Required Neutrals for parallel feeders

Status
Not open for further replies.
E

electriciansnet

Member
We are installing an 800 amp 3 phase 208v custom panel board for a machine shop which we have totally engineered in the field. This is being installed on the back side of an existing 1200 amp service and my drawings have been approved by the county.
My drawings indicate 3 -400 mcm alu - ungrounded and 1- 400 mcm grounded paralleled X 3.
The only neutral load is the existing 200 amp 3 phase lighting panel which now will be re fed from the new panel board.
Am I actually required to drag a neutral over for each set or can I go with 1 or 2?
Do I have to rate the neutral for the entire 800 amp service with such a small neutral load?
 
You need to keep the characteristics of each parallel set the same. A,B,C, and N could all be different _inside_ of a parallel set, but Phase A from one set must match Phase A from the other sets, B match B, etc.

If you don't pull the same neutral in each set, then the characteristics would be different. Even though you could make the argument that _all_ of the N conductors had the same characteristics, if you had some phase conductors run with N and some run not with N, then the phase conductors would have different characteristics.

_If_ you can reduce the size of your neutral (and I am not going to try to answer that question), then you should use the same reduction across all of the parallel sets.

-Jon
 
When can we derate except for res. and kitchen loads?

When can we derate except for res. and kitchen loads?

Thanks Jon- I found your reference in 310.4 but is there any hard and fast rule that syas we cannot size the neutral in each set to the size of the lighting load which is 200 amp in this case? I know that 220.61 (C)(1)-(2) basically says that you cannot reduce the neutral size in a 3 phase system but it says its supposed to be sized to the max unbalance load and not necessarily the size of the phase conductors. Why can't we run a 4/0 ALU neutral (size of the lighting circuit)with each set of 400? Can you clarify this?
p.s. We are talking about a wye system here.
 
electriciansnet said:
Do I have to rate the neutral for the entire 800 amp service with such a small neutral load?
Click to expand...
No. See 220.61(A) and (B)(2)

Essentially, your neutral only has to be sized to handle the maximum unbalanced load between any one ungrounded conductor and the neutral.

However, if I recall correctly, it cannot be any smaller than the required size for the GEC.
 
electriciansnet said:
Why can't we run a 4/0 ALU neutral (size of the lighting circuit)with each set of 400?
Click to expand...

You can do this. 220.61 (2008 NEC)
It seems to me that, at one time, a neutral could be no less than 12 1/2% of the ungrounded conductors, but I cannot find it now.

Finally, there is no requirement to take a neutral at all. The determining factor is 'need'. If you need a neutral, there are rules. If you have no 'line to neutral' loads, you needn't provide a neutral.
 
thinfool said:
...

Finally, there is no requirement to take a neutral at all. The determining factor is 'need'. If you need a neutral, there are rules. If you have no 'line to neutral' loads, you needn't provide a neutral.
Click to expand...
Not true. This is a 208Y/120V service (assumed, as the OP only stated "208v" and later states there is a "200 amp 3 phase lighting panel" and an "800 amp service").

250.20(B)(1) requires the system to be grounded.

250.24(C) requires the grounded conductor to be brought to the service disconnect and 250.24(C)(1) says the grounded conductor cannot be smaller than required size of the GEC.
 
Last edited:
Required Neutrals for parallel feeders
Click to expand...

We are installing an 800 amp 3 phase 208v custom panel board for a machine shop which we have totally engineered in the field. This is being installed on the back side of an existing 1200 amp service
Click to expand...

It does not sound like a service to me, nothing requires a feeder to have a neutral but if you do run a neutral it must be the larger of either the calculated load or Table 250.122.
 
So Frank- we will have to run triple 400 kcml for our 200 amp lighting panel? I have allowed for this with 2- 4" conduits; the material is on the job and it is what has already been approved by the county. Its just that the existing through that our feeders end up at is getting knda stuffy.
 
Are you tapping the 1200 amp bus or is there an 800 amp overcurrent device in the existing gear?

Do you know the maximum imbalance of the 200 amp panel (The actually current the neutral could be expected to carry?

There is a good chance you could run 3/0 Al x 3 for the neutral.
 
yes smart it is 120/208 and it is part of the building"s service. Not to get off track but to give you guys a visual on how this is: It is an 800 amp service with ct meter for one tennant (our customer) on the back side of the buildings's sole service. It is an 800amp NEMA-1 panelboard with a shunt trip main and a panic button at the location where the existing 200 amp, 208/ 120 meter was. The load that this meter used to serve is the only neutral load which now has a spot on the new panelboard. Kind of crazy but the only way to meet customer's needs since the county would not allow special permission for seperate service of same voltage siting lack of utilization of the existing service. Plus the only way to stay withing 6 throws
 
electriciansnet said:
It is an 800 amp service with ct meter for one tennant (our customer) on the back side of the buildings's sole service.
Click to expand...

My hats off to Smart for seeing that.


So your tapping the existing 1200 amp service before the metering?

If that is the case you will not have an EGC and your minimum neutral size will be based on 250.66
 
Just pulled as-builts and existing riser shows our 200 amp panel to be fed with 3-250 and 1-4/0 Alu from meter that will go away. Load calcs show 17.4 amps on neutral
 
electriciansnet said:
Just pulled as-builts and existing riser shows our 200 amp panel to be fed with 3-250 and 1-4/0 Alu from meter that will go away. Load calcs show 17.4 amps on neutral
Click to expand...


Based on that the grounded conductor will have to be just as Smart pointed out.

250.24(C) requires the grounded conductor to be brought to the service disconnect and 250.24(C)(1) says the grounded conductor cannot be smaller than required size of the GEC.
Click to expand...

Look at 250.24(C)(2) for how to deal with the sizing when using parallel sets.

There will be no need for an EGC as you must bond the new service disconnect.
 
electriciansnet said:
Just pulled as-builts and existing riser shows our 200 amp panel to be fed with 3-250 and 1-4/0 Alu from meter that will go away. Load calcs show 17.4 amps on neutral
Click to expand...
But is that 17.4 amps the maximum unbalanced current? That sounds more like the maximum with all phases energized and conducting at calculated kva.

To determine maximum unbalanced current, you need to consider each phase's single phase loads—as these are the only ones that contribute to neutral current—as if the other two phases single phase loads are denenergized. Do that for all three phases, and the phase with the highest load is the maximum unbalanced neutral current.
 
Last edited:
iwire said:
...

There will be no need for an EGC as you must bond the new service disconnect.
Click to expand...
...and don't forget about bonding a grounding electrode to the grounded conductor at the outside transformer, i.e. if it is in fact outside and under NEC purview.
 
Status
Not open for further replies.
Top