square d subpanel - isolating neutral / ground

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  #1  
Old 01-09-07, 07:12 AM
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Smile square d subpanel - isolating neutral / ground

hi all-

i am hoping to install my subpanel today (from my square d 200 amp main to 100 amp sub panel w/ breaker).

i know i need to isolate ground and neutral in my subpanel, but i am uncertain as to how to accomplish this task on the square d (qo series) panel?

can you look at the picture and provide me with some comments?

thanks!

kevin

http://img443.imageshack.us/my.php?image=squaredv2rc9.jpg
 
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  #2  
Old 01-09-07, 10:48 AM
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You will need to install an auxillary ground bar kit. This may or not be included in the panel itself, but are easily purchased seperately. This would install in the tapped holes located in the gutter space of the panel.

You would NOT install the green bond screw.

NOTE: You should have run a 4 wire feed to this sub-panel if it is in the same structure as the main panel.
 
  #3  
Old 01-10-07, 01:12 PM
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Smile

Thanks pcboss!

For future reference to others, this is my setup:

1. One, QO132M100C QO 100 Amp 32 Space/32 Circuit Main Breaker Load Center used as a sub-panel (100 Amp main breaker pre-installed)

2. Two, PK18GTA ground bar kits (above panel already has two neutral bus bars that are bonded to each other but isolated from the panel frame/ground)

3. Used 70 ft of #4 copper romex (black, white, red, bare ground) to connect to main panel, mated to a double pole, 100amp breaker

Kevin
 
  #4  
Old 01-10-07, 01:25 PM
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> #4 copper romex...100amp breaker

#4 Romex (NM-B) is limited to 70A -- you should replace the 100A breaker with a 70A breaker.

Table 310.15(B)(6) does not apply to NM cable.
 
  #5  
Old 01-10-07, 01:47 PM
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thanks ipbooks but I am confused;

When I spoke with my township's electrical inspector, he said #4 is fine with 75ft as did the counter person at the distributor where i bought the wire.

The wire i purchased has a black sheathing, and has the four cables i mentioned earlier? I also used one of those javascript wire size calculator and it says the minimu size i should use is #8?

Am I describing the wire correctly or did I just waste $240 bucks buying this wire (I need the 100amp service)?
 
  #6  
Old 01-10-07, 01:54 PM
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The type of insulation on the cable is very important, and it should be printed on the cable jacket. If it is type SE or USE, then it is good for 100A as installed. Types NM or UF (commonly called "Romex") are only good for 70A.

If the inspector passed it I suppose you can let it slide, but that wouldn't pass in many jurisdictions assuming you actually have NM or UF.
 
  #7  
Old 01-10-07, 04:31 PM
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Ib is totally correct here....

You sound like you are speaking of something like 4/3 w G which is romex...the NEC says using Section 310.16 that since you are using less than # 1 AWG....the ampacity table for your application is 60 degrees based on what you are telling us.

In that understanding IB is correct...is 70A and would not allow a 100A OCPD to protect it...in this case your local inspector is incorrect.

(a) Termination provisions of equipment for circuits
rated 100 amperes or less, or marked for 14 AWG through
1 AWG conductors, shall be used only for one of the following:
(1) Conductors rated 60C (140F).
(2) Conductors with higher temperature ratings, provided the
ampacity of such conductors is determined based on the
60C (140F) ampacity of the conductor size used.
(3) Conductors with higher temperature ratings if the equipment
is listed and identified for use with such conductors.
(4) For motors marked with design letters B, C, or D, conductors
having an insulation rating of 75C (167F) or
higher shall be permitted to be used, provided the ampacity
of such conductors does not exceed the 75C
(167F) ampacity.

(b) Termination provisions of equipment for circuits
rated over 100 amperes, or marked for conductors larger
than 1 AWG, shall be used only for one of the following:
(1) Conductors rated 75C (167F)
(2) Conductors with higher temperature ratings, provided
the ampacity of such conductors does not exceed the
75C (167F) ampacity of the conductor size used, or
up to their ampacity if the equipment is listed and identified
for use with such conductors

Either way it sounds like the inspector is using 310.15(B)(6) which in his case he is again..incorrect.

Now I would have run # 1 AWG AL SER....which with the cost of copper would run you much less....and be perfectly fine for 100A
 
  #8  
Old 01-10-07, 06:40 PM
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Question

Hi-

I just got home and checked the cable and it says "NM-B 4 W/G CERTEX UL"

http://img150.imageshack.us/my.php?image=subpanelwiregh8.jpg

In addition, I spoke (again) with a (different) township electrical inspector and discussed the "finer points" between SER/SE and NM or Romex. He confirmed that that he was basing his approval on 310.15.b and that #4 copper with 75 ft was perfectly okay for the 100 amp sub panel.

