dsmith411

Hello, Could someone tell me if a chart says a 4/0 aluminum triplex is good for 315 amps does that mean for both legs combined or just one leg? different people try to explain it different ways.Thanks for any replies

brian john

I am no Code expert and others may fill in my holes here but....The table you are referring to is 310.17?, If so, note the title of this table SINGLE CONDUCTOR I FREE AIR, I do not believe triplex falls into this category.
2nd how are you terminating this conductor the 90 degree column may not be applicable to your installation.

But in answer to your basic question if you use the right coloum in the right table, the rating is based upon a single conductor.

I think table 310-20 might more applicable to your installation, which would give your 4/0 AL 224 amps per conductor based upon 75 degrees C.

After posting I noticed you work for a power company, I assume a local utility? In which case the NEC may/or may not apply, depending on the installation.

MDShunk

DSmith... are you working under NEC or NESC rules for this install? The ampacity charts for the NESC are wildly different than the NEC. I ask, because your profile says you're with a POCO.

dsmith411

I'm not really trying to go by either code.I'm just using this for an example.The charts we have are ones the distriutors gave us.Some say if the chart says 4/0 3 wire triplex is good for 315 amps that means both hot legs combined are just good for that amount.Others say each hot leg is good for 315 amps which would mean the two combined would be good for 630 Amps.So what is the right way to read the chart? Thanks !

Speedy Petey

You DO NOT combine amperages.
It may be 315 amps @ 240v. Or 315 amps @ 120 volts times two. It would never be considered 630 amps.

As has been stated though. There is pretty much NO situation where you can use the 90 deg C column in figuring actual conductor ampacity.

nec decoder

so does NEC article 310 section 15 have anything to do with triplex or not?

wildleg

let me try with an example:

a single phase service rated at 200 amps would have two hots and a neutral. each conductor would have to be rated at 200 amps or better.

a three phase service rated at 400 amps would have 3 hots and a neutral. each conductor would have to be rated at 400 amps or better.

the power, in watts would add for each phase, but the rating of the service is as described above. it is a convention.

triplex that was owned by the power company would be rated by the poco engineers, and they do not use NEC 310.15 to size the wire. they would presumably use the manufacturer's info, although they may use other info.

triplex that was on the customer's side would be rated using the correct table in NEC 310.

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Mrtofu82

I think that could be a possibility considering one parallel run.

JohnR

if you look at 2008 NEC Table 310.20

"Ampacities of Not More Than Three Single Insulated Conductors, Rated 0 Through 2000 Volts, Supported on a
Messenger, Based on Ambient Air Temperature of 40°C (104°F)"

The 75° scale the ampacity of 1 conductor is 224 A

The 90° scale says 262A
This is for one of the conductors not the total for all three.

480sparky

Wow. Three years, two months, and this thread is revived.

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JohnR

ROTFL I didn't even notice.

wildleg

lol forgot to look at date of op. I guess NEC decoder is a troll ?

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frenchelectrican

Yeah but it is NOT a halloween time yet

Merci,
Marc

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