Wiring method for residential feeder

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JDBrown

Senior Member
Location
California
Occupation
Electrical Engineer
Okay, Residential gurus, I'm in need of some assistance. In a single family home where the branch circuits are wired with Romex, what wiring method do you typically use for feeders?

There's an overhead service drop which now does not meet the required clearances because somebody put in a patio without pulling a permit. Since the homeowner is putting in a new A/C he needs a larger service anyway, so we're just going to put the new service in a different location to fix the clearance issues. The sub-panel in the house has accessibility issues (it's buried behind the pantry shelves), so we'll be moving all those circuits to a new sub-panel as well (which will almost certainly mean adding AFCIs). What this means for me is that there's no way we can re-use the existing sub-panel's feeder. Unfortunately, I don't know what wiring method you would normally use for this application, as almost all of my experience is commercial.

My first instinct is to specify SE cable for the feeder, but I'm hoping for some input from those more experienced than I, to either confirm this or point me in the right direction. The house is in Southern California, if that matters.

For those who might be interested in the back story (as I usually am), the customer wanted to install a new A/C, so he calls up his electrician buddy to connect it for him. The electrician takes a look at the service and subpanel, and informs his friend that he's going to have to do a lot more than just connect the new A/C. The trouble is, the electrician hasn't done residential work in over a decade, so he calls up his engineer buddy (who happens to be my boss). The boss delegates to me, and I find myself trying to prepare some residential drawings on my own for the first time. :D
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
A bit more detail might make a difference, such as the relative location of the service and subpanel and the size circuit to the sub.
In this area, unless it's fairly close by,. most E/Cs use SE (SER) cable.
You need to be careful and understand how your AHJ approaches 338.10 (If the 60? rating apples it can make a difference in how you approach the install also.)
 

Redcliffe

Member
Location
NJ USA
My choices would be SER 3 conductors and 1 ground or NM/Romex 3 conductors and 1 ground...length and amperage would determine the choice.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Most guys around here would use aluminum SER cable which would be the cheapest wiring method. NM cable would work too but you're limited in size and the conductors would be copper which is more expensive.
 

JDBrown

Senior Member
Location
California
Occupation
Electrical Engineer
Thanks for the input, guys. It looks like no matter how hard I try to include all of the relevant information in the original post, I always end up forgetting something. :ashamed1:

The new service will be either 150A or 200A (still waiting to hear back from POCO on that one, so the drawings will probably just say "150 Amps minimum). The new subpanel will be 100A, and is about 50' away from the new service. The subpanel feeder will be run through the attic.

I've been looking at Southwire's website, and it looks like I would want to go with either Copper SER Three Conductor with Bare Ground (3#1 & 1#3 EGC), or Aluminum SER Three Conductor with Bare Ground (3#1/0 & 1#2 EGC). Does that sound like the best way to approach this to you guys?

Also, as I understand it, I have to use the 60 degree ampacity column for an SE cable feeder or branch circuit in the attic, per 338.10(B)(4)(a). Am I understanding that correctly?
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
You could also compare the cost of running 1.25" flex metal conduit versus the SER (which may have to be larger with its 60C limitation). A drawback to flex in this size if you need a rather large hole in a 2x4 top plate, so nail plates or other protection will be required. Also depends on how many bends you need to make or else you'll have so many junction boxes it may not save you anything (remember, the conductors are #4 or larger so the junction boxes will be at least 8x8 and maybe 10x10).

The pro of this approach is you can pull THHN wires and use the 75C column for terminations and 90C for thermal derating. You could also have an insulated ground should there be a swimming pool or other specialty issues.

If it was a 55A subpanel, I'd go with 6-3 NM cable. Another possibility is pulling any big loads out of the subpanel (e.g. electric range, clothes dryer, water heater) and limiting it to 55A. Then just run a separate circuit to those things you pulled out.

Finally, do you really need the subpanel? It is usually just as easy to run a bundle of #14, #12, #10, and #8 cables as it is to run a larger feeder (just maintain some spacing so you don't have to derate a "bundle"). But those smaller cables go down a wall much easier, and you'll have having to run them anyway from the subpanel.
 

Ohms law

Senior Member
Location
Sioux Falls,SD
Okay, Residential gurus, I'm in need of some assistance. In a single family home where the branch circuits are wired with Romex, what wiring method do you typically use for feeders?

There's an overhead service drop which now does not meet the required clearances because somebody put in a patio without pulling a permit.

When you say clearances do you mean the height of the service conductors? If that's the case can't you raise the service mast higher? Even if you have to go through a soffit, I don't know if this is your case, you would just have to add one or two guy supports depending on how high the mast go's past the roof line while considering the roof pitch.
 

JDBrown

Senior Member
Location
California
Occupation
Electrical Engineer
You could also compare the cost of running 1.25" flex metal conduit versus the SER (which may have to be larger with its 60C limitation). A drawback to flex in this size if you need a rather large hole in a 2x4 top plate, so nail plates or other protection will be required. Also depends on how many bends you need to make or else you'll have so many junction boxes it may not save you anything (remember, the conductors are #4 or larger so the junction boxes will be at least 8x8 and maybe 10x10).

The pro of this approach is you can pull THHN wires and use the 75C column for terminations and 90C for thermal derating. You could also have an insulated ground should there be a swimming pool or other specialty issues.

