How do I determine the ampacity of two aluminum 3-conductor cables with ground , rated 0 to 600 volts buried in PVC conduit 24 inches below the surface and adjacent to each other?

What size cable?

As long as they are not physically bundled and touching in the trench their proximity is not an issue.
You say in conduit. Why cable in conduit?
You do mean one per conduit right?

This question is actually very vague. Is it some sort of test question?

It might be a dumb question, but it's not a test question. There are two 3-conductor cables with ground in each of two conduits.They are in conduit to prevent damage from gophers. One cable is for a 100 amp service from a cabin to a bunkhouse. The other is for a branch circuit from the cabin to a well pump motor. Table 310.84 shows the ampacity of two 3-conductor cables of #1 aluminum directly buried 24 inches apart to be 145 amps. It also shows the ampacity of a single directly buried cable to be 155 amps. This amounts to a derating of 94% because of the second cable 24 inches away. Mine will be adjacent. Table 310.16 implies that raceways, cable, and direct burial are equivalent insofar as number of conductors is concerned. Should I use table 310.15(B)(2)(a) to get an adjustment factor of 80%, or since I am in conduit rather than direct burial should I use the 100 amp figure for #1 aluminun from table 310.16 without any adjusting factor?

After rereading my comment I realize it is confusing. There are two buried conduits - each with a 3-conductor cable.

Not necessarily a dumb question, but I think your over thinking things.

Why are you using Table 310.84?

Did you read the heading of that table?And the voltage rating of the listed cables?

This is not even close.

This table is for Medium Voltage, 2001-35000 volts, cables, not conduits, buried in a duct bank. I am not going to explain further, as it is beyond the scope of this forum.

No offense, but this is what happens with a Code book in laymens hands.

Two 120/240 volt 100 amp or less feeders laying in a trench does not constitute a duct bank.

For residential wiring, all you need to use is Table 310.16 as a start, using the 60 Deg C column for Romex, and the 75 Deg C column for MC cable or pulled conductors in conduit, then Table 310.15 (B)(6) as applies to residential feeders, if applicable, to help use smaller conductors.

What you don't offer is the insulation of the conductors.


Is the #1 used to reduce voltage drop due to distance?

Let's not worry about derating, one, since that applies to more than 4 current carrying conductors in one race way, and does not apply here, and since buried in earth I can actually use temperature correction factors at the bottom of Table 310.16, and not worth doing the calcs, or even using this correction. Note, 70-77 Deg F ambient I can increase the ampacity anywheres from 4% to 8% higher than the ampacities listed in Table 310.16.

I get you have one #1 set of conductors for a 100 amp service, and another for a well pump feeder. What are the breakers ratings for each feeder?

I think I answered your original question, actually I am sure of it, but not sure if you see that.

Please answer my questions, and why are you asking?

I wasn't using table 310.84. I only mentioned it as an example of where the NEC requires a buried cable to be derated if there is one buried near it. Yes I read the heading on that table and knew that it didn't apply to my 0 to 600 volt case. I thought perhaps there was a similar table or paragraph somewhere that applied to 0 to 600 volts and wasn't able to find it - hence the question. I didn't offer the insulation because I hadn't got that far in cable selection. I chose #1 type USE aluminum rather than #2 type USE-2 aluminum because of voltage drop due to distance.

I am planning to use type USE #1 aluminum 3-conductor cable with ground protected by a two pole 100 amp breaker for the 100 amp service and a type UF #12 copper 3-conductor with ground protected by a two pole 20 amp breaker for the line to the pump.

Thank you for your help. You answered my question. I hope I have answered yours although I am not sure I addressed the last one where you ask "Why are you asking?".

I'm good. Looks like you are also, Happy camping.