Neutral sizing 220.61

[darren12]

220.61 states that the neutral load shall be the maximum unbalance of calculated between the neutral conductor and any one ungrounded conductor.

If my panel voltage and capacity is 120/240VAC 400Amp load center and the loads connected are all 2 wire 240Volt appliances with balanced current on the 2 ungrounded conductors ex. electric heaters and water heaters what do I use to size my neutral to the panel?

 

[Dennis Alwon]

If this is a service then the neutral can be sized as small as the grounding electrode conductor. If it is a feeder then size the neutral no smaller than the equipment grounding conductor but if there are no neutral loads then you don't need a neutral for a feeder.

 

[darren12]

If this is a service then the neutral can be sized as small as the grounding electrode conductor. If it is a feeder then size the neutral no smaller than the equipment grounding conductor but if there are no neutral loads then you don't need a neutral for a feeder.

What article calls out the service neutral can be sized as small as the grounding electrode conductor? What article calls out the feeder neutral shall be no smaller than the equipment grounding conductor? I just want to read it and understand it.
I do have a couple 20 amp lighting and receptacle loads on this panel so I know I have to at least size it to carry the load of them but it didn't seem right to size it so small and the ungrounded conductors to carry 400amps. What if someone in the future pulls out the balanced 240v loads and connects 120V loads?

 

[Dennis Alwon]

The 2017 has changed a bit so that Table 260.66 has been split up and now the grounded conductor is sized by the bonding jumper which is T. 250.102 which is basically the same as T250.66

230.23(C) Grounded Conductors. The grounded conductor shall
not be less than the minimum size as required by 250.24(C).

(C) Grounded Conductor Brought to Service Equipment.
Where an ac system operating at 1000 volts or less is grounded
at any point, the grounded conductor(s) shall be routed with
the ungrounded conductors to each service disconnecting
means and shall be connected to each disconnecting means
grounded conductor(s) terminal or bus. A main bonding
jumper shall connect the grounded conductor(s) to each service
disconnecting means enclosure. The grounded conductor(
s) shall be installed in accordance with 250.24(C)(1)
through 250.24(C)(4).


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

 

[JFletcher]

What if someone in the future pulls out the balanced 240v loads and connects 120V loads?

In that case the neutral will carry the difference of the currents of leg A and leg B. For that to fry a typically downsized neutral, this would require the astronomically impossible odds of all 240-volt equipment being disconnected, and something like 20 1500 watt space heaters or 75 400w high pressure sodium lights to all be plugged in at once on just a single leg.

Normally the 120 volt loads will be fairly balanced on each leg, which is required under 210.11(b), and also fairly low in many calculations compared to the 240 volt loads. if you had a perfect 50/50 split, there would be zero amps on the neutral... You would not even need a neutral under a perfect load split distribution.

 

[LarryFine]

You would not even need a neutral under a perfect load split distribution.

That would be true only if there are no loads with a neutral at all. Balance can not be guaranteed otherwise.

 

[darren12]

The 2017 has changed a bit so that Table 260.66 has been split up and now the grounded conductor is sized by the bonding jumper which is T. 250.102 which is basically the same as T250.66



(C) Grounded Conductor Brought to Service Equipment.
Where an ac system operating at 1000 volts or less is grounded
at any point, the grounded conductor(s) shall be routed with
the ungrounded conductors to each service disconnecting
means and shall be connected to each disconnecting means
grounded conductor(s) terminal or bus. A main bonding
jumper shall connect the grounded conductor(s) to each service
disconnecting means enclosure. The grounded conductor(
s) shall be installed in accordance with 250.24(C)(1)
through 250.24(C)(4).


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

I was looking at a feeder for a “sub panel” so I see that 215.2 (A) (2) says to use 250.122 and looking at 250.122 shows for a 400amp OPD use 3 AWG.

Sound right??

 

[Dennis Alwon]

I was looking at a feeder for a “sub panel” so I see that 215.2 (A) (2) says to use 250.122 and looking at 250.122 shows for a 400amp OPD use 3 AWG.

Sound right??

Yep--250.122 is the equipment grounding conductor. If there was a line to neutral short then the neutral would need to be sized the same as the equipment grounding conductor in order to safely trip the breaker. Imagine a long run with a #12 equipment grounding conductor, if there were a 100 amp breaker the chances are the #12 would not be capable of opening the breaker