neutral load calculation

[martyg]

When explaning to my students how to use the form to calculate a residential service load, I noticed, that when calculating the neutral load, the appropriate VA total was divided by 240 volts instead of 120 volts. It seems to me that the neutral conductor would only carry the unbalanced current from 120 volt circuits. Why then is the neutral load calculated by dividing by 240 volts? Any suggestions before one of my 'over-achieving' students catches this, and ask me about it in class?

 

[charlie b]

The current flowing in the neutral started out life by flowing on the two phase conductors. The power associated with it is the power flowing in the one phase conductor added to (or perhaps more properly, subtracted from) the power flowing in the other. The power associated with the two phase conductors is driven by a 240 volt source, so the current value is calculated by dividing by 240. The fact that some of this power flows in the neutral, rather than back via one of the phase conductors, does not change the fact that it is a 240 volt source that is driving the whole parade.

How does that work for you?

Welcome to the forum.

 

[handy10]

What if, for some strange reason, one of the phases is not used at all. That is, zero current flows in one phase. Then, should one still divide the load by 240 in order to size the neutral? Last winter, one phase burned out in one of my customers house. POCO removed the meter and shorted the working phase to the burned out one. Only remained that war for three months.

 

[Smart $]

I'd have to know exactly what you mean by "the appropriate VA total" before I could answer your question with certainty.

The only appropriate way to calculate a VA value to a neutral conductor per 220.61, is to first know what Line-Neutral loads are connected to which Line. Add VA up for all connected to Line 1. Add up all connected to Line 2 separately. The larger of these two totals is the corrct value to use... and yes, you divide by 120.

 

[martyg]

neutral load calculation

First of all, thanks to all who responded to my question.
The form we used to calculate the service load for a single family dwelling goes like this.

1. General lighting + 2 small appliance circuits + 1 laundry circuit
All 120 volt stuff, VA's totalled and applied to the phases, this total also applied to the neutral ( this sounds like worse case to me, ie all 120 volt circuits are driven by 1 of the 2 phases).

2. Fixed appliances (dishwaher, dispposer, attic fan etc)
VA's totalled, 75% demanad factor applied, result applied to phases , and also to neutral.

3. Dryer
100% of the VA's applied to the phases, 70% applied to the neutral.

4. Cooking Equipment
100% of the VA's applied to the phases, 70% applied to the neutral.

5. Heating / AC
Largest VA load applied to the phases, nothing to the neutral.

6. Largest Motor
VA X 25%, result applied to phases and to the neutral.

All phase VA's totalled, then divided by 240 volts.
All neutral VA's totalled, then divided by 240 volts.

I agree that the best approach for sizing the neutral(220.61) is to determine the worst case unbalanced load, then divide that by 120 volts, but the form provided in our text book has no provisions for this approach. I submit that one cannot know to what extent the unbalanced load will be, when doing the service load calculation, so the worst case is applied

If dividing the neutral VA load by 240 volts is merely some scheme to derate the total by 1/2, I can accept that.

Marty G.

 

[mull982]

First of all, thanks to all who responded to my question.
The form we used to calculate the service load for a single family dwelling goes like this.

1. General lighting + 2 small appliance circuits + 1 laundry circuit
All 120 volt stuff, VA's totalled and applied to the phases, this total also applied to the neutral ( this sounds like worse case to me, ie all 120 volt circuits are driven by 1 of the 2 phases).

2. Fixed appliances (dishwaher, dispposer, attic fan etc)
VA's totalled, 75% demanad factor applied, result applied to phases , and also to neutral.

3. Dryer
100% of the VA's applied to the phases, 70% applied to the neutral.

4. Cooking Equipment
100% of the VA's applied to the phases, 70% applied to the neutral.

5. Heating / AC
Largest VA load applied to the phases, nothing to the neutral.

6. Largest Motor
VA X 25%, result applied to phases and to the neutral.

All phase VA's totalled, then divided by 240 volts.
All neutral VA's totalled, then divided by 240 volts.

