Derating more than 3 current carrying conductors in a conduit

[infinity]

You're confusing yourself by making this too overly complicated. ALWAYS count a neutral as a CCC. EVERY INDIVIDUAL CONDUCTOR in a raceway counts as one CCC. EVERY NEUTRAL in a raceway counts as one CCC. Grounds HAVE NOTHING to do with this calculation. only worry about them when figuring for box fill,,,,,,in which ALL GROUNDS combined equals the same fill as ONE of the largest conductors in the box.

We should worry about them when figuring conduit fill too.

 

[erickench]

Hey I learned something. So why is a singe phase 240v. load neutral not counted, but a three phase 240v. load it is? Does it have something to do with the transformer?

I do not see how a neutral in a wye configuration can be counted. The neutral in this case is carrying the unbalanced current of the three phases. However, if two of the three phases are used to carry the load then the neutral becomes current carrying and is therefore counted. If you add the current phasors for two off these phases you will see that it will carry as much current as the other phases.

In = IaCos(60) + IbCos(60) = Ia(.5) + Ib(.5); assume Ia = Ib & Ic=0

In = Ia(.5) + Ia(.5) = Ia = Ib

 

[erickench]

Okay, if a wye which consists of a four wire three phase configuration has a load where the major portion consists of nonlinear loads then the neutral is counted as current carrying.:smile:

 

[LarryFine]

However, if two of the three phases are used to carry the load then the neutral becomes current carrying and is therefore counted. If you add the current phasors for two off these phases you will see that it will carry as much current as the other phases.

True, but, with one phase omitted, that's still only 3 CCC's.

 

[brother]

No need. Picture a balanced 4-wire 3ph MWBC with, say, 10a per phase; no neutral current. Now, reduce the load on one phase to, say, 5a. The neutral current increases by the exact same amount, to 5a. Take that phase to zero, and the neutral current is now 10a.

can you DRAW a picture please?

 

[LarryFine]

Not easily, but re-read what I posted. I don't know how else to say it, except to reiterate:

For every amp reduced on one conductor of an otherwise-balanced 3-ph MWBC, the neutral current increases by that same one amp.

Take such a balanced circuit with 10a on each line and 0a on the neutral. If you drop 1a from one line, you'll have 1a on the neutral.

 

[flashlight]

... However, if two of the three phases are used to carry the load then the neutral becomes current carrying and is therefore counted.

OK, so I have a 3 wire MCBC in a commercial space, which comes from a single phase 110/220 V panel. Since the neutral is only carrying the unbalanced current, I don't count it.

However, this panel is fed from 2 of the 3 legs, plus neutral, of the service in the basement--therefore I do count it ?

 

[infinity]

OK, so I have a 3 wire MCBC in a commercial space, which comes from a single phase 110/220 V panel. Since the neutral is only carrying the unbalanced current, I don't count it.

However, this panel is fed from 2 of the 3 legs, plus neutral, of the service in the basement--therefore I do count it ?

From a 120/240 volt single phase supply, you are correct the 3 wire MWBC would only count as two CCC's. If that same 3 wire MWBC is fed from a 208Y/120 volt, 3 phase supply all three conductors are CCC's. If your 3 phase system is 120/240, 4 wire Delta then you would still only have 2 CCC's on the 3 wire MWBC.

 

[flashlight]

From a 120/240 volt single phase supply, you are correct the 3 wire MWBC would only count as two CCC's. If that same 3 wire MWBC is fed from a 208Y/120 volt, 3 phase supply all three conductors are CCC's. If your 3 phase system is 120/240, 4 wire Delta then you would still only have 2 CCC's on the 3 wire MWBC.

I work in various municipalities around here. If, at the panel, I
get voltage readings of, say, 111 from line to neutral, and 219 between phases.
Can I extrapolate from this which 3 phase system it is, or do I need to contact POCO ? Thanks in advance !

 

[hardworkingstiff]

I work in various municipalities around here. If, at the panel, I
get voltage readings of, say, 111 from line to neutral, and 219 between phases.
Can I extrapolate from this which 3 phase system it is, or do I need to contact POCO ? Thanks in advance !

If you are getting those voltages, someone needs to start looking at connections and/or wire sizes. 111 is too low.

 

[RUWired]

can you DRAW a picture please?

Attached is a picture with the associated articles for the delta and wye.

View attachment Counting CCC.sdr.doc

 

[daleuger]

can you DRAW a picture please?

Not easily, but re-read what I posted. I don't know how else to say it, except to reiterate:

For every amp reduced on one conductor of an otherwise-balanced 3-ph MWBC, the neutral current increases by that same one amp.

Take such a balanced circuit with 10a on each line and 0a on the neutral. If you drop 1a from one line, you'll have 1a on the neutral.

Basically what Larry is saying is that the load on the neutral will be the difference between the phases.

 

[daleuger]

Attached is a picture with the associated articles for the delta and wye.

View attachment 3842

Worth 1,000 words :grin:

 

[infinity]

I work in various municipalities around here. If, at the panel, I
get voltage readings of, say, 111 from line to neutral, and 219 between phases.
Can I extrapolate from this which 3 phase system it is, or do I need to contact POCO ? Thanks in advance !

In a normally operating system you should be able to figure out what the supply system type is just by taking voltage measurements at the panel. Even a visual inspection is all that's really needed in most cases. For the most part it should be one of these:

3 phase, 3 wire ungrounded Delta
3 phase, 3 wire corner grounded Delta
3 phase, 4 wire Delta
3 phase, 3 wire WYE
3 phase, 4 wire WYE
Single phase, 3 wire 120/240

 

[charlie b]

. . . so a good rule of thumb is just keep your conduit fill to 9 wires (+ equip ground) and avoid the problem.

You do realize, I presume, that this rule of thumb only applies to #12 wire with a 90C insulation rating? If you use #10 (even if you are merely upsizing for voltage drop considerations on a long run), the "9 wire rule" becomes a "6 wire rule."

 

[infinity]

You do realize, I presume, that this rule of thumb only applies to #12 wire with a 90C insulation rating? If you use #10 (even if you are merely upsizing for voltage drop considerations on a long run), the "9 wire rule" becomes a "6 wire rule."

For #10 THHN you still could use "Augie's rule" of 9 CCC's if the loads were 28 amps or less and not multi-receptacle circuits. But as Charlie noted for a full 30 amps on #10 AWG, 90 degree conductors, 6 CCC's would be your limit.

40 amps * 70% = 28 amps next size up 30 amps.

 

[jap]

Although I agree with all of the mathematical explanations of why you wouldnt need to count the Neutral as a Current carrying conductor. I would think that in actuality there is hardly ever a case where there is a completely balanced load where a neutral would be carrying no current whatsoever.

 

[infinity]

Although I agree with all of the mathematical explanations of why you wouldnt need to count the Neutral as a Current carrying conductor. I would think that in actuality there is hardly ever a case where there is a completely balanced load where a neutral would be carrying no current whatsoever.

The requirement for not counting a neutral as a CCC doesn't mean that it never carries current, it means that it only carries the unbalanced load. This can change when the neutral is loaded with harmonic currents which are additive in the neutral. Even with harmonics present the neutral would have to carry more than 50% harmonic current before it would need to be considered a current carrying conductor for derating pruposes. Take a look at 310.15(B)(4).

 

[jap]

I just feel that if this were worded differently there would be much less confusion about it.

 

[infinity]

I just feel that if this were worded differently there would be much less confusion about it.

What parts of the wording do you find confusing?