3PH disconnect with 1PH load centers

ericstac

Member
Location
Houston, TX
I have a 40 unit multifamily building that has drawings which show 3 phase service and single phase load centers. I have never seen this done like this before, is this even possible?
 

brantmacga

Senior Member
Location
Georgia
Occupation
Electrical contractor
Yes. Single phase load centers divided amongst the 3 phases.


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GoldDigger

Moderator
Staff member
The voltage to ground is never more than 120 and the current in the neutral bus will never be greater than the max current on a single line input. That allows a single phase 120/240 panel to be used.
You can also run shared neutral circuits from it, but the neutral must be *counted* as a CCC.
(In a 120/240 MWBC the neutral is also current carrying, but it is not counted for adjustment.)
 

iwire

Moderator
Staff member
Location
Massachusetts
I have a 40 unit multifamily building that has drawings which show 3 phase service and single phase load centers. I have never seen this done like this before, is this even possible?
Yes, that is normal. With 40 units 13 will connect to phases A & B, 13 will connect to phases A & C, 13 more will connect to phases B & C with the remaining unit connecting to any two phases.

The house / landlord panel will likely be three phase.

Keep in mind this is. 208 volt so the reduced size wire allowances for dwelling units will not apply.
 

ericstac

Member
Location
Houston, TX
Yes, that is normal. With 40 units 13 will connect to phases A & B, 13 will connect to phases A & C, 13 more will connect to phases B & C with the remaining unit connecting to any two phases.

The house / landlord panel will likely be three phase.

Keep in mind this is. 208 volt so the reduced size wire allowances for dwelling units will not apply.

I gues I should of added that the service is split between a north and south service location. 20 on each side.. the house panels are on its own 3phase service with the house panels being 3ph as well. We have done 24 unit buildings and have always been engineered with single phase service so this was a surprise to me when I saw it.
 

brantmacga

Senior Member
Location
Georgia
Occupation
Electrical contractor
3PH disconnect with 1PH load centers

I take it this configuration is done primarily for cost purposes, or is there a rule against supplying 3ph to dwellings?
Cost ..... You wouldn't need 3 phase in the dwelling units.


You can have 3-phase in a dwelling as long as it's not over 240v.

I've worked on large custom homes that had 208Y/120 3 phase services, the largest being 1200A. We were there to do the pool; it had 100A and 125A sub-panels. The electricians did the whole house in EMT. It was 1-story with a crawlspace over a slab. Those guys were under there for 6 months on creepers running pipe. I wish I had taken pics of that. They did an awesome job.


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kwired

Electron manager
Location
NE Nebraska
Did I just make that up?

Maybe I'm thinking of 120v limitations to outlets in residential branch circuits?


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I think you will find limitations for luminaires, otherwise you just are not all that likely to need 277 or 480 volts in a dwelling.
 

jaggedben

Senior Member
I visited a large rural property recently with step-up and step-down transformers to bring power more than 1000ft from the service to the dwelling. Granted, the higher voltage wasn't brought inside, but it could have been. I understand this sort of thing is fairly common.
 

kwired

Electron manager
Location
NE Nebraska
I visited a large rural property recently with step-up and step-down transformers to bring power more than 1000ft from the service to the dwelling. Granted, the higher voltage wasn't brought inside, but it could have been. I understand this sort of thing is fairly common.
Not common around here, and I have worked in a lot of rural homes, large and small. But we can generally get the POCO to put their transformer within a reasonable distance from the home.
 

brantmacga

Senior Member
Location
Georgia
Occupation
Electrical contractor
Same here .... Poco will brings distribution voltage to the house and set transformer; multiple if needed for additional structures.


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ericstac

Member
Location
Houston, TX
Can yall help clarify this for me? We have never installed single phase on a 3 phase and we dont' understand how it will work. How do you attach the single phase meter stacks to the main so that its all balanced?
 

ericstac

Member
Location
Houston, TX
Yes, that is normal. With 40 units 13 will connect to phases A & B, 13 will connect to phases A & C, 13 more will connect to phases B & C with the remaining unit connecting to any two phases.

The house / landlord panel will likely be three phase.

Keep in mind this is. 208 volt so the reduced size wire allowances for dwelling units will not apply.

we have never installed something like this before and not really understanding how you would connect the single phase meterstacks to the main, can you help with this?
 

ericstac

Member
Location
Houston, TX
we have never installed something like this before and not really understanding how you would connect the single phase meterstacks to the main, can you help with this?

So looking at everything to me it sounds like we install a 3 phase main, 3 phase meter stacks and then at the meter we switch to single phase when connecting the SER? and balance the load by how we connect the wires.

Does this sound right?
 

iwire

Moderator
Staff member
Location
Massachusetts
So looking at everything to me it sounds like we install a 3 phase main, 3 phase meter stacks and then at the meter we switch to single phase when connecting the SER? and balance the load by how we connect the wires.

Does this sound right?
Typically you would have a 3 phase four wire main breaker section, the vertical meter sections will plug onto the main, there are three phases buses through all of it but the meters themesleves will be single phase and connected to alternating phases.

There will be a place to plug in a two pole breaker for the single phase feeder heading to the units.
 
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