I agree that the best approach for sizing the neutral(220.61) is to determine the worst case unbalanced load, then divide that by 120 volts, but the form provided in our text book has no provisions for this approach. I submit that one cannot know to what extent the unbalanced load will be, when doing the service load calculation, so the worst case is applied

If dividing the neutral VA load by 240 volts is merely some scheme to derate the total by 1/2, I can accept that.

Marty G.

Think of it this way. For a given load on a 240/120V panel you calculate the maximum current by dividing the kVA by 240V. This will give you the maximum L-L current for the given load. This current values is the value used for sizing panel, main breaker etc. So lets say for example then when performing this calc you come up with 100A. This tells you that for the maximum load you will have at maximum 100A L-L current and breaker and panel will be rated accordingly. Now with a 100A main breaker you can also view this as saying this same breaker will allow me to have a 100A L-N load on either of the two 120V legs. So ignoring harmonics, p.f. and other things that add to neutral current you can simply say that the worst case scenario for an unbalanced neutral would be 100A (with only 1 of the 120V legs serving L-N loads) You would then use this 100A as your worst case neutral current.

So you can see from this that for a given load you essentially use the 240V to determine the maximum current for the load, which will in turn lead to the sizing of devices which will dictate maximum current allowed for any given L-N load on a single phase which is taken as the worst case scenario.

 

[martyg]

neutral load calculation

Thanks to mull982's explanation, I now thoroughly understand why the neutral load current is calculated by dividing by 240 volts. The world is now back in order.

Thank you so much,

Marty G.

 

[kingpb]

240/120V I thought the topic was single phase residential service calcs?

120/240V and 240/120V are not interchangeable terms :roll:

 

[charlie b]

120/240V and 240/120V are not interchangeable terms :roll:

Nonetheless, they are going to be treated as interchangeable by a great many people. There is a difference between the precise language, the precise use of terms, associated with any profession, and the language and terms that I like to call "conversational English." Most of us use the later language most of the time, and in that language it is not worth the time to try and remember which comes first (and why), the 120 or the 240.

 

[kingpb]

Nonetheless, they are going to be treated as interchangeable by a great many people. There is a difference between the precise language, the precise use of terms, associated with any profession, and the language and terms that I like to call "conversational English." Most of us use the later language most of the time, and in that language it is not worth the time to try and remember which comes first (and why), the 120 or the 240.

Personally, I like to take pride in what I do, and that includes even the small things such as stating voltages correctly. I guess some may regard the differences between a single phase 3-wire system, and a 3-phase high leg delta system trivial, but I do not.

There is a reason that ANSI, IEEE, NEMA, and other standards were developed; so that we are all talking the same language and have consistency. But then I think anyone in the US should have to speak English, too; so, maybe I'm just goofy that way.

Next time you go to the doctor, ask him/her if they practice conversational medicine, or if they are going to stick to the technical stuff and have a grasp of the whys, and how's. I'll bet you will appreciate the fact that the doctor remembers to apply the anesthetic before he starts to cut. If he can't remember, don't blame him, he's just being conversational.

 

[charlie b]

Personally, I like to take pride in what I do

I don?t. Complete waste of time and energy! :roll:

Whether or not we pay attention to the fact, we all speak two separate languages. So do doctors, and any other professional. When my doctor speaks with one of his colleagues, I suspect they will use terms that I will not understand. When he speaks with me, he will use other terms, or perhaps use the same terms in a different way. My point is that we need to be aware, when we write, speak, listen, read or otherwise apply technicalish terms, which of the two languages is being used.

 

[kingpb]

I don?t. Complete waste of time and energy! :roll:

Charlie, I know your joking, and I am sure others do too, but on the serious side, making a statement like that, in writing, could come back to bite you in the, you know where.

Lawyers will use anything to win a case, and they do like to talk technical

 

[Smart $]

...

2. Fixed appliances (dishwaher, dispposer, attic fan etc)
VA's totalled, 75% demanad factor applied, result applied to phases , and also to neutral.

