Example D2(a)

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antenna2001

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I attempted to solve Example D2(a) on page 70-717 of the 2005 NEC. I did fine until I tried the Feeder Neutral Load. I included all the loads shown on the solution and I also included:
1. Space heating @ 9000 VA
2. Water heater @ 2500 VA.

The solution in the NEC excludes these two factors. Why?

Thanks
 
How about range and dryer?

How about range and dryer?

bphgravity said:
Those two appliances are 240V loads and place no demand on the neutral...
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Yes, that is what I thought, but if that is the case, can we not assume that the range and the clothes dryer are both, more than likely, 240 V loads too?

Thanks for the reply
 
antenna2001 said:
Yes, that is what I thought, but if that is the case, can we not assume that the range and the clothes dryer are both, more than likely, 240 V loads too?

Thanks for the reply
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220.61(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
 
Look at the schematic of a typical Range or Dryer and you will see the where the neutral comes into play for the 120 v loads of each appliance.

In other words, they are 120 / 240 volt supplied units

Roger
 
jwelectric said:
220.61(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
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There is no section that permits reductions (demand factors) applied to the space heating and water heater loads. Therefore, shouldn't these loads be entered 100 % in the calculations?

If I think of a Y-connected transformer providing, say, phases A and B, to supply these loads, then whatever current that flows on those phases, that is not equally balanced by a current flowing in phase C, would flow in the neutral. If this is correct, then all the loads, regardless of voltage cause an unbalance in the system and they should be included in the neutral load calculation. Is this faulty reasoning?
 
roger said:
Look at the schematic of a typical Range or Dryer and you will see the where the neutral comes into play for the 120 v loads of each appliance.

In other words, they are 120 / 240 volt supplied units

Roger
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Thanks, this makes perfect sense. The 120 V loads would be the unbalance from the point of view of phases a and b. The question of phase C that I posted in my reply to jwelectic's reply is still unanswered. Any difference in current between phases A, B, and C flows in the neutral, as far as I know
 
The water heater is entered at 100 % and see 220.82(C)(6) for the space heating reduction.

D2(a) is only addressing a 120/240 volt single phase service, but you are right that the neutral carries the same current as the two phases of a wye if the third phase is not in play.

Roger
 
antenna2001 said:
There is no section that permits reductions (demand factors) applied to the space heating and water heater loads. Therefore, shouldn't these loads be entered 100 % in the calculations?

If I think of a Y-connected transformer providing, say, phases A and B, to supply these loads, then whatever current that flows on those phases, that is not equally balanced by a current flowing in phase C, would flow in the neutral. If this is correct, then all the loads, regardless of voltage cause an unbalance in the system and they should be included in the neutral load calculation. Is this faulty reasoning?
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How can current flow in a conductor that is not connected?
 
roger said:
The water heater is entered at 100 % and see 220.82(C)(6) for the space heating reduction.

D2(a) is only addressing a 120/240 volt single phase service, but you are right that the neutral carries the same current as the two phases of a wye if the third phase is not in play.

Roger
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That is what I thought, there most be a mistake (impossible?) in the answer to D2(a) since they don't include them.
 
Example D2(a) Optional Calculation for One-Family Dwelling, Heating Larger Than Air Conditioning [see 220.82]
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antenna2001 said:
If I think of a Y-connected transformer providing, say, phases A and B, to supply these loads, then whatever current that flows on those phases, that is not equally balanced by a current flowing in phase C, would flow in the neutral. If this is correct, then all the loads, regardless of voltage cause an unbalance in the system and they should be included in the neutral load calculation. Is this faulty reasoning?
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Where is the three phase wye coming from?

edited to remove a word that was not needed.
 
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jwelectric said:
How can current flow in a conductor that is not connected?
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This is a superb question. I don't have the answer for it right away. I have to review some theory and that will take a couple of hours. However, from having asked the same question myself before, I know that I answered it satisfactorily and I can asure you that the current does flow in the neutral even though it is not connected to the appliance. I will come back with an explanation later.
 
antenna2001 said:
This is a superb question. I don't have the answer for it right away. I have to review some theory and that will take a couple of hours. However, from having asked the same question myself before, I know that I answered it satisfactorily and I can asure you that the current does flow in the neutral even though it is not connected to the appliance. I will come back with an explanation later.
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The basic rule for current flow is that it has to have a path. If the neutral is not part of that path then current can not flow on the neutral.
 
jwelectric said:
How can current flow in a conductor that is not connected?
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I sent a reply to this question, but some reason I don't see it posted. In summary, what I said was that you asked an excellent question and that I don't have a complete explanation right of the top of my head, but I asure you that the difference of the currents flowing in phases A, B, and C does flow in the neutral even though the neutral might not be directly connected to any appliance. I will come back with an explanation in a couple of hours. I will go back and review the theory.
 
jwelectric said:




Where is the three phase wye coming from?

edited to remove a word that was not needed.
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The 3-phase wye transformer is at the top of the utility's power pole. From this transformer, the utility sends three wires (two phase wires and a neutral)to supply the 120/240V residential load. Even though you don't see the third phase, it is still part of the system in the sense that it balances (or unbalances) the system. The system, in this case, is the y-connected transformer.
 
Antenna, a wye configuration of windings can not produce 120/240 it would be 120/208.

As Mike said, current will not flow in a conductor that is not part of the circuit.

If you can find some information on Kirchoffs Current laws you will see this.

In short, current will not leave home (source) if it can not get back home, and to do so it must have a complete circuit. A neutral (or any conductor) that isn't connected to a load as part of this circuit will not have current flowing on it, if it does we have a problem.

Roger
 
roger said:
Antenna, a wye configuration of windings can not produce 120/240 it would be 120/208.

As Mike said, current will not flow in a conductor that is not part of the circuit.

If you can find some information on Kirchoffs Current laws you will see this.

In short, current will not leave home (source) if it can not get back home, and to do so it must have a complete circuit. A neutral (or any conductor) that isn't connected to a load as part of this circuit will not have current flowing on it, if it does we have a problem.

Roger
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You are right. Thanks for setting me straight. The system must be a delta transformer with a tap conductor taken from the middle of a phase winding (or windings). In this case, no unbalance current can flow in the neutral if it is not connected.

This, clarifies everything. The space heating and water heater are not included in the neutral calculations because they do not contain any 120 V loads connected to the neutral. The range and the dryer do contain 120 V circuits and that is why they are included.

I hope I have it right this time. Please tell me if I don't.
 
Antenna, you've got it.
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BTW, welcome to the forum.

Roger
 
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