Residential 200A ATS and MSP Neutral wiring

B

Battlebornelectrician

Member
Location
Reno
Occupation
Electrical contractor
I have a residential 400A MSP that is needing Generator backup. Inside there are 2-200A MCB. Both are fed from the bussing straight off the meter and load out to 2 separate subpanels inside the home via sub feed cables inside the MSP. I would wire in 2- 200A ATS, one for each feed. Standard. HOWEVER, I have been getting really tripped up on how many neutrals are needed going back and forth between the MSP and ATS. Lets take just one of the 200A feeds and ATS for example. If I run one of those 240V 200A circuits over to the ATS (L1 L2 N & G), and that neutral is landed on the MSP neutral bar and on the neutral bar in the ATS, do i need a 2nd neutral leaving the transfer switch and landing back on the same neutral bar it originated from in the MSP? It seems like I wouldn't if both sets were in the same conduit (feeder rules) and the subpanels neutrals are connected to the same neutral bar in the MSP, It seems redundandt to run a neutral there and back, the ATS is 2 pole generac and not switching the neutral. The subpanel would get it's neutral Via the neutral bar in the MSP still with just one neutral conductor landed between the MSP and ATS. Can someone explain this to me if I am wrong in my thinking on this? Is it wrong to think about this the same as a light switch not needing the neutral there, you can just leave it where the load is and switch the hots.? I know a light switch is not the same as feeders though. Doing it my way means 2 less 3/0 neutrals I need to find a home for in the MSP and less cost on unesecessary wire. This is not so difficult when the 2-200A MCB's are regulated to just feeding subfeeds separately in the MSP without landing on a distribution section becuase I can just unland all the wiring for both (L1 L2 N GR) and polaris tap the backfeed from the ATS directly onto those subfeeds which creates 2 open slots on the neutral bar for my neturals leaving the MSP to go feed the ATS. BUT, this is not always the case and usually one of the 2- 200A MCB's in these 400A MSP's load side land on a distribution section and does not have a neutral conductor I can remove to create a space to land in... Hope this question makes sense, it is difficult to explain. Advice would be much appreciated!!!
I have attached 2 different 400A MSP configs I am dealing with for reference.
0
0
 
Welcome to the forum.

I would love to help you, but I can't follow your description.

Any chance you can draw a diagram and post a pic of it?
 
The generator feed has a neutral, and needs to get to the main panel.

If you run the in and out from the ATS in the same conduit, either as a nipple less than 24”, or with derating, then you only need one neutral.

If you are using two separate conduits, one for the feed to the ATS, and for the fed from the ATS, you need a neutral in each conduit.

In no case can you treat an ATS like simple switch, because you have a neutral from the generator.
 
Battlebornelectrician said:
I have a residential 400A MSP that is needing Generator backup. Inside there are 2-200A MCB. Both are fed from the bussing straight off the meter and load out to 2 separate subpanels inside the home via sub feed cables inside the MSP. I would wire in 2- 200A ATS, one for each feed. Standard. HOWEVER, I have been getting really tripped up on how many neutrals are needed going back and forth between the MSP and ATS. Lets take just one of the 200A feeds and ATS for example. If I run one of those 240V 200A circuits over to the ATS (L1 L2 N & G), and that neutral is landed on the MSP neutral bar and on the neutral bar in the ATS, do i need a 2nd neutral leaving the transfer switch and landing back on the same neutral bar it originated from in the MSP? It seems like I wouldn't if both sets were in the same conduit (feeder rules) and the subpanels neutrals are connected to the same neutral bar in the MSP, It seems redundandt to run a neutral there and back, the ATS is 2 pole generac and not switching the neutral. The subpanel would get it's neutral Via the neutral bar in the MSP still with just one neutral conductor landed between the MSP and ATS. Can someone explain this to me if I am wrong in my thinking on this? Is it wrong to think about this the same as a light switch not needing the neutral there, you can just leave it where the load is and switch the hots.? I know a light switch is not the same as feeders though. Doing it my way means 2 less 3/0 neutrals I need to find a home for in the MSP and less cost on unesecessary wire. This is not so difficult when the 2-200A MCB's are regulated to just feeding subfeeds separately in the MSP without landing on a distribution section becuase I can just unland all the wiring for both (L1 L2 N GR) and polaris tap the backfeed from the ATS directly onto those subfeeds which creates 2 open slots on the neutral bar for my neturals leaving the MSP to go feed the ATS. BUT, this is not always the case and usually one of the 2- 200A MCB's in these 400A MSP's load side land on a distribution section and does not have a neutral conductor I can remove to create a space to land in... Hope this question makes sense, it is difficult to explain. Advice would be much appreciated!!!
I have attached 2 different 400A MSP configs I am dealing with for reference.
0
0
Click to expand...
well, i suppose that depends upon whether you're coming and going in the same conduit/nipple.

