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?
Yes, I think you've got it!
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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:
Rob
Moderator
All responses based on the 2014 NEC unless otherwise noted
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.
Rob
Moderator
All responses based on the 2014 NEC unless otherwise noted
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.
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.
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.
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
Rob
Moderator
All responses based on the 2014 NEC unless otherwise noted
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”
Last edited by david; 10-13-17 at 11:44 AM.
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