...

6. Largest Motor
VA X 25%, result applied to phases and to the neutral.

...

Thanks to mull982's explanation, I now thoroughly understand why the neutral load current is calculated by dividing by 240 volts. The world is now back in order.

Thank you so much,

Marty G.

Doing the calculation as such is inappropriate by Code. It assumes the panel's L-N loading will be completely balanced. It is a rare occurence when that actually happens. Continuing to do it as you're suggesting does nothing more than perpetuate using a flawed method. That said, it will likely get overlooked, as many simply misunderstand the appropriate method because so many perpetuate using the flawed method.

Additionally, 220.61(B) is the only permitted reductions to maximum unbalanced neutral current. Number 2 in your form appears to pass the demand factoring on to the neutral loading. That is not permitted.

Number 6 appears to account for 25% of the largest motor load... but where is the other 100%??? If it is in the Heating?A/C determination, fine... but what if it is not? Also, don't forget that many central air systems use the furnace blower motor when the A/C is running.

 

[mull982]

Doing the calculation as such is inappropriate by Code. It assumes the panel's L-N loading will be completely balanced. It is a rare occurence when that actually happens. Continuing to do it as you're suggesting does nothing more than perpetuate using a flawed method. That said, it will likely get overlooked, as many simply misunderstand the appropriate method because so many perpetuate using the flawed method.

Additionally, 220.61(B) is the only permitted reductions to maximum unbalanced neutral current. Number 2 in your form appears to pass the demand factoring on to the neutral loading. That is not permitted.

Number 6 appears to account for 25% of the largest motor load... but where is the other 100%??? If it is in the Heating?A/C determination, fine... but what if it is not? Also, don't forget that many central air systems use the furnace blower motor when the A/C is running.

I was only giving my example to show how it is possible that the 240V was used in the calc the OP was looking at.

I'd like to understand exactly how the neutral should be sized. Is 220.61 the place to look to size neutral properly? What is the difference between using this section and the table in 250.66 which I have also seen or heard can be used for sizing the neutral?

 

[LarryFine]

I guess some may regard the differences between a single phase 3-wire system, and a 3-phase high leg delta system trivial, but I do not.

Well, other than the high leg, they're identical, but, I agree it's not trivial.

Also, don't forget that many central air systems use the furnace blower motor when the A/C is running.

We keep ours set that way for several reasons, such as better air filtering and better comfort, and maybe even saving a few cents.

 

[Smart $]

I was only giving my example to show how it is possible that the 240V was used in the calc the OP was looking at.

I'd like to understand exactly how the neutral should be sized. Is 220.61 the place to look to size neutral properly? What is the difference between using this section and the table in 250.66 which I have also seen or heard can be used for sizing the neutral?

Several sections contribute to sizing of a neutral conductor properly. Section 220.61 is the one which "a size" is determined by calculation. Other sections provide minimum size requirement and the not-required-to-be-larger-than size. I have recapped below the most pertinent section parts by Article...

ARTICLE 215
Feeders
?
215.2 Minimum Rating and Size.

(A) Feeders Not More Than 600 Volts.

(1) General. ?
?
Exception No. 2: Grounded conductors that are not connected
to an overcurrent device shall be permitted to be
sized at 100 percent of the continuous and noncontinuous
load.
The size of the feeder circuit grounded conductor shall
not be smaller than that required by 250.122, except that
250.122(F) shall not apply where grounded conductors are
run in parallel.
?

(B) Feeders over 600 Volts. ?
?
Where installed, the size of the feeder-circuit
grounded conductor shall not be smaller than that required
by 250.122, except that 250.122(F) shall not apply where
grounded conductors are run in parallel. ?

ARTICLE 220
Branch-Circuit, Feeder, and Service
Calculations
?
III. Feeder and Service Load Calculations
?
220.61 Feeder or Service Neutral Load.

