10-3 Wire Ok For 100 Amp Sub

Reply

  #1  
Old 03-02-06, 04:05 PM
Member
Thread Starter
Join Date: Jun 2001
Location: Deer Park, N.Y.
Posts: 155
10-3 Wire Ok For 100 Amp Sub

I have a 200 amp main panel and i wanted to install a 75 ft run to a 100 amp lug subpanel in the basement.
is #10-3 wire good enuf?
no major circuits going into this subpanel, just lighting, receptacles, and a dedicated electric heat line, all 12-2 wiring on 20 amp breakers...
 
Sponsored Links
  #2  
Old 03-02-06, 04:14 PM
Speedy Petey's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Nov 2003
Posts: 2,455
Yes. It's good enough for 30 amps.

Does it make sense? No.
30 amps is not nearly anough for a sub-panel with the type of circuits you describe.
A better bet would be to use 6/3 and use a 60 amp breaker in the main.
 
  #3  
Old 03-02-06, 05:40 PM
ElectricalMan's Avatar
Member
Join Date: Feb 2005
Location: Harrisonburg
Posts: 744
I agree with Speedy on this. If you are going to be running the wire anyway I would have to agree with again speedy's choice.
 
  #4  
Old 03-02-06, 06:11 PM
Member
Join Date: Nov 2005
Location: North Carolina
Posts: 948
10/3 makes a real good extension cord. go with pete's # 6 recomendation
 
  #5  
Old 03-02-06, 06:14 PM
ElectricalMan's Avatar
Member
Join Date: Feb 2005
Location: Harrisonburg
Posts: 744
lol.....not if it is Romax 10/3 it wont make a good cord..not flexible, not protected from physical damage and the cord caps are probably not rated for NM..........lol..So best bet not to use it as an extention cord...
 
  #6  
Old 03-03-06, 07:38 AM
Member
Thread Starter
Join Date: Jun 2001
Location: Deer Park, N.Y.
Posts: 155
thanks for the replys guys...
so let me get this straight, i want a 100 amp main feed to the sub and to do so, i need #6/3 wire?
hmmmmmmm
glad i came here first, i must have read wrong somewhere that #10-3 was good
 
  #7  
Old 03-03-06, 07:54 AM
Member
Join Date: Aug 2004
Location: NA
Posts: 1,065
So let me get this straight, i want a 100 amp main feed to the sub and to do so, i need #6/3 wire?
No.... #6 awg copper will give you 60 amps (reread Speedys post) to the 100 amp panel. If you need the rated amperage of the panel (100 amps) you need #3 awg copper thhn or an SE cable. (Ampacity using NEC 310.16)
 
  #8  
Old 03-03-06, 08:26 AM
Member
Join Date: Nov 2005
Location: Greeley, Colorado
Posts: 135
What size would the non-current-carrying conductor (EGC)
from the main to the sub be? Is 6 AWG okay, or should it be 4 AWG? I know you
can go bigger than needed, but I was just concerned with cost.
I too have a 200 amp main feeding a 100 amp sub, only
about 8 feet away from the main panel.
Thanks in advance.
 
  #9  
Old 03-03-06, 08:30 AM
Member
Join Date: Sep 2000
Location: United States
Posts: 18,497
Michael, don't confuse the rating of the panel with the rating of the feeder. A 100-amp panel can handle a feeder rated anything at 100 amps or below. The actual rating of the feeder is based on the size of the breaker to which it is connected in the main panel, which in turn determines the size of the wire you need.

The size of the required EGC is based on a table in article 250 of the code. It is based on the size of the overcurrent protection and the size of the current carrying conductors. Willg54, if you provide both those pieces of information, we can answer your question (in a new thread please, so as not to confuse this one).
 
  #10  
Old 03-03-06, 08:32 AM
Member
Join Date: Jan 2006
Posts: 131
minimum size of the egc is based on 250.122 and goes by the size of the overcurrent device. it shall not be required to be larger than the circuit conductors. where a raceway or armor is used as the egc it shall comply with 250.4(A)(5) or (B)(4) i didnt mean to repeat john as i posted right after him
 
  #11  
Old 03-03-06, 02:36 PM
Member
Thread Starter
Join Date: Jun 2001
Location: Deer Park, N.Y.
Posts: 155
great info guys, thanks a bunch. just one more, i promise....


my subpanel is needed to feed 6 circuits in my basement for lighting, receptacles, and a 240 volt electric heater...
the only upgrade in power needs will be central air in a year or two.

will i need to upgrade from a 60 amp subpanel using 6/3 wire as well?
or will this be more than enuf?
 
  #12  
Old 03-03-06, 03:54 PM
Member
Join Date: Jan 2006
Posts: 131
since you have the 100 amp sub already i would go with a 100 amp breaker and run 3-2's plus ground. in your first post you mention no major things going in and now you mention central air.
 
  #13  
Old 03-03-06, 03:59 PM
bolide's Avatar
Member
Join Date: Jan 2006
Location: PA
Posts: 1,909
my subpanel is needed to feed 6 circuits in my basement for lighting, receptacles, and a 240 volt electric heater... the only upgrade in power needs will be central air in a year or two.
What is the total load for lights and receptacles?

> will i need to upgrade from a 60 amp subpanel using 6/3 wire as well?

I thought you said you have a 100A subpanel.

