parallel feeders in one conduit

[ashtrak]

Could someone tell me if there is a problem with pulling parallel feeders in ONE conduit.
I've got Eight 3/0's (two blacks /two reds/ two blues and two neautrals 120/208 3phase) in an 3-1/2 in conduit.I was told that someone at a code class said this was discussed and derateing would come in to effect. I can't understand the reasoning behind that as the parallel conductors are electrically the same conductor.
I can't find any thing in the code book that says the conductors would require two conduits.

Thanks

 

[infinity]

Derating does apply. The answer is in 310.15(B)(2)(a) last sentence.

 

[petersonra]

Could someone tell me if there is a problem with pulling parallel feeders in ONE conduit.
I've got Eight 3/0's (two blacks /two reds/ two blues and two neautrals 120/208 3phase) in an 3-1/2 in conduit.I was told that someone at a code class said this was discussed and derateing would come in to effect. I can't understand the reasoning behind that as the parallel conductors are electrically the same conductor.
I can't find any thing in the code book that says the conductors would require two conduits.

Thanks

2 conduits is not required, but you do have to derate if more than 3 CCC. It may be more cost effective to use multiple conduits.

 

[Power Tech]

I personally would rather have 2 easier pulls rather than one hard.

Derating will apply, I have been through this at the BD.

I would run 2 conduits.

 

[cal1947]

retired electrician

retired electrician

you have to derate after more than three current carrying conductors in a raceway

 

[infinity]

So if your #3/0's are copper and have 90 degree insulation and your neutrals are not CCC's then you would end up with:

225 * 2 * 80%= 360 amps

If your connected load is 360 amps or less then you cold go up to the next standard size 400 amps.

 

[charlie b]

I can't understand the reasoning behind that as the parallel conductors are electrically the same conductor.

Electrically, referring to the carrying of current, yes they are essentially the same conductor. But thermally, referring to the generation of heat, they are not. You have six conductors that are each creating heat around themselves, as opposed to having only three conductors generating heat in one conduit, and three others generating heat in a separate conduit.

 

[Pierre C Belarge]

Electrically, referring to the carrying of current, yes they are essentially the same conductor. But thermally, referring to the generation of heat, they are not. You have six conductors that are each creating heat around themselves, as opposed to having only three conductors generating heat in one conduit, and three others generating heat in a separate conduit.

Charlie
This is a very good explanation. I am going to "borrow" it from you.

 

[ashtrak]

Thank you

Thank you

Thanks Charlie and all who replied.
My main breaker is a 400 and my connected load isn't such that I couldn't derate and still be Ok.
I do realize that I must derate for more than three ccc's, but I didn't think my parallel feeders qualified as more than one each.
I would think there would be less heat with the two conductors, but what Charlie said makes sense,heat is now being disipated from more total area around the two conductors.

Thank you all again for the informitive response

 

[charlie b]

Charlie. This is a very good explanation. I am going to "borrow" it from you.

No problem Pierre. The usual rates apply. :grin:

 

[ksmith846]

So if your #3/0's are copper and have 90 degree insulation and your neutrals are not CCC's

Please correct me where I am wrong.......I thought that neutrals are considered CCC's unless you could prove that all 3 phase conductors are perfectly balanced. That never is the case and those Neutrals are considered CCC's.

 

[roger]

Please correct me where I am wrong.......I thought that neutrals are considered CCC's unless you could prove that all 3 phase conductors are perfectly balanced. That never is the case and those Neutrals are considered CCC's.

That is not the case.

A neutral conductor of a MWBC in a single phase service would not be considered a CCC.

This would also be true of the grounded conductor in a center grounded Delta MWBC.

It would also be true in a 4 wire MWBC of a Wye service where the loads are mostly non-linear (and even in most that are linear)

Where only two phases of a Wye are used in a MWBC the neutral conductor would be a CCC and the grounded conductor is always a CCC in a two wire circuit.

In reality a neutral conductor almost never needs to be counted as a CCC regardless of the circuits loading


Roger

 

[LarryFine]

Please correct me where I am wrong.......I thought that neutrals are considered CCC's unless you could prove that all 3 phase conductors are perfectly balanced. That never is the case and those Neutrals are considered CCC's.

Nope. Any current in the neutral must be current that is not being carried by a line conductor. There can never be more than three conductors' worth of current.

 

[bfrosty]

What about?

What about?

Harmonics (3rd order) non resistive loads?

 

[LarryFine]

Harmonics (3rd order) non resistive loads?

If of a great-enough magnitude, yes, the NEC requires the neutral to be a CCC.

 

[charlie b]

Harmonics (3rd order) non resistive loads?

I think you meant "non-linear." A load's being "non-resistive" does not mean that it will create harmonics.

 

[winnie]

Please correct me where I am wrong.......I thought that neutrals are considered CCC's unless you could prove that all 3 phase conductors are perfectly balanced. That never is the case and those Neutrals are considered CCC's.

Keep in mind that we are not talking reality, we are talking about counting for the purpose of applying code. If the phase conductors are not perfectly balanced, then there will be _some_ current flowing on that neutral.

Derating for number of conductors bundled together is because of heating. As LarryFine notes, in the systems where you don't have to count the neutral as a CCC, whenever you do have some current on the neutral, you have less current some place else. The net result is less total heating for the entire bundle, and thus for purposes of heat generation you have fewer effective CCCs than actual conductors with current on them.

-Jon

 

[bfrosty]

How about non (purely) resistive load. OK, linear will work as well.

 

[sameguy]

75 deg. terms?
both runs the same length?
A=A,B=B,C=C, must be the same respectivly or one pipe set will pull more than the other.
code book is at the shop.
400a=600mcm yes?

 

[Mr. Wizard]

Could someone tell me if there is a problem with pulling parallel feeders in ONE conduit.
I've got Eight 3/0's (two blacks /two reds/ two blues and two neautrals 120/208 3phase) in an 3-1/2 in conduit.I was told that someone at a code class said this was discussed and derateing would come in to effect. I can't understand the reasoning behind that as the parallel conductors are electrically the same conductor.
I can't find any thing in the code book that says the conductors would require two conduits.

Thanks

I had a very similar situation last week. My application was 400 amp, single phase, and I ran 2-two inch rmc's for my risers, and installed 3- 3/0's in each. Overall cost for two seperate conduits was cheaper, and easier than one larger conduit. And yes, as mentioned by others, derating does apply