Feeder Neutral Question

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boku0003

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Given an 800 ampere 480/277 service with a load of 70,000 VA and a non-linear load of 30,000 VA. What is the feeder neutral?

It looks like it is a Wye system looking at the voltage. Since there is 30kVA of non-linear load, it looks like I have to count that as current carrying. Can I disregard the 70kVA since that would only be the unbalanced current?
 

Smart $

Esteemed Member
Location
Ohio
Given an 800 ampere 480/277 service with a load of 70,000 VA and a non-linear load of 30,000 VA. What is the feeder neutral?

It looks like it is a Wye system looking at the voltage. Since there is 30kVA of non-linear load, it looks like I have to count that as current carrying. Can I disregard the 70kVA since that would only be the unbalanced current?
By the way it is stated, 70kVA is your total load, whether balanced or not.

30kVA of non-linear loads is not "a major portion" of the total load, so the neutral is not counted as a current-carrying conductor. Ref: 310.15(B)(4)(c)
 

boku0003

Member
response

response

221.61 FPN 2 says that "nonlinear loads may necessitate that the power system design allow for the possiblity of high harmonic neutral-conductor currents."

So the harmonic current "may" necessitate inclusion. However, looking at 310.15(B)(4)(c), it says "where the **major** portion of the load consists of nonlinear loads..." For a connected load of 70k, the harmonic load would need to be 35k or more, which it is not. So the harmonic load is not included...?

That being said how would you solve this? Take 70k and divide by 277 or 480? Would you also divide by 1.73...?
 

ray cyr

Senior Member
Location
Yakima, Wash.
I dont think so. Read 220.61 FPN No.2

I think the real answer depends upon whether this is a test question or a real life situation. It sounds like a test question and since FPN's are not enforcible, it would not be used to answer the question. I think that the question is in reference to 220.61(C)(2).
 

Smart $

Esteemed Member
Location
Ohio
...

That being said how would you solve this? Take 70k and divide by 277 or 480? Would you also divide by 1.73...?
So is this a test question or real life situtation?

In either case, there is not enough information to determine the size of the neutral without making assumptions. First, the designation 480/277 denotes a 3? 4W wye system, but the the load details fail to mention whether they are 3?, 1? Line-to-Line, 1? Line-to-Neutral, or a combination thereof.

If we assume they are all 1? Line-to-Neutral loads and in a perfectly balanced configuration, you would divide 70kVA by 480V and again by the 3? factor of 1.732 for a balanced Line current of 84.2A. This amount of balanced Line current would also be the maximum unbalanced Neutral current. Using 110.14(C) temperature limitations at 75?C, the Neutral would be sized at #4 Cu or #2 Al minimum and based on mentioned assumptions.
 

slavan

Member
Given an 800 ampere 480/277 service with a load of 70,000 VA and a non-linear load of 30,000 VA. What is the feeder neutral?

It looks like it is a Wye system looking at the voltage. Since there is 30kVA of non-linear load, it looks like I have to count that as current carrying. Can I disregard the 70kVA since that would only be the unbalanced current?

TOTAL LOAD - BALANCED LOAD + NONLINEAR LOAD = NEUTRAL LOAD

So, total load and nonlinear load are given except the balanced load.

My guess would be:

70,000 VA (total load)- 30000 VA (nonlinear load) = 40000 VA

40000 / 480 = 83.333 amps or 85 amps (neutral load)
 

slavan

Member
No 250.66 is use for sizing of the grounding electrode conductor.

85 amps X 1.25 (continuous load) = 106 amps is the minimum allowable conductor size ampacity.

310.16 would required a #4 conductor size under the 75*column.
 

slavan

Member
No 250.66 is use for sizing of the grounding electrode conductor.

85 amps X 1.25 (continuous load) = 106 amps is the minimum allowable conductor size ampacity.

310.16 would required a #4 conductor size under the 75*column.

Let me rephrase that. For this particular question, 250.66 would not be use to size the neutral. However, if you are given the conductor size, then you would reference to 250.66

i.e: If you are given two 500 kcmil conductor in parallel, what is the size of the grounding electrode conductor?

In this instance, you would combine the two 500 kcmil to a total of 1000 kcmil; from 250.66, a size of 2/0 copper or 4/0 aluminum would be required.
 

Smart $

Esteemed Member
Location
Ohio
TOTAL LOAD - BALANCED LOAD + NONLINEAR LOAD = NEUTRAL LOAD

So, total load and nonlinear load are given except the balanced load.

My guess would be:

70,000 VA (total load)- 30000 VA (nonlinear load) = 40000 VA

40000 / 480 = 83.333 amps or 85 amps (neutral load)
Assumptions abound!!!

Please read Post #6

Additionally, you divided 40kVA by 480V. Line-to-Neutral loads are 277V and there are 3 Lines.
 

Smart $

Esteemed Member
Location
Ohio
Don't you need to use table 250.66 for sizing the neutral?

No 250.66 is use for sizing of the grounding electrode conductor.

85 amps X 1.25 (continuous load) = 106 amps is the minimum allowable conductor size ampacity.