I tend to be cautious and try to go "above" code whenever I can so I am perplexed as to what is the right thing to do here. Is this a matter of difference in opinion as to how the NEC codes/sub sections are interpreted?

Please let me outline exactly how this installation was planned so I would greatly appreciate your comments/suggestions:

1. 200amp main panel is located in the garage (where its unfinished attic is fully accessible)

2. I planned on routing the new subpanel feeder cable across the attic floor, then drop it behind a kitchen drywall into the basement (~70 ft altogether)

3. Then I planned to connect the cable to a subpanel with 100amp breaker

I would like to note that the attic space is unheated and it gets hot/cold during the summer/winter.

Because I am planning to feed the majority of circuits for my new kitchen as well as amp-intensive appliances like sump pump/treadmill/furnace from this subpanel, I would really like to keep the 100amp breaker and NOT downsize to a 70amp breaker.

So, what is the correct wire size I should use in this scenario? Please keep in mind that I prefer NOT to use a conduit since the attic space is not used.

what are the pros and cons between aluminum or copper when using as a feeder cable?
 
  #9  
Old 01-10-07, 11:48 PM
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Hello

Though this follows some very experienced members, I thought maybe I could spend some time explaining the seventy amp requirement for NM-B. First thing I would like to mention is it is common in some parts of the country for local inspectors to allow #4 copper to carry 100 amps to a sub-panel using table 310.15(b)(6) normally used only for dwelling "main" power feeders. There is always a lot of controvery allowing this table to be used for sub-panel feeders. However I do not believe I have ever heard of any inspectors allowing #4 NM-B to carry 100 amps for a sub feeder. Article 334 of the nec covers the requirements for nm-b cable. NEC Art. 334.80 specifically states that the ampacity of nm-b shall be that of a 60 C conductor... which from table 310.16 is 70 amps. You actually need a #3 copper cable that is not a restricted 60 C conductor to get 100 amp capability. An Ser cable in #4 copper would only give you 85 amps and it is a 75C conductor for ampacity purposes, unless table 310.15(B)(6) is allowed for the SER cable.
It would be very difficult for inspectors to allow nm-b to carry 310.15(b)(6) amperages. They would have to ignore a rather specific restriction set forth in 334.80 to do so. In fact they have to be very flexible to allow 310.15(b)(6) for any sub panel feeders.

For your interest look here......

http://appprod.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet6

look at the ampacity for #4 nm-b per the manufacturers listing and also the fine print at the bottom of the table as it specifically points out 334.80.

Now look here for an SER rated cable in copper.....

http://appprod.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet11

Notice #4 using the 75C column is rated 85 amps but notice there is another column called "dwelling" this shows 100 amps and is specifically utilizing 310.15(B)(6) to allow 100 amps as a main power feeder to the "main" panel of a dwelling. Also note that the "dwelling" column does not appear for NM-b cable. Only ampacities from the 60 C column of table 310.16 appear for nm-b cable.

Now all that said I really think that 70 amps is going to serve your needs if you consider diversity of loads. For example 70 amps will allow for 16,800 watts of power. Almost all range/stoves are allowed 8000 watts for load purposes, dishwashers 1200 watt, waste disposal 1000 watts, sump pump 600 watts, professional grade treadmill 1800 watts, microwave (over range) 1500 watts....etc. Bear in mind that all of these would have to be in operation at the same time in order for the loads to be additive. So lets say they were all in use you would have 8000 + 1200 + 1000 + 600 + 1800 + 1500 = 14,100 watts, which is reasonably close to having the feeder loaded to 80%. Of course your loads will be different and I'm not covering every possible screnario but maybe you see my point. 70 amps is a pretty good power supply.

Since it is unlikely that you can return the NM-b I would just use a 70 amp breaker to protect the nm-b and see how things go. The 100 amp main breaker in the new sub can be your disconnect for that sub panel.

Roger
 

Last edited by Roger; 01-11-07 at 12:09 PM.
  #10  
Old 01-11-07, 02:03 AM
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ElectricalMan - just for clarification, would you please provide a reference point for the NEC information you provided?

When you say "Section 310.16", did you mean "Table 310.16"?

Also, the code portions you provided:

(a) Termination provisions of equipment for circuits
rated 100 amperes or less, or marked for 14 AWG through
1 AWG conductors, shall be used only for one of the following:
(1) Conductors rated 60C (140F).
(2) Conductors with higher temperature ratings, provided the
ampacity of such conductors is determined based on the
60C (140F) ampacity of the conductor size used.
(3) Conductors with higher temperature ratings if the equipment
is listed and identified for use with such conductors.
(4) For motors marked with design letters B, C, or D, conductors
having an insulation rating of 75C (167F) or
higher shall be permitted to be used, provided the ampacity
of such conductors does not exceed the 75C
(167F) ampacity.

