Paralleling - This hurt my brain, it can't be correct.

[Sea Nile]

I've been going over this material to a point, then I go bact to the beginning and do it again.
If I ever feel like I'm not understanding, I go back to the fundamentals and cover it again.
But this slide challenges everything I know about the fundamentals.





Mr. Holt says there is no violation in this scenario....

But... 310.10(G)(2) says the paralleled condunctors of each (phase) must comply with (1)-(5).

If a phase consists of two different phase conductors then a phases are L1-L2, L2-L3, and L3-L1.
So all three phases are in each raceway.

Since the phases must comply with (G)(2), I would interpret the meaning of G(3) to be each set of conductors paralleled in a raceway must be the same, but they don't have to be the same as a set of paralleled conductors in another raceway.

From a fundamentals aspect, in order to cancel out the inductive reactance in each raceway, the above graphic dosen't make sense.

If I'm wrong, then please help me get back on track.

 

[jap]

In the graphic the (2) different phase conductors for (Line1 or A Phase or the Black Conductor) are the (2) 250 KCMIL and both are 100' long although they are in 2 separated conduits.

JAP>

 

[Sea Nile]

In the graphic the (2) different phase conductors for (Line1 or A Phase or the Black Conductor) are the (2) 250 KCMIL and both are 100' long although they are in 2 separated conduits.

JAP>

I'm not confused by what the slide is showing, I'm confused about why the slide is showing it.

How can you cancel out the inductive reactance of a raceway if Line 1 is 250 Kcmil Aluminum, Line 2 is 3/0 Copper, Line 3 is 3/0 Copper but a different length. Maybe I'm just going to have to accept this as is for now and revisit it when I'm more experienced.

 

[EC Dan]

The magnetic field is a factor of the current in the wire, not the wire material, so if it's a balanced 3-phase system regardless of the wire size or material for each phase, then the fields cancel out.

 

[tom baker]

The rule states each set (2 wires) must be the same. Mike used this example, which is correct, to illustrate a misunderstood rule. He does this often to get you to think

 

[Sea Nile]

Well it worked, I'm thinking LOL. Ok, let me see if I have this right. Since each phase conductor is the same, theoretically the same current will flow through each raceway to balance the inductance of the current in the other conductors?

 

[kwired]

I'm not confused by what the slide is showing, I'm confused about why the slide is showing it.

How can you cancel out the inductive reactance of a raceway if Line 1 is 250 Kcmil Aluminum, Line 2 is 3/0 Copper, Line 3 is 3/0 Copper but a different length. Maybe I'm just going to have to accept this as is for now and revisit it when I'm more experienced.

If each "parallel conductor" is same size type and length, they will have equal impedance - the idea is current will divide proportionally. We don't really care about what size, length or material the other two phase conductors are for this aspect.

Only thing that matters with ferrous raceways is that you have one conductor of each phase and neutral if applicable in each raceway so that their fields will cancel out and not cause inductive effects on the raceway, this will depend on parallel sets having equal division. If that division is even on all lines involved then net field within a raceway will be zero though. (if you clamped a ammeter around all the conductors in conduit #1 it should read zero even if not all those conductors are carrying any current)

 

[kwired]

What you can't do is run 250 al for phase A in conduit one and 3/0 copper for phase A in conduit 2. They will have different impedances and current will not divide equally.

 

[Sea Nile]

If each "parallel conductor" is same size type and length, they will have equal impedance - the idea is current will divide proportionally. We don't really care about what size, length or material the other two phase conductors are for this aspect.

Only thing that matters with ferrous raceways is that you have one conductor of each phase and neutral if applicable in each raceway so that their fields will cancel out and not cause inductive effects on the raceway, this will depend on parallel sets having equal division. If that division is even on all lines involved then net field within a raceway will be zero though. (if you clamped a ammeter around all the conductors in conduit #1 it should read zero even if not all those conductors are carrying any current)

You must have been writing this the same time I was writing what I wrote, is what I said the same as what you said?