(A) Basic Calculation. 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 conductor
and any one ungrounded conductor.
Exception: For 3-wire, 2-phase or 5-wire, 2-phase systems,
the maximum unbalanced load shall be the maximum
net calculated load between the neutral conductor and any
one ungrounded conductor multiplied by 140 percent.

(B) Permitted Reductions. A service or feeder supplying
the following loads shall be permitted to have an additional
demand factor of 70 percent applied to the amount in
220.61(B)(1) or portion of the amount in 220.61(B)(2) determined
by the basic calculation:
(1) A feeder or service supplying household electric
ranges, wall-mounted ovens, counter-mounted cooking
units, and electric dryers, where the maximum unbalanced
load has been determined in accordance with
Table 220.55 for ranges and Table 220.54 for dryers
(2) That portion of the unbalanced load in excess of 200
amperes where the feeder or service is supplied from a
3-wire dc or single-phase ac system; or a 4-wire,
3-phase, 3-wire, 2-phase system; or a 5-wire, 2-phase
system

(C) Prohibited Reductions. There shall be no reduction of
the neutral or grounded conductor capacity applied to the
amount in 220.61(C)(1), or portion of the amount in (C)(2),
from that determined by the basic calculation:
(1) Any portion of a 3-wire circuit consisting of 2 ungrounded
conductors and the neutral conductor of a
4-wire, 3-phase, wye-connected system
(2) That portion consisting of nonlinear loads supplied
from a 4-wire, wye-connected, 3-phase system
FPN No. 1: See Examples D1(a), D1(b),
D2(b), D4(a), and D5(a) in Informative Annex D.
FPN No. 2: A 3-phase, 4-wire, wye-connected
power system used to supply power to nonlinear
loads may necessitate that the power system design allow
for the possibility of high harmonic neutral-conductor
currents.

ARTICLE 230
Services
?
II. Overhead Service-Drop Conductors
?
230.23 Size and Rating.
?
(C) Grounded Conductors. The grounded conductor shall
not be less than the minimum size as required by
250.24(C).
?

III. Underground Service-Lateral Conductors
?
230.31 Size and Rating.
?
(C) Grounded Conductors. The grounded conductor shall
not be less than the minimum size required by 250.24(C).
?

IV. Service-Entrance Conductors
?
230.42 Minimum Size and Rating.
?
(C) Grounded Conductors. The grounded conductor shall
not be smaller than the minimum size as required by
250.24(C).
?

 

[Smart $]

(Continuation)

ARTICLE 250
Grounding and Bonding
?
II. System Grounding
?
250.24 Grounding Service-Supplied Alternating-
Current Systems.
?
(C) Grounded Conductor Brought to Service Equipment.
?
(1) Routing and Sizing. This conductor shall be routed
with the phase conductors and shall not be smaller than the
required grounding electrode conductor specified in Table
250.66 but shall not be required to be larger than the largest
ungrounded service-entrance phase conductor(s). In addition,
for sets of service-entrance phase conductors larger than 1100 kc-
mil copper or 1750 kcmil aluminum, the grounded conduc-
tor shall not be smaller than 121⁄2 percent of the area of the
largest service-entrance phase conductor. ?

(2) Parallel Conductors. Where the service-entrance phase
conductors are installed in parallel, the size of the grounded
conductor shall be based on the total circular mil area of the
parallel conductors as indicated in this section. Where in-
stalled in two or more raceways, the size of the grounded
conductor in each raceway shall be based on the size of the
ungrounded service-entrance conductor in the raceway but not
smaller than 1/0 AWG.
FPN: See 310.10(H) for grounded conductors connected in
parallel.

(3) High Impedance. The grounded conductor on a high-
impedance grounded neutral system shall be grounded in
accordance with 250.36.

?

250.36 High-Impedance Grounded Neutral Systems.
?
(B) Grounded System Conductor. ?
?.
The grounded system conductor shall have an ampacity
of not less than the maximum current rating of the grounding
impedance. In no case shall the grounded system conductor
be smaller than 8 AWG copper or 6 AWG aluminum
or copper-clad aluminum.