(b) Termination provisions of equipment for circuits
rated over 100 amperes, or marked for conductors larger
than 1 AWG, shall be used only for one of the following:
(1) Conductors rated 75C (167F)
(2) Conductors with higher temperature ratings, provided
the ampacity of such conductors does not exceed the
75C (167F) ampacity of the conductor size used, or
up to their ampacity if the equipment is listed and identified
for use with such conductors

where in the NEC did these come from? Knowing the reference will help enlighten all of us.

Thanks!!

Kevin2010: if you need the extra amperage, you might consider one of the following:

1. If there is space in the panels, install another feeder and associated OCPDs sized to provide the additional current you need from the main to the sub.
2. install an additional 30 amp subpanel, fed from your main.
3. install some of the new branch circuits you need directly from your existing main panel.

Best wishes!
 
  #11  
Old 01-11-07, 03:09 AM
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Thanks for the feedback folks.

Just one more question: if I were to purchase a new set of wires to properly feed 100amp subpanel (keeping in mind that the feeder cable will be traversing through the attic space where it can easily exceed 100 deg. fahrenheit during the summer months), what is the correct wire sizing I need to go with? It would be great if you can provide the type and sizing for both copper and aluminum...

thanks!

kevin
 
  #12  
Old 01-11-07, 10:30 AM
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Sorry to put you through this hassle so late in the game of your project.

As for my recommendation, I will echo Roger. Just replace the 100A breaker with a 70A; 70A is really a lot of power. If you ever trip the 70A, then consider replacing the feeder with a larger one, but until then I'm confident that 70A will meet your needs.


> if I were to purchase a new set of wires to properly feed 100amp subpanel

Based on proper application of 310.15(B)(6), you can use an aluminum type SE cable of size #2-2-4-6; or a copper SE cable of size #4-4-6-8 (minimum sizes reflect hot-hot-neutral-ground). These sizes also apply if you were to install THHN conductors in conduit.

To stay with type NM cable, you would need #2 copper (95A ampacity which may be rounded up to 100A).

> what are the pros and cons between aluminum or copper when
> using as a feeder cable?

Copper: Now much more expensive, but you can use a smaller AWG which is a bit easier to work with. It's more resistant to water damage and corrosion, and requires no special technique to terminate.

Aluminum: Cheaper, but the larger AWG is more cumbersome to work with. Easily damaged by water if insulation is ripped/cut. Requires care when terminating: strip wire, polish conductors shiny with wire brush/emory paper, apply a lot of non-oxidation grease, torque lug to manufacturer's spec.

Both are safe when installed and sized correctly.
 
  #13  
Old 01-11-07, 11:30 AM
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Electricalman is refering to NEC 110.14(C)(1)(a) and (b). Calculating ampacities involves a lot more than wire size and there really isnt a short answer. NEC 110.14 is about temperature limitations as associated with the ampacity of conductors and their insulatuion ratings. It is also about the temperature ratings of the equipment to which the wires are connected. Other issues such as what the wire serves, derating, limitations as to smaller guages of wire like #14,#12 and #10 copper and aluminum plus other factors come into play.....


Specifically for a sub-panel feeder..... (not considering table 310.15(b)(6).... or downsizing the neutral as Ben shows in his examples......

1.) Modern load centers and circuit breakers are terminal rated to be connected at 75C. This means that the wire must have at least a 75C insulation rating to be connected to the equipment. NM-b is 90 C rated but for ampacity purposes it can only use the 60 C ampacity ratings of table 310.16. Further all 90 C rated wire cannot use more than the 75C rated ampacities of table 310.16 in virtually all cases. 90 C can only be used for derating purposes and ambient adjustments if the wire is so rated.

2.) Panels can be fed at their listed rating or less (in your case 100 amps) and usually in a case like yours 60 amps is the minimum by code.

3.) In order to use the 75 C ampacity of table 310.16 you must use wire and a wiring method on equipment that permits that temperature rating.

4.) Calculation of ampacities are determined by art. 310.15 and others so it isnt a simple deal of just looking at wire size.

5.) Your must know the application limitations and insulation ratings of the wire you choose. Not as simple as it sounds.

6.) Copper is a better conductor and can be used in smaller sizes than aluminum for the same ampacity. Cost however is higher. Example.... to provide 100 amps to a sub-panel using at least 75C rated wire (excluding nm-b and uf-b) and 75C rated equipment you would need a #3 awg copper conductor or a #1 awg aluminum conductor. You must also be sure to use wire that can be used inside a dwelling which generally means it must be fire retardant.