 

[Sea Nile]

I think I usnderstand, You guys are great. Thank you

 

[LarryFine]

The basic rule is that all paralleled conductors of any given phase must match each other.

The matched group of each phase need not match the matched group of another phase.

 

[wwhitney]

From a fundamentals aspect, in order to cancel out the inductive reactance in each raceway, the above graphic dosen't make sense.

Is there a code requirement to cancel the inductive reactance in each raceway? Is there a practical downside to not doing so?

For example, say you have 2 conduits and want to run 3 sets of a 4-wire circuit A-B-C-N. So you have A1, A2, A3, B1, B2, B3, C1, C2, C3, and N1, N2, N3. 310.10(G)(3) prevents you from putting 8 conductors in one conduit (say the 1s and 2s) and 4 conductors in the other conduit (the 3s).

But as written, it doesn't rule out the following arrangement: A1, A2, B1, B2, C1, N1 in one conduit, and A3, B3, C2, C3, N2, N3 in the other conduit. It's 6 conductors in each conduit. Is that arrangement a problem?

If so, then 310.10(G)(3) needs to be revised to call for each "each phase, polarity, neutral, grounded circuit conductor, equipment grounding conductor, or equipment bonding jumper" to have the same number of conductors within each conduit.

Cheers, Wayne

 

[tom baker]

The conductors sets are different lengths but the resistances are the same. If you did this on a job the AHJ may not approve, until you showed him/her the graphic

 

[kwired]

You must have been writing this the same time I was writing what I wrote, is what I said the same as what you said?

Let me reiterate it doesn't matter if phase a is carrying 5 amps, B is carrying 20 amps, C is carrying 15 amps... What matters is that both A phases are same impedance so that current divides evenly, same for B, C and N.

Remember (I'll go with 120/240 single phase for simplicity here) if you have 10 amps on A and 20 amps on B the neutral carries 10. If you clamp a meter around all three you get zero - which also means no inductive effects if it would be in a ferrous raceway. Now properly parallel those in two raceways, A in each raceway carries 5 amps, B in each raceway carries 10 amps and each neutral carries 5 amps. Still net zero effect if you clamp all three of either raceway. Won't change a thing when it comes to induction on the raceway if phase A is 250 al and phase B is 3/0 copper, as long as current of each "conductor" divides equally.

 

[Sea Nile]

Let me reiterate it doesn't matter if phase a is carrying 5 amps, B is carrying 20 amps, C is carrying 15 amps... What matters is that both A phases are same impedance so that current divides evenly, same for B, C and N.

Remember (I'll go with 120/240 single phase for simplicity here) if you have 10 amps on A and 20 amps on B the neutral carries 10. If you clamp a meter around all three you get zero - which also means no inductive effects if it would be in a ferrous raceway. Now properly parallel those in two raceways, A in each raceway carries 5 amps, B in each raceway carries 10 amps and each neutral carries 5 amps. Still net zero effect if you clamp all three of either raceway. Won't change a thing when it comes to induction on the raceway if phase A is 250 al and phase B is 3/0 copper, as long as current of each "conductor" divides equally.

Yep, I'm good now.

 

[LarryFine]

Is there a code requirement to cancel the inductive reactance in each raceway?

Yes.

For example, say you have 2 conduits and want to run 3 sets of a 4-wire circuit A-B-C-N. So you have A1, A2, A3, B1, B2, B3, C1, C2, C3, and N1, N2, N3. 310.10(G)(3) prevents you from putting 8 conductors in one conduit (say the 1s and 2s) and 4 conductors in the other conduit (the 3s).

But as written, it doesn't rule out the following arrangement: A1, A2, B1, B2, C1, N1 in one conduit, and A3, B3, C2, C3, N2, N3 in the other conduit. It's 6 conductors in each conduit. Is that arrangement a problem?

Yes.