when you run a feeder through a conduit, once it has amperage flowing on it, those conductors are going to be giving off magnetic fields. without the neutral carrying right along them in the same conduit, you're looking at having an unbalanced magnetic field coming from the set of conductors traveling in a metal conduit. if that's the case, there's a good chance that those non-cancelling magnetic fields are going to induce current on that metal pipe, and this will cause at least 2 issues i can think of. 1 - heat on the conduit,and 2 - resistance to current flow through reactance.

so when running any circuit, especially higher ampacity feeders, we gotta make sure our magnetic fields all get cancelled out. this is just like when running 3-way switches inside the house. we gotta make sure those travelers have either the neutral or the power travelling with them, so that fields will cancel each other out.

does this help?
 
Seven-Delta-FortyOne said:
The generator feed has a neutral, and needs to get to the main panel.

If you run the in and out from the ATS in the same conduit, either as a nipple less than 24”, or with derating, then you only need one neutral.

If you are using two separate conduits, one for the feed to the ATS, and for the fed from the ATS, you need a neutral in each conduit.

In no case can you treat an ATS like simple switch, because you have a neutral from the generator.
Click to expand...
This makes sense and is in essence what I was thinking. I know I need at least one neutral connecting MSP to ATS for the connection of the generators neutral back to the MSP to feed loads, but I wasn't sure If i needed to bring a second one back out of the ATS. I am planning on running 4/0 THHN ( for derating) in (2) 2-1/2" PVC conduits underground each containing the ins and outs of the 200A feeder circuits. So inside each conduit would be L1 L2 N G (line side) heading into ATS and then just L1 L2 (Load side) in same conduit coming out of ATS back into MSP to connect to the sub panel feeders in there, and the neutral for the subpanel feeder is staying landed on the neutral bar inside the MSP. If I put 2 Neutrals in that pipe ( one in and one out) that would be redundant right? The 1 neutral covers both return paths in normal grid operation and in generator operation because both can't happen at the same time so you only need the 1. If they're in the same conduit of course.
 
Master Nater said:
well, i suppose that depends upon whether you're coming and going in the same conduit/nipple.

when you run a feeder through a conduit, once it has amperage flowing on it, those conductors are going to be giving off magnetic fields. without the neutral carrying right along them in the same conduit, you're looking at having an unbalanced magnetic field coming from the set of conductors traveling in a metal conduit. if that's the case, there's a good chance that those non-cancelling magnetic fields are going to induce current on that metal pipe, and this will cause at least 2 issues i can think of. 1 - heat on the conduit,and 2 - resistance to current flow through reactance.

so when running any circuit, especially higher ampacity feeders, we gotta make sure our magnetic fields all get cancelled out. this is just like when running 3-way switches inside the house. we gotta make sure those travelers have either the neutral or the power travelling with them, so that fields will cancel each other out.

does this help?
Click to expand...
Yes that helps. They would be run in the same conduit.
 