Examples for a 100amp feed to a sub-panel from a main panel using table 310.16.

1.) Nm-b rated 90 C dry location ......must use 60 C ampacities...you would need an nm-b cable with #1 awg copper conductors....problem.... they dont make nm-b of that size.

2.) Ser copper (service entrance round type) 90 C rated dry or wet location...can use 75 C ampacities if terminations (art.110.14(C)(1)(a) allow.
You would need #3 awg copper conductors. Typically the cable will be a 3-3-3-4 awg. The ground may vary by manufacturer but will not be less than #8 copper for 100 amp ocpd.

3.) Thhn/thwn 90 C wet or dry rated individual wires in conduit. Can use the 75C ampacities if equipment allows. You would need #3 awg copper for the 3 conductors H-H-N and a #8 copper ground if using 100 amp protection. The ground wire is sized according to table 250.122 based on the ocpd upstream protecting the feeder also must take into account whether copper or aluminum. If aluminum you would need a #6 awg ground.

4.) NM-b is not made in aluminum anymore you might find some but it would be very unusual.

5.) Ser aluminum you would need a cable that has #1 conductors typically 1-1-1-6 awg cable. Some cables will have a #4 ground depending on the manufacturer.

6.) Individual wires in conduit thhn/thwn al... #1 awg conductors and #6 ground wire.

Aluminum must be treated at terminations with anti-oxidant paste and torqued according to the panel and breaker specifications.

If you choose to derate using a ambient of 100 F the ampacity of a 100 amp rated 90 C wire or cable would be derated using the ampacity from the 90 C column. For a copper #3 wire/cable that would be 110 amps x .91 = 100.1 amps... so even derating for attic temperature you still can have 100 amp capability for a #3 copper cable or wire. If that derating would have been for 105 F then you would have had 110 x .87 = 95.7 amps. this would be your new ampacity for that #3 copper wire. If you wanted to have 100 amps then you would have to increase your wire size to #2 awg copper.

Conversely if the ampacity after derating was 105 amps for example you could not use that ampacity for equipment rated with 75 C terminations you would have to use 75C ampacities or 100 amps.

Hope this helps

Roger
 

Last edited by Roger; 01-11-07 at 12:07 PM.
  #14  
Old 01-11-07, 04:37 PM
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Wow, you guys are simply awesome. Thanks for provding me with such insightful comments.

In the end, I decided to bite the bullet and buy a new aluminum SER cable (2-2-2-4) and install my 100 amp sub-panel. Inspectors at my township told me I was wasting money, but I take pride in my work and just wanted to make sure that it was done correctly.

For future readers, please let me restate why I am proceeding with this solution:

1. Initial plan to use 4 AWG NM-B (or Romex) to feed the 100 amp sub-panel was no good because the cable was rated only for 70 AMPS;

2. Thought of using a main-lug-only solution at the sub-panel was no good because of line-of-sight issue (subpanel in the basement, 70 feet away from the main which is located in the garage);

3. Thought of using the 70 AMP breaker at the main and using the already-purchase 4 AWG NM-B was no good because I already purchased a QO subpanel with a built in 100 amp main-breaker (apparently mis-matched breakers are not permissble in my town)

4. Asked Square D to see if they had a main-breaker rated at 70 amps but unfortunately I was told that they don't make this particular item

So, to make the long story short (and after wasting 240 bucks for 70ft of 4 awg NM-B), what are the lessons I learned?

A. Definately post in this forum to get some feedback and comments BEFORE purchasing anything!!

B. Don't assume that township inspectors will always have the correct answer; in my case, I wasn't interested in obtaining solutions to pass inspections, I wanted to build it right

C. Don't put too much value on information given by local electrical supply distributors because it's a crap shoot!

I especially want to thank ipbooks for first commenting on my cable problem; you saved me from tearing down a new wall.

Thanks again guys.

Kevin
 
  #15  
Old 01-11-07, 08:49 PM
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> (apparently mis-matched breakers are not permissble in my town)

How odd that they would care about a ridiculous non-issue like mismatched breaker sizes when it's obvious that the 100A main is used as a disconnect and the 70A as an OCPD, but would blatantly disregard the most basic rules about cable ampacity!

> I especially want to thank ipbooks for first commenting on my cable
> problem; you saved me from tearing down a new wall.

I'm glad you're happy with the outcome, but sorry it cost you a pile of wasted cable -- hope you can recoup some cost selling it in the classifieds or even for scrap.
 
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