helpful. I'll bet a steak dinner you have enough capacity to add the sub panels unless there is gobs of electric heat or some growing going on.
If I had to do a load calc without a doubt I would be using help from the annex. It is one of the better ideas the NFPA has put into the code book.
 

cpinetree

Senior Member
Location
SW Florida
Is that the feeders wires coming in from poco on the right side?

Hope not, if so they should be on the left under the separation barrier to keep fused and unfused wiring separate.
 

iwire

Moderator
Staff member
Location
Massachusetts
Is that the feeders wires coming in from poco on the right side?

Hope not, if so they should be on the left under the separation barrier to keep fused and unfused wiring separate.
The NEC does not prohibit fused and unfused conductors in the same enclosure.
 

cpinetree

Senior Member
Location
SW Florida
The NEC does not prohibit fused and unfused conductors in the same enclosure.
Pretty sure the instructions that come with those units have you install the poco line inside the barrier on the left when using an underground feed.


And what about this? I guess the panel may not be a raceway but, doesn't seem like a good idea when provisions are made by the manufacturer to avoid it.

230.7 Other Conductors in Raceway or Cable
Conductors other than service conductors shall not be installed in the same service raceway or service cable.
Exception No. 1: Grounding conductors and bonding jumpers.
Exception No. 2: Load management control conductors having overcurrent protection.

Handbook commentary
All feeder and branch-circuit conductors must be separated from service conductors. Service conductors are not provided with overcurrent protection where they receive their supply; they are protected against overload conditions at their load end by the service disconnect fuses or circuit breakers. The amount of current that could be imposed on feeder or branch-circuit conductors, should they be in the same raceway and a fault occur, would be much higher than the ampacity of the feeder or branch-circuit conductors.


Siemens info from this PDF: http://w3.usa.siemens.com/powerdistribution/us/en/speedfax-product-catalog/Documents/sf-11-sect-03-025-033.pdf

pg 4 (document pg# 3-28) in the description area -

Underground feed accomplished by use of
removable gutter trough


Note - I do not know the actual model number for this installation but all the ones we have used (Usually Square D's) list the gutter to be used for the underground feed.

Edit to add 230.7 & hand book commentary
 
Last edited:

iwire

Moderator
Staff member
Location
Massachusetts
230.7 does not apply to enclosures.

Most service panels do not have a seperater at all.

Perhaps a local power company requirment.
 

cpinetree

Senior Member
Location
SW Florida
See also this thread: http://forums.mikeholt.com/showthread.php?t=117656

Is the panel a wireway then? and by extension a raceway?

Raceway. An enclosed channel of metal or nonmetallic
materials designed expressly for holding wires, cables, or
busbars, with additional functions as permitted in this
Code. Raceways include, but are not limited to, rigid metal
conduit, rigid nonmetallic conduit, intermediate metal conduit,
liquidtight flexible conduit, flexible metallic tubing,
flexible metal conduit, electrical nonmetallic tubing, electrical
metallic tubing, underfloor raceways, cellular concrete
floor raceways, cellular metal floor raceways, surface
raceways, wireways, and busways.
Now I'm curious what others think.

Granted you always have fused and unfused wire in the same area when feeding from a meter to a main breaker panel, but the above installation allows for a way to avoid it by using the gutter inside the meter breaker combo unit.
 

kwired

Electron manager
Location
NE Nebraska
In reality it does not, 'this code is not an instruction manual for the untrained' etc. but I will gladly use the annex.
and putting them in the annex puts them outside of the official document which is NFPA 70. I suppose they could put that content in an informational note somewhere in art 220 and accomplish about same thing.

Is that the feeders wires coming in from poco on the right side?

Hope not, if so they should be on the left under the separation barrier to keep fused and unfused wiring separate.
230.7 does not apply to enclosures.

Most service panels do not have a seperater at all.

Perhaps a local power company requirment.
That panel appears to have a meter compartment above the loadcenter (see the meter seal near top of image?).

Many POCO will not accept this metering equipment if it leaves customer access to the unmetered conductors so that is why most manufacturers of this kind of equipment provide a cover for supply side conductors like that. You can not remove that cover without removing the meter cover - so the meter seal is what prevents customer access to those unmetered conductors - without leaving some seal tampering evidence behind anyway.
 

iwire

Moderator
Staff member
Location
Massachusetts
and putting them in the annex puts them outside of the official document which is NFPA 70. I suppose they could put that content in an informational note somewhere in art 220 and accomplish about same thing.





That panel appears to have a meter compartment above the loadcenter (see the meter seal near top of image?).

Many POCO will not accept this metering equipment if it leaves customer access to the unmetered conductors so that is why most manufacturers of this kind of equipment provide a cover for supply side conductors like that. You can not remove that cover without removing the meter cover - so the meter seal is what prevents customer access to those unmetered conductors - without leaving some seal tampering evidence behind anyway.
So when I say 'perhaps a local power company requirment' you see a need to quote me and repeat it. :D
 

sw_ross

Senior Member
Location
NoDak
Feeder wires to sub-panels?

Feeder wires to sub-panels?

My thought, when looking at the photo, was that those wires were what the OP was to use to feed the 2-100 amp sub-panels?

Not sure how he is going to create the breaker space for those 2-100 amp breakers in a mostly full main panel!?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The NEC does not prohibit fused and unfused conductors in the same enclosure.
No it, doesn't but the UL standard for service equipment has been changed to require separation like shown in the picture. The change has been accepted by UL, but not sure when it takes effect for manufacturers.
 
Top