310.16 would required a #4 conductor size under the 75*column.

Let me rephrase that. For this particular question, 250.66 would not be use to size the neutral. However, if you are given the conductor size, then you would reference to 250.66

i.e: If you are given two 500 kcmil conductor in parallel, what is the size of the grounding electrode conductor?

In this instance, you would combine the two 500 kcmil to a total of 1000 kcmil; from 250.66, a size of 2/0 copper or 4/0 aluminum would be required.
boku0003,

Sizing a service or feeder neutral is a two part qualification. First you must calculate the maximum unbalanced current on the neutral and determine the size necessary to carry such current. Next, you determine the minimum size of a grounding electrode conductor per Table 250.66 based on the service or feeder line conductor size. The neutral must be sized to the larger of the two size determinations. The larger of the two sizes is used to ensure the service or feeder neutral is capable of carrying the full amount of trip current in the event of a short-circuit.

Generally speaking, if all of your total load is line-to-neutral loads and balanced, the maximum unbalanced neutral current will require the neutral to be sized larger than a minimum permitted GEC size, because the neutral conductor will be the same, or nearly the same size as the line conductors (because neutral conductors, without OCP, are permitted to be sized at 100% of the continuous current, whereas line conductors have to be sized using 125% of continuous current).
 

boku0003

Member
Neutral

Neutral

First, this is an exam question. I actually received it from a study partner, so that is all the info that I got.

According to 250.24(C)(1) "...this conductor (neutral) shall be routed with the phase conductors and shall not be smaller than the required grounding electrode conductor specified in T250.66."

So how do you know when to use it to size the neutral? My guess would be when the neutral is not counted as a current carrying conductor. As Smart$ referenced 310.15(B)(4)(c), which states "On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor."

From the question, you can figure out that this is a 4-wire, 3-phase wye circuit. The harmonic load accounts for less than half of the total load. This to me would suggest that the neutral is not considered current carrying. So use 250.66. First calculate the service (ungrounded) conductor size.

70kVA/(480x1.73) = 84.2A. Not sure if you would need to multiply by 1.25. Doesn't specify if it is a continuous load. (by the way do you even multiply by 1.25 for the neutral...multiplying by 1.25 is for the benefit of the breaker, which the neutral is not on a breaker). So looking at table 310.16 in the 60 degree C column, since it's less than 100A, this would require a 3 AWG conductor. Since the neutral is not current carrying conductor, it does not need to be bigger than the GEC from 250.66, which is 8AWG copper.

Don't know if there are going to be any "right" answers since you have to make assumptions due to the lack of info in the question. But I have learned a lot so far from your responses. I greatly appreciate it. Let me know what your thoughts are.
 

Smart $

Esteemed Member
Location
Ohio
First, this is an exam question. I actually received it from a study partner, so that is all the info that I got.

According to 250.24(C)(1) "...this conductor (neutral) shall be routed with the phase conductors and shall not be smaller than the required grounding electrode conductor specified in T250.66."

So how do you know when to use it to size the neutral? My guess would be when the neutral is not counted as a current carrying conductor. As Smart$ referenced 310.15(B)(4)(c), which states "On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor."

From the question, you can figure out that this is a 4-wire, 3-phase wye circuit. The harmonic load accounts for less than half of the total load. This to me would suggest that the neutral is not considered current carrying. So use 250.66. First calculate the service (ungrounded) conductor size.

70kVA/(480x1.73) = 84.2A. Not sure if you would need to multiply by 1.25. Doesn't specify if it is a continuous load. (by the way do you even multiply by 1.25 for the neutral...multiplying by 1.25 is for the benefit of the breaker, which the neutral is not on a breaker). So looking at table 310.16 in the 60 degree C column, since it's less than 100A, this would require a 3 AWG conductor. Since the neutral is not current carrying conductor, it does not need to be bigger than the GEC from 250.66, which is 8AWG copper.

Don't know if there are going to be any "right" answers since you have to make assumptions due to the lack of info in the question. But I have learned a lot so far from your responses. I greatly appreciate it. Let me know what your thoughts are.
Regarding red highlighted portion of your post, see 220.61?where you start regardless of the neutral being counted or not as a ccc. for sizing of neutral. 250.24(C)(1) only sets the minimum and maximum sizes for a grounded service conductor.
 

boku0003

Member
Neutral

Neutral

I did a little reading online:
http://www.ecmag.com/?fa=article&articleid=8664

This points to 220.61(C). Someone mentioned the FPN in this section. (1) says "Any portion of a 3-wire circuit consisting of 2-phase wires and the neutral of a 4-wire, 3-phase, wye connected system".

(2) says "That portion consisting of nonlinear loads supplied from a 4-wire, wye-connected, 3-phase system."

Well, (1) seems to say that we count all 70KVA. So perhaps we do use the full 70 kVA.

Can someone post what type of wiring systems would have the neutral as non-current carrying? Perhaps there are 2 separate applications. I believe the unbalanced current is always required to be sized in the neutral. When it is not counted as a current carrier, then you don't count it for T310.15(B)(2)(a), so perhaps using 310.15(B)(4) should not be used for this question.
 

slavan

Member
I did a little reading online:
http://www.ecmag.com/?fa=article&articleid=8664

This points to 220.61(C). Someone mentioned the FPN in this section. (1) says "Any portion of a 3-wire circuit consisting of 2-phase wires and the neutral of a 4-wire, 3-phase, wye connected system".

