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# Maxing Empty Flex Conduit to the main panel

#1
02-25-07, 01:17 PM
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Join Date: May 2005
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Maxing Empty Flex Conduit to the main panel

Hi,
My main electrical service was recently upgraded to 200A and the electrician installed 2 @ 1" flex conduits for future circuit expansion (they are currently empty). Given this:

1) how many 20A circuits could I run on each flex legally or am I limited by space?
2) if I want to add a subpanel, will I be able to run the #4 wires thru this flex in order to route them to the panel?

Thanks much for any information.
Jason

#2
02-25-07, 02:05 PM
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Join Date: Apr 2006
Location: New Bern, NC
Posts: 1,623
is that metal conduit?

OOps I just looked and it does not matter.

The most number 4 wires you can put in that conduit is 3. You would need at least 3 and a ground for a sub panel.

The limit on the 20 amp circuits would be when you need to de-rate for ampacity. Four circuits would be the most before you would need to put a number 12 wire on a 15 amp breaker.

If you use number 10 wire you can get 7 circuits on 20 amp breakers.

(i did not share neutrals for any calculation.)

Edit: correction. the flex can get 4 number 4, or the 3 number 4 and one number 6 that you would need for a sub panel.

#3
02-25-07, 02:07 PM
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For finding conduit conductor fill allowances, you may use the tables in Annex C of the NEC. In the case of flexible metal conduit (FMC), you would use Table C.3

Using the information you have provided, here are a few examples:

Question 1: assuming the use of type THHN #12AWG insulated conductors, Table C.3 indicates a total of 24 conductors allowed in a 1-inch FMC. Assuming three wires per circuit, the total would be eight 20A circuits.

Question 2: assuming the use of type THHN #4AWG insulated conductors, Table C.3 indicates a total of four conductors allowed in 1-inch FMC. This is the number of conductors you will need for a subpanel.

Check the allowable ampacity of conductors based on the type used in Table 310.16 of the NEC.

Best wishes!

#4
02-25-07, 03:49 PM
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Location: welland ontario
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You could fit 24 conductors but you could not use them for 20 amps if you put that many in there. You would need to derate them after 9 conductors.

#5
02-25-07, 06:30 PM
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ok guys. isn;t there a rule about less than a 24 in nipple that the conductors do not need to be derated?

Don't have a code handy but it seems to ring a bell.

If true, we would need to no how long the flex is before advising the requirement of derating

#6
02-25-07, 08:11 PM
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Nap, you are correct about the the 24-inch exception (see NEC 310.15(B)(2) exception no. 3).

jdu - adjustment factor information can be found in NEC 310.15(B)(2). There are a variety of things that affect deratings, so you'd have to provide more details about the type of conductors (e.g., THHN), and what the circuits would be used for before an exact number of circuits at the desired ampacity could be computed.

With this in mind, let me re-phrase my last post: according to Annex C, Table C.3, you may fit 24, 12AWG THHN conductors in a 1-inch FMC. However, while 12AWG is used for a 20A circuit, the actual number of 20A circuits you may safely support with 12AWG THHN conductors in 1-inch FMC depends on calculating any required adjustment factors.

As for the subpanel feeder issue, I think you are OK with four THHN 4AWG conductors in the 1-inch FMC.

#7
02-25-07, 08:25 PM
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Originally Posted by nap
ok guys. isn;t there a rule about less than a 24 in nipple that the conductors do not need to be derated?

Don't have a code handy but it seems to ring a bell.

If true, we would need to no how long the flex is before advising the requirement of derating
No one said anything about the conduits being 24 inches or less so I didn't consider it.

#8
02-25-07, 08:43 PM
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I was speculating.

If the panel is 6+ feet to the top and the ceiling is anything less than 8 feet, the conduit could very easily be less than 24 inches. If it goes down, it is probably more than 2 feet.

#9
02-26-07, 04:35 PM
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Someone has got to do it. "May as well be me".

Is speculation the same as assumption?

I've been "conditioned" (& some scolded) here not to ASSUME.

But I do,Due to the fact that I "assume", most have a BASIC knowledge of what they are dealing with and the potential HAZARDS of it (Electricity).

If they don't, I would recommend that they get it. Even the most basics could save a lot of hurt and property loss.

Changing a light switch could kill you. Give it the respect deserved. No matter how minor the task.

#10
02-26-07, 04:57 PM
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I agree with educating oneself. My speculation was not a matter of safety though. It was merely trying to let folks know that the deration being recommended so heavily may not be required.

It was also a very germane point considering the OP's original question. It would affect the answer very directly.

#11
02-26-07, 05:16 PM
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Off/on topic

We are a bit off topic now/but it all is directly on. (ever feel like a politician?)

There are many things (code) that come into play with so many issues in this feild. That sometimes a yes or no answer cannot suffice.
Many variables are in play, and I hope that posters can understand that. With that said EVERY ounce of information is important.

This will(should) make the application both safe and code compliant.

I hope that makes sence.

#12
02-27-07, 07:55 AM
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Posts: 22
All,
Thanks for all the information, I really appreciate it. I will take every bit of information into consideration. I have decided to add a subpanel and route the wires thru one of the two available 1" FMC. The distance from the main to the subpanel is about 40 ft and I will route it under the house. I figure that I will run #4AWG THHN and make the subpanel a 70A.

Questions:

1) Is it safe to say that in the future, I could upgrade this to a 100A subpanel with that wiring and distance?

2) What is the best conduit to use from the end of the 1" FMC to the subpanel, 40 feet away? The 1" FMC is about 2-3ft (dist from main panel to under the house).

Again, thanks for the help.
Jason

#13
02-27-07, 08:48 AM
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> Is it safe to say that in the future, I could upgrade this to a 100A subpanel
> with that wiring and distance?

Assuming your jurisdiction allows table 310.15(b)(6) for subpanels (most do), copper feeders sized #4-4-4-8 are good up to 100A as a subpanel feeder. A 100A breaker is the same price as a 70A breaker, and 100A panels are very cheap so you might as well make it a 100A panel now.

> What is the best conduit to use from the end of the 1" FMC to the subpanel,
> 40 feet away?

I would run 1" or 1-1/4" schedule 40 PVC conduit. Glue it, let it dry, then pull the wires in; it helps to install a twine or rope as you assemble the conduit. Make sure you have a pull access (using LB or boxes) for every 360 degrees worth of bends, and support the horizontal conduit with half-straps every 3' under the house.

#14
02-27-07, 03:01 PM
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Ben, could you explain how that section applies to the OP's situation.

I don't do a lot of resi but even when reading that section, I don;t see where it allows a feeder to a subpanel to be sized as they allow in a service entrance or main panel feeder.

School me if you would.

#15
02-27-07, 03:29 PM
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I believe that this sentence is the one that allows 310.15(b)(6) to apply to subpanels. It permits any feeder between the main disconnect and the branch circuit panels to be sized using the table.

"...For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch circuit panelboard(s). ..."

I certainly understand how someone might read it differently, which is why I always recommend checking with the local inspector on that issue.

#16
02-27-07, 03:51 PM
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I think I'm with you on this, Ben.

As well, since the SE conductors would only need to be #4 (for 100amp) I can't see why you would need any panel feeder larger than the #4. It only makes sense.

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