Seven-Delta-FortyOne said:
It needs to be sized to the MCB. It handles fullload current on utility power.
Click to expand...
Have only skimmed over the long descriptions, but I'm picturing it the way that Larry is picturing it:

Utility power: Ungrounded conductors carry current from MSP to ATS back through MSP (as pull box) to interior panels. Grounded conductor carries current MSP to interior panels directly.

Generator power: All conductors carry current from generator to ATS then through MSP (as pull box) to interior panels.

Cheers, Wayne
 
Master Nater said:
well, i suppose that depends upon whether you're coming and going in the same conduit/nipple.

when you run a feeder through a conduit, once it has amperage flowing on it, those conductors are going to be giving off magnetic fields. without the neutral carrying right along them in the same conduit, you're looking at having an unbalanced magnetic field coming from the set of conductors traveling in a metal conduit. if that's the case, there's a good chance that those non-cancelling magnetic fields are going to induce current on that metal pipe, and this will cause at least 2 issues i can think of. 1 - heat on the conduit,and 2 - resistance to current flow through reactance.

so when running any circuit, especially higher ampacity feeders, we gotta make sure our magnetic fields all get cancelled out. this is just like when running 3-way switches inside the house. we gotta make sure those travelers have either the neutral or the power travelling with them, so that fields will cancel each other out.

does this help?
Click to expand...
Here is an example drawing I made
 

Attachments

  • ATS to MSP wiring example.pdf
    202.2 KB · Views: 3
Battlebornelectrician said:
Here is an example drawing.
Click to expand...
Then I am correct. As long as the line and load conductors to the ATS are in one nipple, the service neutral need not be routed through the ATS.

The black dotted line in your diagram will never see more than the generator's neutral current. I wired one exactly like this a few years ago.

And the ATS is not the service disconnect and need not be service rated because of the disco ahead of it.
 
Last edited:
I have done a couple of similar setups, ie two ATSes. After some discussion with the inspector, we also thought a neutral bond between the two ATSes was also needed, as if you ever disconnected the MSP for service/replacement whatever, and you were on generator, you'd need a neutral bond between them, instead of relying on the neutral bond at the MSP.
 
VirutalElectrician said:
After some discussion with the inspector, we also thought a neutral bond between the two ATSes was also needed, as if you ever disconnected the MSP for service/replacement whatever, and you were on generator, you'd need a neutral bond between them, instead of relying on the neutral bond at the MSP.
Click to expand...
If you are referring to a N-G bond, you are not allowed to have more than one during normal operation. So in your "what if" scenario where the MSP N-G bond is disconnected (because say the MSP is being replaced) you'd need to install a temporary N-G bond at the generator to allow running on generator.

Cheers, Wayne
 
wwhitney said:
If you are referring to a N-G bond, you are not allowed to have more than one during normal operation. So in your "what if" scenario where the MSP N-G bond is disconnected (because say the MSP is being replaced) you'd need to install a temporary N-G bond at the generator to allow running on generator.

Cheers, Wayne
Click to expand...
No, I'm referring to bonding the two neutrals in the two ATSes together.

But yes, You'd also need to add a temporary NG bond in that case as well.
 
LarryFine said:
Then I am correct. As long as the line and load conductors to the ATS are in one nipple, the service neutral need not be routed through the ATS.

The black dotted line in your diagram will never see more than the generator's neutral current. I wired one exactly like this a few years ago.

And the ATS is not the service disconnect and need not be service rated because of the disco ahead of it.
Click to expand...
Ok. But don't you need at least 1 service neutral in that conduit with all those line conductors to cancel out the magnetic fields so as not to create unbalanced magnetic fields and induced heat as stated above by master nater? In your scenario the only neutral in that conduit would be the one coming back from the generator, which is connected to the neutral bar in the MSP, so if it was sized only to match Generator output would that be sufficient to cancel out the magnetic fields of the larger conductors and not cause issues?
 
You must log in or register to reply here.
Top