Wire sizing/derating

P

Pinnie

Senior Member
Location
Ohio
Occupation
Commercial Electrician
Learning how to derate/ size conductors.

Can you put more conductors in a conduit than the tables in Annex c allow and the derate? Or is that always a violation.
 
Pinnie said:
Can you put more conductors in a conduit than the tables in Annex c allow and the derate? Or is that always a violation.
Click to expand...
Yes and no. Annex C conduit fill tables do not take into account short raceway nipples that are 24" or less because the permitted fill for those nipples is 60% instead of the standard 40% for more than two conductors. So when applying the tables if your raceway is more than 24" the number of conductors listed is the maximum permitted within the raceway. Raceway fill limits are to prevent the conductors from being damaged during installation. Derating has no bearing on the maximum fill. Derating is required when the number of CCC's exceeds three for raceways longer than 24". This may help when trying to figure out how many conductors are actual CCC's.

Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC:
3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*
Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception,
*if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.

1Ø- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.
Click to expand...
 
infinity said:
Yes and no. Annex C conduit fill tables do not take into account short raceway nipples that are 24" or less because the permitted fill for those nipples is 60% instead of the standard 40% for more than two conductors. So when applying the tables if your raceway is more than 24" the number of conductors listed is the maximum permitted within the raceway. Raceway fill limits are to prevent the conductors from being damaged during installation. Derating has no bearing on the maximum fill. Derating is required when the number of CCC's exceeds three for raceways longer than 24". This may help when trying to figure out how many conductors are actual CCC's.
Click to expand...
So when using all phases of a system in a circuit (at least in the provided systems) the neutral conductor only carries the unbalanced load making is a non CCC. (With exception of nonlinear loads.)

Why on a theory level does the neutral function that way in those scenarios?
 
When a neutral is only carrying unbalanced current it does not add to any heating.
 
We've had a several discussions on this topic. My take is found in these two:

FAQ: When is a neutral a current carrying conductor?

I'd like to propose an addition to the FAQ. When does a neutral count as a current carrying conductors for purposes of 310.15(B)(4)? Code requires that the ampacity of conductors be adjusted if several conductors are bundled together, for example in a single raceway. The reason is _heat_...
forums.mikeholt.com

Counting neutrals as current carrying conductors

Good afternoon! 2017 NFPA 70, 310.15 (5) A neutral conductor that carries only the unbalanced load from conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(3)(a). For NFPA 70 2020 THE REFERENCE IS 310.15(C)(E). Both basically say the same...
forums.mikeholt.com
 
Pinnie said:
Why on a theory level does the neutral function that way in those scenarios?
Click to expand...
Any current in the neutral is current not being carried by one or more lines, so the total current can never exceed that which a fully-loaded balanced system would carry.
 
infinity said:
Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC:
3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*


1Ø- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Click to expand...
Click to expand...
why do the 3 wire circuits on both systems differ on amount of CCC’s?
 
Pinnie said:
why do the 3 wire circuits on both systems differ on amount of CCC’s?
Click to expand...
It has to do with the total resistive heat generated among the conductors.

Given a balanced system, whether 3-phase or split-phase, consider equal current (I0) on all live conductors, and no current on the neutral. The total heating, is proportional to 3*I0^2 for 3-phase, or 2*I0^2 for split phase. Explore possible distributions of current with neutral current and unbalanced loads, which must have a vector sum of zero (per Kirchhoff's current law). You'll see that the sum Ia^2+Ib^2+Ic^2+In^2 among all the conductors, which directly determines the heating, will never exceed 3*I0^2. Likewise for split-phase. This assumes harmonics are negligible, which if they weren't, is another unrelated condition that can cause neutral to count as a CCC.

By contrast, if neutral is a mandatory part of the return path even when loads are balanced, the neutral is guaranteed to be a heat generating conductor. E.g. two phases plus neutral derived from a 3-phase service supplying an assortment of single phase loads, or just single phase-to-neutral loads on their own. The vector sum of currents can't add up to zero, without the neutral helping, which is why it's a mandatory part of the return path.
 
Pinnie said:
why do the 3 wire circuits on both systems differ on amount of CCC’s?
Click to expand...

Giving a slightly different answer.

In the single phase 3 wire system, the current going to the neutral from leg A is 180 degrees out of phase from the current going to the neutral from leg B. If leg A and leg B carry equal amps, then the current on the neutral is zero. You have 3 conductors but in the worst case (both leg A and leg B fully loaded) the neutral carries zero current.

