Subpanel project review

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  #1  
Old 02-04-06, 05:42 AM
mxw128
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Subpanel project review

Hello All! I've had many useful comments and answers on this board over the past year or so, (Thanks!) and after seeing so many sub-panel discussions, I figure I'd put the detail of my latest out there for comment. So please let me know if you see anyhting I've missed or got it wrong...

I'm finishing my basement (approx 1100 finished square feet) and although my main service panel (200 Amp service) could accomodate the 7 circuits I'm installing (filling it up), I decided to go the sub-panel route for the added fexibility and expandability. I have a cutler hammer "main-lug" subpanel (i.e. no main breaker. The subpanel itself is rated for 125A max). I installed it so the "mains" are at the bottom of the panel and it is mounted directly next to the service panel. This way I can run my line to the service panel out of the bottom of the load center and into the bottom of the service panel to the feeder breaker. The neutral and ground bus bars come isolated for use as a subpanel and I will not connect them. The bonding bar is NOT connected to the neutral bus.

QUESTION: My service panel is set up in what I would call a "standard"config, with all of the wires and feed coming through the top. I know that you can install it either way (top or bottom cable entry) but is there any convention or "Standard practice" that all of the cable should enter from the top or bottom once you start that way. I figure it's just a matter of convenience, since the feeder breaker will sit near the bottom of the service panel... but I don't WANT to look like a total amateur!

My 7 circuits are:
Two 15A circuits - Mostly lights with one or two outlets
Five 20A circuits.
- 1 of these is the dedicated bathroom circuit and will service only the GFCI, light and fan (so really not much load here).
- 2 more are workshop circuits (occasional use of a radial arm saw or router type of equip) and will proabaly have a refridgerator plugged in to one of these. One of these ciruits also services outlets (4) in a workout room
- 1 will service the bar area an outdoor outlet (all after the GFCI) and a few outlets around the bar
- 1 will service outlets in the general living area including one behind the bar cabinents to service (MAY have a small refridge behind bar)

I tried to keep outlets under a total of 15 for a 20A circuit. Most will not be used at all (there for code compliance) or be used simultaneously.

I don't see my load being too great here with no major appliances apart form the refridgerator, so I'm going to run #4 copper (3 conductor of course..) from the subpanel to the service entrance which should accomodate 80 amps, but I'm going to downrate that and use a 60A feeder breaker. 60 amps should be more than enough, but I figure if I want to or need to upgrade later, the wire will already be in place to bump up to an 80 Amp feed.

What did I miss? I did find a problem with the already installed wiring, that I need to get an opinion on, but that'll be another post. Thanks again!
-Mike
 
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  #2  
Old 02-04-06, 07:34 AM
bolide's Avatar
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Originally Posted by mxw128
I installed it so the "mains" are at the bottom of the panel and it is mounted directly next to the service panel. This way I can run my line to the service panel out of the bottom of the load center and into the bottom of the service panel to the feeder breaker.
Good.

My service panel is set up in what I would call a "standard" config, with all of the wires and feed coming through the top. I know that you can install it either way (top or bottom cable entry) but is there any convention or "Standard practice" that all of the cable should enter from the top or bottom once you start that way.
Top entry was arguably a bad idea since branch circuits usually exit the top.

I figure it's just a matter of convenience, since the feeder breaker will sit near the bottom of the service panel...
There is no rule. It would be nice to have them the same.
But its not clear that this is a concern...

1 of these is the dedicated bathroom circuit and will service only the GFCI, light and fan (so really not much load here).
Wrong. NEC 210.11(C)(3) & 210.23(A) says bath receptacle must be the only outlet - no lights or fan.

2 more are workshop circuits (occasional use of a radial arm saw or router type of equip) and will probably have a refrigerator plugged in to one of these. One of these ciruits also services outlets (4) in a workout room
Sounds overloaded to me. NEC 210.50(B)(1).


1 will service the bar area an outdoor outlet (all after the GFCI) and a few outlets around the bar
Could be same problem.


1 will service outlets in the general living area including one behind the bar cabinets to service (MAY have a small refridge behind bar)
Same problem.



I tried to keep outlets under a total of 15 for a 20A circuit. Most will not be used at all (there for code compliance) or be used simultaneously.
True. And there is no limit.
But when you have more than 6-10, you can guess that you are asking for trouble. What if you put a treadmill in the exercise room?
What if you bring in a live band? What if you need power outside?



I don't see my load being too great here with no major appliances apart form the refrigerator, so I'm going to run #4 copper (3 conductor of course..)
You need 4 conductors.


from the subpanel to the service panel which should accomodate 80 amps,
100A.

but I'm going to downrate that and use a 60A feeder breaker.
Okay, any reason other than cost?




Anyway, since the panels are adjacent, are you planning just to use the main lugs in the subpanel?
 
  #3  
Old 02-04-06, 10:46 AM
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The very short distance between the two enclosures does not justify a Feeder rated at less than 100 amps.
 
  #4  
Old 02-04-06, 11:25 AM
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Your one 20 amp circuit serving the bathroom is fine. As long as the circuit serves nothing outside one (and only one bathroom) it may serve the lights, fan and receptacle in the bathroom.
 
