Service Neutral Grounded Conductor Ampacity?

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hitehm

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Got a test question not sure how to answer: "A 120/240-volt, 200-ampere service requires a neutral with a minimum ampacity of:".

Answers to choose from are: 140 160 175 or 200 amps.

At first I thought well 230.42 Exception says" Grounded conductors not connected to a OC device shall be permitted to be sized at 100% of non and continuous load so that would be 200 amps. However...

230.42C says "The grounded conductor shall not be smaller than required by 250.24C which says it can't be smaller than the GEC in table 250.66. Table 250.66 gives wire sizes based on the Largest ungrounded service conductor, but it does not chart minimum ampacities and the question doesn't say the type of cable or even if it's residential. So how do I determine the "minimum ampacity" of the service neutral? And then there is 220.61 which says the service neutral load shall be the max unbalanced load between any one ungrounded conductor and neutral, so wouldn't that be 100 amps in a 200 amp service?

What am I missing?
 
hitehm said:
...And then there is 220.61 which says the service neutral load shall be the max unbalanced load between any one ungrounded conductor and neutral, so wouldn't that be 100 amps in a 200 amp service?

What am I missing?
Click to expand...

The 200 A in that service could be 200A from hot-to-hot or hot-to-neutral, though it would be decidedly rare for that latter to occur. You'd have to purposely power everything on one side and not the other, and many panels many not even have that many loads, but they CAN. Even a randomly distributed panel is likely to never see a load near that unbalanced.

I'm not addressing the full context of the test question, just that principle.

Look at how a 20 A single-pole breaker and a 20 A double-pole breaker are both 20 A, but one is hot-to-hot, and the other is hot-to-neutral. Same amperage at different voltage.

Now put ten 20 A single pole breakers on the same buss, and load them up. You are going to have 200 A from hot-to-neutral on the feeders of that 200 A, 240 V panel. Again, this would almost have to be done on purpose to ever see it in real life.
 
MAC702 said:
Now put ten 20 A single pole breakers on the same buss, and load them up. You are going to have 200 A from hot-to-neutral on the feeders of that 200 A, 240 V panel. Again, this would almost have to be done on purpose to ever see it in real life.
Click to expand...

And would also mean the neutral is carrying the entire load, i.e. no imbalance.
 
hitehm said:
Got a test question not sure how to answer: "A 120/240-volt, 200-ampere service requires a neutral with a minimum ampacity of:".

Answers to choose from are: 140 160 175 or 200 amps.

At first I thought well 230.42 Exception says" Grounded conductors not connected to a OC device shall be permitted to be sized at 100% of non and continuous load so that would be 200 amps. However...

230.42C says "The grounded conductor shall not be smaller than required by 250.24C which says it can't be smaller than the GEC in table 250.66. Table 250.66 gives wire sizes based on the Largest ungrounded service conductor, but it does not chart minimum ampacities and the question doesn't say the type of cable or even if it's residential. So how do I determine the "minimum ampacity" of the service neutral? And then there is 220.61 which says the service neutral load shall be the max unbalanced load between any one ungrounded conductor and neutral, so wouldn't that be 100 amps in a 200 amp service?

What am I missing?
Click to expand...

The question is impossible to answer.
 
Thanks for replies

Update on this: The writer has the correct answer as 140 amps, which means he's incorrectly interpreting the 70% demand factor rule on the service neutral load, which only applies the the portion of the unbalance load "in excess of 200 amps". Although the question is a flawed, it does bring up a good question though. How do you size the "ampacity" of the service neutral when knowing the service amperage size (not conductor size)? Do you:
A: size it exactly the same as the ungrounded service conductors
B: If you know the conductor type (thhn, thwn etc) do you use table 310.15b7 for the service conductor size and then apply that to table 250.66 for the neutral wire size and then go BACK to 310.15b7 for the ampacity using that wire size? (that seems nutty)
or C: Something other?

What is the minimum info needed to answer his flawed question?
 
hitehm said:
Got a test question not sure how to answer: "A 120/240-volt, 200-ampere service requires a neutral with a minimum ampacity of:".

Answers to choose from are: 140 160 175 or 200 amps.

At first I thought well 230.42 Exception says" Grounded conductors not connected to a OC device shall be permitted to be sized at 100% of non and continuous load so that would be 200 amps. However...

...
... And then there is 220.61 which says the service neutral load shall be the max unbalanced load between any one ungrounded conductor and neutral, so wouldn't that be 100 amps in a 200 amp service?

What am I missing?
Click to expand...
It looks like you are falling into a common misconception. A 200A service is not 100A on each leg. It is 200A on each leg. 200A at 240V.



Sent from my XT1585 using Tapatalk
 
GoldDigger said:
It looks like you are falling into a common misconception. A 200A service is not 100A on each leg. It is 200A on each leg. 200A at 240V.



