Main to Sub: feeders and conduit?

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  #1  
Old 03-27-06, 03:18 PM
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Main to Sub: feeders and conduit?

Main 200amp panel....to. . .
Sub panel in garage, 125amp...
Using 125amp feeder breaker in main panel . .to . . .125amp breaker in sub.
About 75 feet from panel to panel.
Gonna run underground feed in PVC, sch 40 for horizontal(underground),
sch 80 for verticle (below and above ground.)

feeder size recommendations.
Would #1AWG copper be too big, too little, or just right?
Would 2/0 aluminum be too big, too little, or just right?
Would #6 copper EGC be too big, too little, or just right?

Conduit size recommendations for 2 hots, 1 neutral, and one EGC:
Sch 40: 1 1/4 inch for #1 copper: too big, too little, just right?
Sch 40: 1 1/2 inch for 2/0 aluminum: too big, too little, just right?
Sch 80: 1 1/4 inch for #1 copper: too big, too little, just right?
Sch 80: 1 1/2 inch for 2/0 aluminum: too big. too little, just right?

It's simple, for example, your answer might be, "Use #1 copper
for feeders, #6 copper for EGC, 1 1/2" sch 40, and 1 1/2" sch 80."
Okay, thanks in advance!!
 
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  #2  
Old 03-27-06, 03:57 PM
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Why do you need a 125 amp feeder? A 100 amp is more typical.
For 100 amps I would use #4cu with a #8 EGC in 1.5" conduit.

Sorry if I strayed from your reply decorum.
 
  #3  
Old 03-27-06, 04:47 PM
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Well, I haven't done my load calculations yet, 'cause
I don't know just exactly what I'm gonna have in the garage.
I'll probably have a good sized welder, 240v, and a large
air compressor, 240v, and maybe an electric heater, 240v, etc.
So, I wanted to go with a panel a bit bigger than a 100amp.
Don't even know if they make a 125amp panel, but I think they do.
Didn't want to go bigger than a 125amp, 'cause I figured
the feeder breaker would be costly, and, I probably wouldn't need any mor
than 125amps.

What's your take on this?!?
 
  #4  
Old 03-27-06, 06:24 PM
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For a 125 Ampere panel I would suggest that you use 1/0 THHN copper conductors for the three (3) current-carrying conductors and a #6 equipment grounding conductor. This combination of wire would require 1-1/2" schedule 80 PVC conduit.
 
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Old 03-27-06, 06:29 PM
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Even for 125 amps 1/0cu is way overkill. Why so big?
I would use #2cu.
 
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Old 03-27-06, 07:06 PM
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Given that most of the load is double-pole resistive which cannot produce neutral current, there is no need for a full-size neutral conductor. #4 Cu is enough.
I think you'll be fine with 100A.

At 125A breaker probably costs about double what a 100A breaker costs.
You might want one at both ends.

I added up everything and think you fit within 80A.
My opinion is that welder is not a continuous load.
A water heater is not a continuous load unless you leave the water running for 2.5 hours.
An air compressor is not a continuous load unless you paint with it all day or something like that (very unlikely).
Even allowing for 2000W of lighting, I think you're okay with 100A.
 
  #7  
Old 03-27-06, 07:23 PM
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mmmm?? Can't figure out where everyone is getting their wire sizes from. Are you looking on the "Raceway, Cable, or Earth" allowable ampacities of conductors chart, or the "Free Air"
allowable ampacities of conductors chart?
Remember, I am using separate conductors, enclosed in PVC conduit.

And, which column are you looking in, the 60 degree celcius, 70 degree celcius, or 90 degree celcius
column? I'm definitely confused!!
 
  #8  
Old 03-27-06, 08:38 PM
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Look at at 310.15(B) Services and Feeders, and at 250.112 ECG.
 
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Old 03-28-06, 04:53 AM
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Okay, so if it were you, what insulation type might you use? For dry and damp locations, ie., THHN, or, for dry and wet locations, ie., THHW? (in PVC conduit, remember!) If I know which type to use, I will know which column to use, 60 deg., 75 deg., or 90 deg., right? Or, am I way off base. Is there much more information I need to know before I can choose my conductor size, other than the information I gave in my original post?
Thanks . . .
 
  #10  
Old 03-28-06, 09:37 AM
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Any conductor run underground, even in conduit, must be rated for wet locations. Your insulation of choice is THWN; there are other acceptacle insulation types, but those are usually more expensive. You will use the 75 degree rating of the conductor provided that the breaker terminations are rated for 75 degrees; almost all are.

Most inspectors will let you use table 310.15(B) to size a subpanel feeder; others will not. You have to call your local inspector's office to find out before buying the wire.
 
  #11  
Old 03-28-06, 10:40 AM
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> THHN, or, for dry and wet locations, ie., THHW? (in PVC conduit, remember!)

All THHN is THWN.

If you can use the feeder table, min. #2 THHN for 125A, and min. #4 THHN for the neutral.

For the ECG, min. #6 (I say bare stranded soft copper; but green THHN is fine too; solid is harder to pull if there are bends in the conduit, but fine on a mostly straight run).
 
  #12  
Old 03-28-06, 11:55 AM
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Originally Posted by bolide
A water heater is not a continuous load unless you leave the water running for 2.5 hours.
Take a look at 422.13 when you get a chance.
 
  #13  
Old 03-28-06, 12:32 PM
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> Take a look at 422.13 when you get a chance.

No question, there, most things are limited to 80%.
Same goes for cord and plug connected equipment, etc.

I was referring to its demand on the feeder.
I don't believe that when sharing with other non-continuous loads that it is necessary to count it at 125% unless it is the largest load on the feeder.

Of course if hot water will be running for 2.5 hours or more out of any 4 hours, then clearly it is continuous and should be counted that way as demand on the feeder.
 
  #14  
Old 03-28-06, 02:24 PM
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Bolide, I have to disagree. There is no language to support your idea that 422.13 only applies to water heaters when figuring the ampacity of a branch circuit.

It states that a water heater is a continuous load; nothing more, nothing less.

215.2(A)(1) then tells us we must take continuous loads into consideration when sizing the feeder.
 
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