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Current situation: main service panel (MSP) with 200A service on exterior wall. It feeds a single sub-panel via #2 copper conductors and is protected by a 100A breaker. These three #2 conductors to the sub-panel go from the exterior wall to the crawl space via the following conduit:

enter image description here

Current conduit seems to be ~ 1-1/2" (measured exterior diameter).

How many conductors (2 current carrying+neutral) can I add to this conduit?

Background: I am planning to add another 100A feeder from this MSP which would require three more #2 (two of them current-carrying). I do NOT want more ugly conduits on my exterior wall. I want to understand how many conductors in principle can go in there and if limited, if the limit can be fixed by upgrading to a larger conduit (instead of adding an ugly second one).

Which one is fine with either the current conduit or when replacing to a larger one? (in all examples, only two conductors are current carrying since the 3rd is Neutral):

  1. 3xAWG2 + 3xAWG6 (current subpanel + 60A load, e.g. EVSE)
  2. 3xAWG2 + 2xAWG2 (current subpanel + 100A load, e.g. another sub-panel or EVSE)
  3. 3x4/0 + 2xAWG2 (upgraded 200A sub-panel + 100A load, e.g. another 100A sub-panel or EVSE)
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    Why another 100A feeder rather than a single 200A feeder through the wall, then a 100A breaker feeding a sub-panel?
    – keshlam
    Commented Apr 13 at 10:32
  • What is the length of the conduit, (both here and inside the building? As for "ugly conduit," plant a hedge where you can trim the meter side of it to allow the 3 foot out and 30-inch wide service space, and you won't have to look at conduit or meter ever after...
    – Ecnerwal
    Commented Apr 13 at 11:48
  • 1-1/2" OD is probably 1-1/4" conduit. Conduit, being related to pipe, has a nominal size system based on the hole in it, though in few cases does the hole precisely match the nominal size.
    – Ecnerwal
    Commented Apr 13 at 13:10
  • Aside: It just naturally prompts one to search for the simple solution: pretty conduit.
    – HABO
    Commented Apr 13 at 13:35
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    (You might want to change that padlock for a non-rusting one before you need a Dremel to open it.) Commented Apr 13 at 16:43

2 Answers 2

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If that's 1-1/4" conduit, and XHHW or THWN2 insulation:

  1. passes
  2. passes
  3. requires an upgrade to 2"

Put "conduit fill calculator" into a search engine to investigate any different options yourself.

Option 3 without a service upgrade may be rather pointless with a 200A service. Might as well hang the 100A sub-panel off the 200A sub-panel unless you upgrade the service capacity. Or feed a 200A panel (to get more spaces) with a 100A breaker (for smaller wires.)

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  • Thank you, certified from conduit fill perspective. But there is also de-rating and all kinds of special things I may not be aware of (e.g. 83% allowances, stub rules for small pieces etc etc). So two separate feeds in this one conduit are OK as long as they fit with the fill calculations?
    – divB
    Commented Apr 13 at 21:50
  • 2. I don't understand why "Option 3" without a service upgrade is pointless. Can you elaborate? Currently with the #2 feeder, my sub-panel can only be 100A. If I wanted my sub-panel to enjoy the full 200A service capacity, I'd need to replace with 4/0 wire, right? Why is this pointless?
    – divB
    Commented Apr 13 at 21:51
  • Re 2, note: My sub-panel got upgraded as well (it's a 200A panel) but the feeder is still the #2 wires. So it's effectively only a 100A sub-panel on a 200A service.
    – divB
    Commented Apr 13 at 21:53
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    It's pointless because you don't have 300A service to feed 200+100A panels. So you might as well feed the 100A panel off the 200A panel. Assuming 75°C connections (typical for panels and breakers) that needs 3/0 copper or 250 MCM Aluminum. The 100A feed needs 3 AWG copper or 1 AWG Aluminum. Both in the same conduit needs 4/0 and 2 AWG copper ( or 1/0 and 300 MCM Aluminum) due to thermal derating. If you have only the 200A and your whole service feeds through it, you can use 2/0 copper due to the 83% derate (of the current) allowed by code for that special case.
    – Ecnerwal
    Commented Apr 13 at 23:40
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    And I would note that your question was only about conduit fill. So that's what I answered. Have another question, ask another question. Separately.
    – Ecnerwal
    Commented Apr 13 at 23:42
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Feeder capacity in the first place

Since this is a subpanel feeder, and not the entire service to a dwelling, you must NEC Table 310.16. The 90C column is off-limits except for adjustment factors, which we'll get to. The 75C column is in-play as long as your cable is a type in the 75C column, and note that NM cable goes in the 60C column due to 334.80.

90% of the time, 2 AWG wire used on a 100A subpanel feeder is aluminum wire, because the prior nitwit used the wrong table and concluded 100A=#2 AL. That is wrong, #2 AL is 90A wire per 310.16.

Another 9% of the time, the person made the above mistake but missed the "aluminum" part and used copper. Copper #2, as you can see from 310.16, is 95A wire if NM or UF, and 115A if any other type.

But then we need "too many wires" adjustments.

This happens in NEC 310.15(C)(1). The base numbers in 310.16 assume no more than 3 "wires that matter" (neutral doesn't matter in 120/240V split-phase) in a conduit. Since you want to have two feeders, you have 4 such wires.

4-6 wires requires an adjustment to 80%, but, this comes off the highest temperature the wire itself is permitted, disregarding limits of enclosures and terminals. If you have the happy situation of having a wire type in the 90C column of 310.16, then your #2 copper is 130A and your #2 aluminum is 100A.

So, 80% of that is 104A and 80A, respectively. 104A is not a standard breaker size... so since it's feeder not service wire, you can use the Round Up Rule to go 110A breaker. 80A is a standard breaker size so there you are.

The 104A limit also saves us the trouble of seeing if a smaller copper wire can carry 100A - it can't.

Conduit fill, though...

So now you simply need large enough conduit for six #2 copper wires and 1 or 2 bare #8 grounds.

You claim a 1-1/2" OD of the conduit. That's between sizes. 1" conduit has a 1.315" OD. 1-1/4" conduit has 1.660" OD. 1.5" conduit has a 1.9" OD. I will join Ecnerwal and assume you have 1-1/4" and PVC. (I ran both Schedule 40 and Sched 80, same answer).

I don't crunch numbers, I use a handy conduit fill calculator website. That says.... ROFL not even close. Not gonna happen.

Do you really, really need 100A subpanel though?

In my experience, most people are not driving their quest for a 100A panel out of an actual Article 220 Load Calculation, they are just either a) picking a nice round number out of thin air, or, b) they don't realize a 100A panel is only a redline maximum, and think they HAVE to run 100A to a 100A subpanel.

Since conduit size is our critical constraint, copper wire is the smart play. So let's try #4 copper of a THWN-2 or THHN type giving 95A @ 90C. Conduit fill calc says: Approved.

With our 20% derate, we are now at 76 amps. The Round Up Rule lets us use an 80A breaker.

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