Sent from my XT1585 using Tapatalk
Click to expand...

That is exactly what I was thinking. Does that mean if I'm provided a 200 amp service into a 200 amp panel at 240 volts that I can actually drawl 400 amps if all my loads were 120 volt hot to neutral?
 
hitehm said:
That is exactly what I was thinking. Does that mean if I'm provided a 200 amp service into a 200 amp panel at 240 volts that I can actually drawl 400 amps if all my loads were 120 volt hot to neutral?
Click to expand...
No. That also leads to misconceptions. You can draw up to 200A of 120V loads on one line and up to 200A of 120V loads on the other.
 
Smart $ said:
No. That also leads to misconceptions. You can draw up to 200A of 120V loads on one line and up to 200A of 120V loads on the other.
Click to expand...

I think I'm getting this now. When all the hot to neutral loads are balanced, they actually become pairs of line to line loads each with 2 equal (in theory) loads in series across the entire 240 volts which gives a voltage "drop" across each of 120V.

example: One 6 ohm load across the 120 volts of line A will pull 20 amps but when paired with another 6 ohm load on line B they become 12 ohms total across 240 volts drawing the same 20 amps. Either way, the main breaker will trip when the panel collectively pulls over 200 amps.

I think I'm getting this. For clarity, does this mean the max unbalanced load is 200 amps in a 200 amp panel? One phase using the entire available current and pushing it all through the neutral wire?
 
200 amps * 70% = 140 amps
Right or wrong looks like the question writer was thinking of this:

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 conduc-
tor and any one ungrounded conductor.
Exception: For 3-wire, 2-phase or 5-wire, 2-phase sys-
tems, 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) de-
termined by the basic calculation:
Click to expand...
 
GoldDigger said:
But what are the "following loads"?
The test question does not describe anything about the loads on the service.
Click to expand...

They might not be applicable to the question but who knows what the writer was thinking. I was merely pointing out that the answer given of 140 amps can be derived from using the 70% rule in 220.61(B). If that was the writer's intent then more info should be given. As a test question I would have chosen 140 amps as the answer.
 
Smart $ said:
No. That also leads to misconceptions. You can draw up to 200A of 120V loads on one line and up to 200A of 120V loads on the other.
Click to expand...
On a 120/240 single phase supply that means the neutral is going to be balanced and carry near zero current at that kind of loading. For 208/120 wye only utilizing two ungrounded plus the neutral conductors - the neutral will be carrying approximately 200 amps in that situation.

hitehm said:
I think I'm getting this now. When all the hot to neutral loads are balanced, they actually become pairs of line to line loads each with 2 equal (in theory) loads in series across the entire 240 volts which gives a voltage "drop" across each of 120V.

example: One 6 ohm load across the 120 volts of line A will pull 20 amps but when paired with another 6 ohm load on line B they become 12 ohms total across 240 volts drawing the same 20 amps. Either way, the main breaker will trip when the panel collectively pulls over 200 amps.

I think I'm getting this. For clarity, does this mean the max unbalanced load is 200 amps in a 200 amp panel? One phase using the entire available current and pushing it all through the neutral wire?
Click to expand...
Yes, but keep in mind the three phase why scenario I mentioned using only two phases and a neutral - that neutral will carry approximately same current as the ungrounded conductors because of the 120 degree phase angle.

infinity said:
200 amps * 70% = 140 amps
Right or wrong looks like the question writer was thinking of this:
Click to expand...
But the conditions that follow don't ordinarily apply to a single family dwelling or other small single phase services.



Question as written can't be answered, it must include more details of the installation, including service entrance conductor size and type, as the absolute minimum neutral conductor will be no smaller then the smallest allowed grounding electrode conductor in all cases.
 
kwired said:
But the conditions that follow don't ordinarily apply to a single family dwelling or other small single phase services.



Question as written can't be answered, it must include more details of the installation, including service entrance conductor size and type, as the absolute minimum neutral conductor will be no smaller then the smallest allowed grounding electrode conductor in all cases.
Click to expand...

Why couldn't 1) possibly apply?

(B) Permitted Reductions. A service or feeder supplyingthe following loads shall be permitted to have an additionaldemand factor of 70 percent applied to the amount in220.61(B)(1) or portion of the amount in 220.61(B)(2) determinedby the basic calculation:
(1) A feeder or service supplying household electricranges, wall-mounted ovens, counter-mounted cookingunits, and electric dryers, where the maximum unbalancedload has been determined in accordance withTable 220.55 for ranges and Table 220.54 for dryers
 
infinity said:
200 amps * 70% = 140 amps
Right or wrong looks like the question writer was thinking of this:
Click to expand...

I agree the question is most likely directed at that section of the code

I would answer 200 amps

But with out the load calculations you could not apply the 70% reduction to a 200 amp service

“or portion of the amount in 220.61(B)(2) determined by the basic calculation:

(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”
 
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