If so, then 310.10(G)(3) needs to be revised to call for each "each phase, polarity, neutral, grounded circuit conductor, equipment grounding conductor, or equipment bonding jumper" to have the same number of conductors within each conduit.

Each conduit needs to contain one (or more) conductor of every phase and neutral (and EGC).

Conduit #1: A,B, C, & N
Conduit #2: A,B, C, & N
Conduit #3: A,B, C, & N
Etc.

If there is more than one conductor of each phase per conduit, they all need to contain the same quantities.

 

[synchro]

Is there a code requirement to cancel the inductive reactance in each raceway? Is there a practical downside to not doing so?

For example, say you have 2 conduits and want to run 3 sets of a 4-wire circuit A-B-C-N. So you have A1, A2, A3, B1, B2, B3, C1, C2, C3, and N1, N2, N3. 310.10(G)(3) prevents you from putting 8 conductors in one conduit (say the 1s and 2s) and 4 conductors in the other conduit (the 3s).

But as written, it doesn't rule out the following arrangement: A1, A2, B1, B2, C1, N1 in one conduit, and A3, B3, C2, C3, N2, N3 in the other conduit. It's 6 conductors in each conduit. Is that arrangement a problem?

If so, then 310.10(G)(3) needs to be revised to call for each "each phase, polarity, neutral, grounded circuit conductor, equipment grounding conductor, or equipment bonding jumper" to have the same number of conductors within each conduit.

I think that would be a problem with ferrous conduit because, in general, the magnetic fields of the conductors within each conduit will not cancel out. It will therefore induce heating due to hysteresis and eddy currents within the conduits.

As far as 310.10(G)(3), it depends how you interpret the requirement that raceways with conductors shall have "the same electrical characteristics" vs. just the physical characteristics mentioned later in the paragraph. I think one could argue that they don't have the same electrical characteristics if the distribution of the currents is different between the two conduits.

It also seems that the quote from 300.3(B)(1) below could be used to disallow this particular arrangement, but "all circuit conductors" can be vague as pointed out in your example:
"The requirement to run all circuit conductors within the same raceway, auxiliary gutter, cable tray, trench, cable, or cord shall apply separately to each portion of the paralleled installation"

I do agree that there's room for improvement to clarify these situations more precisely.

 

[wwhitney]

It also seems that the quote from 300.3(B)(1) below could be used to disallow this particular arrangement, but "all circuit conductors" can be vague as pointed out in your example:
"The requirement to run all circuit conductors within the same raceway, auxiliary gutter, cable tray, trench, cable, or cord shall apply separately to each portion of the paralleled installation"

So, I think the above sentence is intended to rule out my example, but it's not clear to me what "portion of the paralleled installation" means. One interpretation is that each conduit must have a portion of each of the circuit conductors, which my example does.

Better language is a bit hard to come up with. Maybe: Each paralleled conductor shall be divided equally across a number of raceways, auxiliary gutters, cable trays, trenches, cables, or cords, each of which shall carry a portion of all circuit conductors.

That still allows 4 sets for one phase, 2 sets for another, divided equally across 2 conduits.

Cheers, Wayne

 

[LarryFine]

So, I think the above sentence is intended to rule out my example, but it's not clear to me what "portion of the paralleled installation" means. One interpretation is that each conduit must have a portion of each of the circuit conductors, which my example does.

I believe they mean differing sections of a run. For example, if you had a length of three parallel 3" conduits, then for some reason, in the middle of the run you transitioned to a length of two 4" conduits, you would have to electrically convert to an appropriate quantity (and perhaps size) of conductors for each section at the transition.

 

[wwhitney]

I believe they mean differing sections of a run.

If "portion" has that meaning, then either the sentence says all the parallel sets have to be in one conduit (the simplest reading, but obviously wrong) or each of the multiple conduits in the portion has to have wires from every one of the circuit conductors. Under the latter interpretation there's no language specifying that they each have an equal number of wires for each of the circuit conductors, and so it doesn't rule out my example.

Cheers, Wayne