# How many 100A/200A conductors can I run through a single conduit from my main service panel?

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:

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)
• Why another 100A feeder rather than a single 200A feeder through the wall, then a 100A breaker feeding a sub-panel? 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... 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. 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
• (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

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.)

• 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
• 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. Commented Apr 13 at 23:40
• 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. Commented Apr 13 at 23:42

## 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.