Down sizing neutral on a service

[Doctor Electric]

Whatsup with the whole downsizing the neutral on a service 200 amp gets 2ot 2ot 1ot 2 hots and 1ot neutral guys tell me not to do this just up the neutral to 2ot can you guys (my trusted elders) point me to a reference in the nec that clarifies this for me so im doing what the code says any thoughts on this will be great aswell thanks!

 

[xformer]

The calculation for the Neutral conductor should be found in Article 220.

 

[don_resqcapt19]

You look at the load calculation and take out all of the line to line loads, then the neutral is sized based on the worst case which would be all of the line to neutral loads that are connected to single leg. It would be very rare that you would need a neutral that is the same size as the ungrounded conductors.

 

[suemarkp]

The odds of a system being 100% unbalanced is pretty low, but the NEC doesn't seem to give allowance for this until the service or feeder is over 200A. So some of the neutral current is going to cancel unless its a 3 phase system with a lot of harmonic loads. Also, most panels have 240 or 208 only loads (most water heaters and heat pumps/air conditioners). These use no neutral current so would not be added to neutral ampacity when doing the calculation. Finally, some 120/240V loads either have minimal neutral current by design (e.g. clothes dryer), or things tend to balance out (electric range). The code gives those a 70% factor for neutral load when doing a load calculation.

 

[Dennis Alwon]

Dropping one or two sizes in the neutral has long been a rule of thumb but not an NEC code. Without calculations one can be pretty sure that dropping one size is never an issue. In fact, if you calculate the neutral load then you may be able to go as small as the main bonding jumper from T.250.102(C)(1). In your case of 2/0 aluminum you could possible go as small as 2 aluminum.

Without calculations and if there are definitely 240v circuits then dropping one size is not an issue.

 

[ActionDave]

I always go down at least one size for the neutral unless job specs don't allow it.

 

[tom baker]

Typically it’s 4-0 4-0 2-0 and that’s what the AHJ is used to. Any smaller on the neutral and they may want to see your calculations.

 

[LarryFine]

Side comment: the service neutral need not even extend past the main OCPD if there are no neutral loads.

A few years ago, I added a 200a ML panel for a tankless water heater, and fed the panel with SE, not SER.

The inspector failed it, I asked him to call his boss, who backed him up, then called back and said pass it.

 

[electrofelon]

Side comment: the service neutral need not even extend past the main OCPD if there are no neutral loads.

A few years ago, I added a 200a ML panel for a tankless water heater, and fed the panel with SE, not SER.

The inspector failed it, I asked him to call his boss, who backed him up, then called back and said pass it.

Yeah I never understood why so many people think you need to run the neutral conductor in a feeder or BC. It's just a system conductor that you may or may not need, no different that pulling single phase stuff off a three phase panel....

 

[Dennis Alwon]

Like I said rule of thumb but it is not in the NEC to drop down one or two sizes. Generally that is not an issue.

 

[dkidd]

Whatsup with the whole downsizing the neutral on a service 200 amp gets 2ot 2ot 1ot 2 hots and 1ot neutral guys tell me not to do this just up the neutral to 2ot can you guys (my trusted elders) point me to a reference in the nec that clarifies this for me so im doing what the code says any thoughts on this will be great aswell thanks!

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

 

[kwired]

If you have a 200 amp single phase 120/240 service and have around 25kVA of 240 volt loads, you don't need to get too complicated with calculations to determine about half that 200 amps is not going to be loading the neutral at all.

Can't remember exactly what code section requires it, but in no case can your service grounded conductor be smaller than the minimum required grounding electrode conductor and for feeders it can't be smaller than the required equipment grounding conductor, when present, you don't need to run a grounded conductor with a feeder if none the loads supplied utilize it.

 

[xformer]

If you have a 200 amp single phase 120/240 service and have around 25kVA of 240 volt loads, you don't need to get too complicated with calculations to determine about half that 200 amps is not going to be loading the neutral at all.

Can't remember exactly what code section requires it, but in no case can your service grounded conductor be smaller than the minimum required grounding electrode conductor and for feeders it can't be smaller than the required equipment grounding conductor, when present, you don't need to run a grounded conductor with a feeder if none the loads supplied utilize it.

230.23(C) / 230.30 (C) - 250.24(C) - 250.102 (C) 1

 

[Dennis Alwon]

Can't remember exactly what code section requires it, but in no case can your service grounded conductor be smaller than the minimum required grounding electrode conductor and for feeders it can't be smaller than the required equipment grounding conductor, when present, you don't need to run a grounded conductor with a feeder if none the loads supplied utilize it.

That used to be true but then they separated T250.66 to 250.102(C)(1) as I mentioned above. The tables are almost identical until you get to sizes of over 1100KCM

250.24(C)(1) Sizing for a Single Raceway or Cable. The grounded
conductor shall not be smaller than specified in Table
250.102(C)(1).