In the three phase 3 wire system, the current going to the neutral from leg A is 120 degrees out of phase from the current going to the neutral from leg B. If leg A and leg B carry equal amps, then the current on the neutral is equal to the current on the hot legs. You have 3 conductors and in the worst case (both leg A and leg B fully loaded) the neutral carries full current.
 
Another way to think about the neutral of 120/208 1ph:

With a balanced 3ph system, neutral carries no current. If you reduce the load on any one line, the neutral carries that much current. If you completely remove the load on any one line, the neutral carries all of that current.

A 20a, B 20a, C 20a, N 0a
A 20a, B 20a, C 15a, N 5a
A 20a, B 20a, C 10a, N 10a
A 20a, B 20a, C 5a, N 15a
A 20a, B 20a, C 0a, N 20a
 
LarryFine said:
Another way to think about the neutral of 120/208 1ph:

With a balanced 3ph system, neutral carries no current. If you reduce the load on any one line, the neutral carries that much current. If you completely remove the load on any one line, the neutral carries all of that current.

A 20a, B 20a, C 20a, N 0a
A 20a, B 20a, C 15a, N 5a
A 20a, B 20a, C 10a, N 10a
A 20a, B 20a, C 5a, N 15a
A 20a, B 20a, C 0a, N 20a
Click to expand...
I’m realizing I’m not sure conceptually how current flows from phase to phase. I understand that it does, and that that is why no neutral is needed on 240 circuits. But I don’t know why. I know ac current oscillates back and forth at 60hz. But how current from one phase conductor to another as a return path without issue is something I haven’t thought through. I guess on an atomic level why not have electrons moving opposing ways? My brains thinks they’ll run into each other though.
 
Pinnie said:
I’m realizing I’m not sure conceptually how current flows from phase to phase. I understand that it does, and that that is why no neutral is needed on 240 circuits. But I don’t know why. I know ac current oscillates back and forth at 60hz. But how current from one phase conductor to another as a return path without issue is something I haven’t thought through. I guess on an atomic level why not have electrons moving opposing ways? My brains thinks they’ll run into each other though.
Click to expand...
At any instant in time, the two phase conductors are at opposite polarities. One is positive while the other is negative.
 
CoolWill said:
Kinda. Instead of being direct opposites, it more like one is more positive than the other.
Click to expand...
i see. That’s why there’s no four phase systems (at least that i know of). You’d have to be very careful when hooking up motors or equipment utilizing such a system, as if you are able to put the phases out of order. In three phase you can only have them reversed. Is that right?
 
Or actually, would the motor just only run backwards, and that scenario too
 
Pinnie said:
Even in three phase?
Click to expand...
Assuming balanced currents:
When phase A is at its peak, phases B and C will both carry exactly half the peak current, but in the opposite direction. One of them is increasing, while the other is decreasing. At any given snapshot in time, the currents among all three phases add up to zero.
Pinnie said:
i see. That’s why there’s no four phase systems (at least that i know of). You’d have to be very careful when hooking up motors or equipment utilizing such a system, as if you are able to put the phases out of order. In three phase you can only have them reversed. Is that right?
Click to expand...

There is 4-phase electricity in theory, with 90-deg phase differences instead of 120-deg phase differences. And with PLUS and SQUARE taking the place of wye and delta respectively. It doesn't serve much practical application, since there are diminishing returns to the advantages of polyphase electricity beyond 3-phase. Plus, if you didn't connect phase A opposite phase C, and likewise B opposite D, you'd have a big problem.

With 3-phase, there are only 2 distinct options for how you can sequence the three phase conductors: clockwise and counterclockwise sequencing. Swap two phases, and you reverse the motor rotation direction. Start on the "wrong" phase, but keep the same sequence (e.g. BCA instead of ABC), and it makes not difference, as they are both the same rotation direction.
 
Yes it's a 50/50 chance of the phasing being correct with 3 phase. You certainly could have a four-phase system but you will see diminishing returns with wire costs vs power delivered.
 
Pinnie said:
Even in three phase?
Click to expand...
With 3ph, there is a genuine phase difference. The alternator has three sets of coils that are physically offset by (360/3) 120 degrees, so their outputs are electrically 120 degrees apart.

When one phase is at its positive peak, the second phase is approaching its negative peak, and the third has passed its negative peak. The three chase each other 'round and 'round.

1741900608915.gif

Read this thread:

Testing ground for voltage

Let's say I working on some electrical and I'm testing for voltage From Ground to black I should get 120v Ground to red I should get 120v Ground to neutral I should get 0v Since ground and neutral are at the same potential But there are times the ground can be energized because there is a...
forums.mikeholt.com
 
You must log in or register to reply here.
Top