If it was a 55A subpanel, I'd go with 6-3 NM cable. Another possibility is pulling any big loads out of the subpanel (e.g. electric range, clothes dryer, water heater) and limiting it to 55A. Then just run a separate circuit to those things you pulled out.

Finally, do you really need the subpanel? It is usually just as easy to run a bundle of #14, #12, #10, and #8 cables as it is to run a larger feeder (just maintain some spacing so you don't have to derate a "bundle"). But those smaller cables go down a wall much easier, and you'll have having to run them anyway from the subpanel.
Thanks for the suggestions; I'll make sure to run those by the boss man. We want to stick with the subpanel because we're trying to avoid re-wiring the entire house (although who knows what will happen when he starts connecting stuff to AFCI breakers?). It's much simpler to re-route the existing branch circuits to a new subpanel a few feet away from the old one.

Thanks for bringing up swimming pools; I had forgotten about the requirement for an insulated ground on the subpanel in that case. This house doesn't have a swimming pool, but it does have an indoor jacuzzi (according to the sketch I've got, it looks like it's about 7' x 4'). Am I correct in thinking that the insulated ground requirement applies to a subpanel feeding an indoor jacuzzi?

When you say clearances do you mean the height of the service conductors? If that's the case can't you raise the service mast higher? Even if you have to go through a soffit, I don't know if this is your case, you would just have to add one or two guy supports depending on how high the mast go's past the roof line while considering the roof pitch.
Yeah, I'm talking about the height of the service conductors. While it might be possible to just raise the service mast higher, the Electrician in charge of the project has already decided he wants to install the new service in a different location. Thank you for the suggestion, though.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
If the tub is a "hydromassage tub", then the pool feeder rules don't apply. If an typical hot tub but mounted inside, code implies that NEC 680 Part II applies unless it is a "staorable spa" (seems to be 42" depth or less).

However, FMC isn't allowed for a pool feeder and you'd probably want to go with EMT in an attic. You could also consider MC cable.
 

renosteinke

Senior Member
Location
NE Arkansas
It's worth having a chat with your AHJ.

I've been several places where the feeder from the outside meter pan (whether there was a fuse or not) to the panel inside was to be in metal pipe. I've also seen the practice of running sub-panel feeds in pipe.

Face it - do you want to risk someone banging a nail into, or attic-walking on, wires with 100-amp fusing?

Code rules aside, in my own home I've run the panel feeds in pipe, while the branch circuits primarily in NM.
 

junkhound

Senior Member
Location
Renton, WA
Occupation
EE, power electronics specialty
Lots of EMT in own and relatives houses. Even some pvc conduit.

Why the aversion to using commercial methods in residential?

Think Chicago still requires conduit in residential (no NM).... (courtesy of Mrs O'Grady and her cow)
 

kwired

Electron manager
Location
NE Nebraska
Lots of EMT in own and relatives houses. Even some pvc conduit.

Why the aversion to using commercial methods in residential?

Think Chicago still requires conduit in residential (no NM).... (courtesy of Mrs O'Grady and her cow)
I agree, some don't seem to think raceways are allowed in dwellings. They also are not as expensive as some think they are. I would probably run PVC in the OP's case and may be able to do it for about the same price as if using SE cable. PVC is fairly easy to install. EMT isn't that bad either but gets a little more difficult if over 3/4 inch. ENT is also a product out there that comes in handy sometimes, but OP would have to use something else for outdoor portions as well as protect it from the heat in the attic.

I guess I am more inclined to use raceways as I use raceways a lot in other work I do and am used to installing them.

I am just about to the point of not wanting to purchase 8-2, 8-3 6-2 or 6-3 NM cable ever again. And I almost never have used anything larger in NM cable. I always end up with so many 10-15 foot pieces left at the end of the reel that I will never use very many of them. Take that times two or three conductors and consider that you are using single conductors in raceways and I have less wasted copper in the long run. I run so much more 6 AWG in raceway at other work I do I would rather run a raceway in a dwelling when the need comes up for 6 AWG and I have run many more raceways for such thing in more recent years.

I recently finished a basement in a house - it was a newer home but basement had never been finished yet. It had 9 foot walls so that ductwork and plumbing could be within suspended ceiling space. I figured why make all my home runs in NM cable and have to drill hundreds of holes through floor joists. I ran about 4 runs of EMT to different areas and all the home runs are in these raceways (including a kitchen at the opposite corner of the floor as where the panel is located so you have two SABC's minimum, a range and a lighting circuit all running nearly the entire length and depth of this house, the raceway means only one EGC or even less copper if using the raceway as the EGC, no holes to drill and you only run the raceway once and pull conductors once. Sure seemed like a time and material saver to me.
 
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JDBrown

Senior Member
Location
California
Occupation
Electrical Engineer
Thanks for all the input, guys. The rule about raceway and insulated EGC for a subpanel that supplies a hot tub was something of a game changer.

I'm going to inform the EC that he has to use an insulated EGC and one of the wiring methods listed in 680.20, and then leave it up to him which method he chooses to use. I expect he'll probably go with EMT since that's what he uses all the time out at the airport, where he does most of his work. But since he's our client on this job (as opposed to being the EC hired by our client, as he is on other jobs), I'm trying to just tell him what the Code requires and then let him do the installation using whatever legal method will be easiest/cheapest for him.

Thanks again.
 
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