6/3 cannot be upgraded above 60A.


> or will this be more than enuf?

It's barely enough.
It you are going to all this trouble, why not just run 100A amp wire (possibly 4-4-4-6 copper), use 100A breaker, and have a 100A subpanel now?
Then you won't have to worry when you add new loads.
 
  #14  
Old 03-04-06, 06:38 AM
Member
Thread Starter
Join Date: Jun 2001
Location: Deer Park, N.Y.
Posts: 155
boy, this thread has gone too long thanks to my confusing the hec out of everybody, im suprised you all still write back to me anymore hehe.

im sorry for all the confustion guys. and to make matters clear for the last time, this is what i wish to do...

install a 100amp subpanel using a 100amp main breaker inside my main panel (which is 200amps)
and i will go with #3/2 wire with ground ..
what size ground cable should i use?
 
  #15  
Old 03-04-06, 07:31 AM
Member
Join Date: Jan 2002
Location: Chicago
Posts: 1,915
You need a 4 wire feeder for a subpanel. 2 hots, 1 neutral and 1 ground.
 
  #16  
Old 03-04-06, 07:40 AM
Member
Thread Starter
Join Date: Jun 2001
Location: Deer Park, N.Y.
Posts: 155
ok, but what size wire? # 3? or #4?
 
  #17  
Old 03-04-06, 08:09 AM
Speedy Petey's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Nov 2003
Posts: 2,455
In most areas you can use #4cu or #2al for a 100 amp feeder.
Unless your local inspector is uptight and doesn't use chart 310.15(B)(6).
In that case you need to use #3cu or #1al.
 
  #18  
Old 03-04-06, 08:17 AM
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Electrical installations are all about the details. The answer to your individual questions will probably not tell the whole story. There are almost certainly details that you don't even know how to ask about.

What you need to do is a 'load calculation' (also called a feeder calculation or a demand calculation) to see how large a feeder to this subpanel is necessary. _Or_ you need to simply oversize the feeder to the subpanel to take care of all reasonably possible future uses. The big unknown is the size of the central air unit, and you never specified how large the 240V heater is.

If you choose the latter approach, then just install a 100A subpanel. There is a _very_ good chance that this subpanel and feeder would be 'overkill', and that a 60A subpanel would be sufficient, but if you include the central AC the 60A subpanel becomes questionable. If you sit down, and calculate out _now_ what size AC you would require (how many 'tons' of cooling), and then get the circuit ratings for AC systems of that size, then you could calculate out how much of a feeder you really will need.

Once you pick the 'ampacity' of feeder that you need, you then need to select the correct wire size to use. But even here there are details to be considered. The ampacity of a conductor is _not_ just based upon its size. You must also consider the temperature rating of the insulation, the wiring method used, and the temperature ratings of the terminations. You must also consider the rules and interpretations of your local code enforcement authority.

Some regions _require_ that all wires be run in conduit, and prohibit the use of non-metallic cables. Other regions permit you to use table 310.15(B)(6) for residential _feeders_. Still others permit non-metallic cables, but do not permit the use of 310.15(B)(6). Depending upon your specific circumstances, you may need to use anything from a cable with 3#1 conductors and 1#6 conductor, to a cable with 4-4-4-6, to individual wires in conduit.

My best guess, given insufficient information, is that you should use type SER cable or individual conductors in conduit, using #3 copper for the hots, #3 copper for the neutral, and #8 copper for the EGC. Protect the cable using a 100A breaker at the main panel, and have a 100A breaker at the subpanel, or use a 'lugs only' subpanel with no main.

You should find out from your local inspection authority if nec 310.15(B)(6) is permitted for residential subpanel feeders (as written, it only applies to the _main_ power feed to a residence, not to subpanels, but many localities permit it for subpanels), and you should find out if there are any restrictions on the wiring methods that you may use. You should also look at your main panel label, and determine if the breakers are rated at 60C or 75C termination temperature. With this information we will be able to refine our answers to you.

-Jon
 
  #19  
Old 03-04-06, 08:55 AM
Speedy Petey's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Nov 2003
Posts: 2,455
Originally Posted by winnie
(as written, it only applies to the _main_ power feed to a residence, not to subpanels, but many localities permit it for subpanels)
NOT TRUE. This section does not explicitly exclude sub-panels as you state. This is one of the main reasons this issue has been debated for years with no clear resolution. The way it is written it can be interpreted either way with very convincing arguments in both directions.

I don't want to start a new discussion about this which is why I stated it could go either way.
Stating one way is right and the other is wrong is a sure way to keep this argumant alive. I'm sure the mods would love that.
 
  #20  
Old 03-04-06, 09:07 AM
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
More than fair enough. I did mis-state: the article in question permits the use of these reduced size feeders to the _main_ panel, and also to any subpanels that constitute the _main_ power feed to a residence, and doesn't say anything about subpanels that are _not_ the main power feed to the residence. Some would read this silence as meaning that you can use 310.15(B)(6) for other subpanels; others would say that since other subpanels are not mentioned then the standard sizing rules (310.16) apply. We could spend lots of time arguing in circles, but I think that both Speedy and I agree that the only entity who's opinion actually matters is that of the inspector.

So I stand pat on advising the OP to call the local inspector, and get the rest of the details, and then we can give more specific answers

-Jon
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
'