(2) says "That portion consisting of nonlinear loads supplied from a 4-wire, wye-connected, 3-phase system."

Well, (1) seems to say that we count all 70KVA. So perhaps we do use the full 70 kVA.

Can someone post what type of wiring systems would have the neutral as non-current carrying? Perhaps there are 2 separate applications. I believe the unbalanced current is always required to be sized in the neutral. When it is not counted as a current carrier, then you don't count it for T310.15(B)(2)(a), so perhaps using 310.15(B)(4) should not be used for this question.


TOTAL LOAD - BALANCED LOAD + NONLINEAR LOAD = NEUTRAL LOAD

I did the calculation wrong. Lets revisit.

The total load is 70,000 VA
the non linear load is 30,000 VA
Balanced load is unknown


70,000 VA + 30,000 VA = 100,000

100,000 / 480 = 208 amperes

208 amperes would need a 250 kCMIL conductor size based on 110.14 (C)...60* column.

208 x 1.25 = 260 amps (mininum conductor ampacity at continous load).

**220.61 (C) 1 prohibit the reduction of the neutral.
**See also, 220.61 (C) 1 FPN No. 2
**the neutral is part of the feeder and must be size for the same load.

**250.24 (C)1 dictates size the neutral conductor to table 250.66 if it is to be used as an EGC.

In this cased, we are hot sizing the neutral to be an EGC. We are sizing and considering it as a current carrying conductor.

am I missing something here?
 

boku0003

Member
I believe 70kVA is the total load, of which 30kVA is nonlinear (40 linear, 30 non-linear). So if you take 70 and add 30, you are adding it twice.

220.61(A) states "The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral and any one ungrounded conductor."

So how much of this would count as unbalanced? I think that is the heart of the problem/question. We all know how to do the math after that. By the way, I think we might have to divide by 277 (line-neutral) since 220.61(A) states "...maximum net calculated load between the neutral and any one ungrounded conductor". Unless you are calculating the size of the ungrounded conductor in order to use 250.66.
 

Smart $

Esteemed Member
Location
Ohio
I believe 70kVA is the total load, of which 30kVA is nonlinear (40 linear, 30 non-linear). So if you take 70 and add 30, you are adding it twice.
I agree.

...

So how much of this would count as unbalanced? I think that is the heart of the problem/question. We all know how to do the math after that. By the way, I think we might have to divide by 277 (line-neutral) since 220.61(A) states "...maximum net calculated load between the neutral and any one ungrounded conductor". Unless you are calculating the size of the ungrounded conductor in order to use 250.66.
As I said earlier, the amount of load that is connected line-to-neutral is not stated. This type of load is the only type which contributes to neutral current. Without knowing, there is only one ultimately safe approach... make the neutral conductor the same size as the phase/line/ungrounded conductors.

Earlier you brought up...
Someone mentioned the FPN in this section. (1) says "Any portion of a 3-wire circuit consisting of 2-phase wires and the neutral of a 4-wire, 3-phase, wye connected system".
An example of this situtation would be a 120/208 1? 3W service that you might see in a multi-dwelling complex... and the grounded conductor for such is always counted as a CCC.

In typical 3? 4W wye systems, the neutral or grounded conductor is seldom considered a CCC. The following are the only conditions where it is counted as a CCC: 1) not all 3 phase/line/ungrounded conductors are included in the circuit, and/or 2) non-linear line-to-neutral loads make up a majority of the load connected to the neutral conductor.
 

boku0003

Member
Feeder

Feeder

I got some more info from my study partner. Here's what he wrote. I asked him about the 30k/70k question. I'll get back to you on that.


"I've look further in my notes on this one. The choices they gave to
pick from was A) 800 B) 600 C) 400 and D) 420.
I don't remember exactly the question but are I remember was: 800
ampere 480/277 service with a load of 70,000 VA and a
non-linear load of 30,000 VA. What is the feeder neutral?"
 

Smart $

Esteemed Member
Location
Ohio
I got some more info from my study partner. Here's what he wrote. I asked him about the 30k/70k question. I'll get back to you on that.


"I've look further in my notes on this one. The choices they gave to
pick from was A) 800 B) 600 C) 400 and D) 420.
I don't remember exactly the question but are I remember was: 800
ampere 480/277 service with a load of 70,000 VA and a
non-linear load of 30,000 VA. What is the feeder neutral?"
IMO, the "more info" makes the question even more confusing...

Again making assumptions, an 800A 480/277V service would require at least parallel 500kcm Cu 75?C conductors for 701A to 760A calculated load, or parallel 600's fro 761 up to 800A calculated load. The minimum neutral service conductor by 250.66 GEC sizes respectively would be 2/0 and 3/0 Cu, neither of which are close to 400, the lowest value answer to choose from.

There has got to be some pertinent info missing :confused:
 
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