  #5  
Old 02-05-06, 07:44 PM
mxw128
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Originally Posted by racraft
Your one 20 amp circuit serving the bathroom is fine. As long as the circuit serves nothing outside one (and only one bathroom) it may serve the lights, fan and receptacle in the bathroom.
Yep, that's the way I read the exception in the NEC too. (and it is the way I wired it, with only the bathroom on that one 20A circuit)

Reagrding the ampacity of the #4 copper: It seems there are some recomendations here to use a 100 Amp breaker. According to to what I see in the NEC, Table 310.16 (since this is cable, not single conductor) the max ampacity for four gauge copper reads as 95 amps. Even if I correct for a cooler ambient temp, it looks like I could only do 4% better or just under 99 amps. According to table 4-1 of Wiring Simplified, it also stated 95 amps for #4 copper. (Also, when I called the county with an estimated max load of 80 amps and asked what size wire they would approve, they said #4 copper or #3 aluminum.) Am I using the tables in the NEC incorrectly?

Another comment that I received was regarding "3 Conductor wire" vs "4 Conductor wire" I may be using incorrect terminolgy here, when I say 3 Conductor, I'm excluding the ground wire. So what I'm using has 3 currect carrying conductors (a black and red hot, and a white neutral) and a bare ground. From the comment, it sounds like someone suggested "4 wire cable". Do they make romex like that? What would you use another conductor for? We may be saying the same thing....

Thanks for the input!
 
  #6  
Old 02-05-06, 08:10 PM
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ampacity of feeder conductors

ampacity of the #4 copper: It seems there are some recomendations here to use a 100 Amp breaker.
Under 800A, you are allowed to round up to the next higher standard breaker size.


According to to what I see in the NEC, Table 310.16
Do you like Table 310.15(B)(6)?




> (Also, when I called the county with an estimated max load of 80 amps
> and asked what size wire they would approve, they said #4 copper or
> #3 aluminum.)

This is completely correct!!


> Another comment that I received was regarding "3 Conductor wire" vs
> "4 Conductor wire" I may be using incorrect terminolgy here, when
> I say 3 Conductor, I'm excluding the ground wire.

The NEC is muddled on this point.
The EGC is a conductor, but not a current-carrying conductor.


> So what I'm using has 3 current-carrying conductors
> (a black and red hot, and a white neutral) and a bare ground.

That is fine. I believe that the ECG can be sized from Table 250.122.



> From the comment, it sounds like someone suggested "4-wire cable".

I did not.


> Do they make romex like that?
Yes.
 
  #7  
Old 02-06-06, 09:52 AM
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There is a bit of confusion here.

1) The ampacity for #4 copper from table 310.16 is _70A_ for 60C temperature limits, and 85A for 75C limits. You cannot use the 95A rating because of the 75C limits on the _terminations_ involved.

2) Table 310.15(B)(6) applies to the 'main power feeder' in a residence, and not to any other feeders. My literal reading of the code is that you cannot use 310.15(B)(6) for an arbitrary subpanel in a home. That said, many inspectors and inspecting authorities do permit 310.15(B)(6) to be used for subpanels, and while I personally would not use 310.15(B)(6) for a residential subpanel, I am not particularly uncomfortable with this use, since many residential subpanels are very lightly loaded.

What 310.15(B)(6) essentially provides is a list of conductors which may be used for _residential_ _services_ of a given amp rating. This is not a table of revised conductor ampacity; but rather a statement that for residential services of calculated load X or lower, protected by a breaker of trip rating X, conductors of ampacity somewhat less than X are permitted. A 100A residential service, protected by a 100A breaker, may be served using conductors of 85A _continuous_ ampacity.

3) You cannot simply 'round up' to the next larger breaker. The NEC permits that if a conductor has an ampacity of X (say 85A), and the load is _less_ than X (say a calculated load of 80A), then you may round up the _protection_ to the next standard size (eg 90A). This does not mean that the conductor may be used at 90A; it is still limited to 85A. But if the _load_ is determined to be less than the 85A ampacity of the conductor, then the _breaker_ may be increased in size.

4) Given the extremely short distance between the main and subpanel, I see no reason not to use a 100A breaker and #3 copper conductors for hot and neutral, and a #8 copper EGC. There are ways in which you can reduce the size of the neutral circuit conductor, however if you are only installing single pole breakers I would not do this.

5) Given the short distance and the size of the conductors, I suggest that you use conduit and loose conductors rather than a cable.

-Jon
 
  #8  
Old 02-06-06, 03:08 PM
mxw128
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Originally Posted by winnie
There is a bit of confusion here.

4) Given the extremely short distance between the main and subpanel, I see no reason not to use a 100A breaker and #3 copper conductors for hot and neutral, and a #8 copper EGC. There are ways in which you can reduce the size of the neutral circuit conductor, however if you are only installing single pole breakers I would not do this.

5) Given the short distance and the size of the conductors, I suggest that you use conduit and loose conductors rather than a cable.

-Jon
Thanks for the reply Jon! I Already have the #4 copper cable (based on input from the county for an estimated 80A max load). So what would be the max breaker size I could use with it?
 
  #9  
Old 02-06-06, 05:00 PM
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I would confirm that your calculated load is less than 85A, and then I would use a 90A breaker.

-Jon
 
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