Multi unit service sizing

Status
Not open for further replies.
Doc13067 said:
So should I just make the neutral the same size as the hots? It’s always 2 sizes smaller in SEU cable so should I do that? This part always confuses me because if I come up with something based on my load calc it’s open for any inspector to say I was wrong. I’d much rather follow a chart or a rule of some sort
Click to expand...
The rule is it needs to be able to handle the maximum possible unbalanced current as determined by what line to neutral loads are connected to it.

250.102(C) is saying regardless what unbalanced load may be it can never be smaller than sizes presented there.

The two sizes smaller thing is sort of an unwritten rule that many inspectors will allow you to use without proving any calculations in most cases. There are times where you might have to use full sized neutral.
There are times when you have minimal line to neutral load and it doesn't take much to prove that 250.102(C) values are higher than your actual maximum neutral current.
 
kwired said:
The rule is it needs to be able to handle the maximum possible unbalanced current as determined by what line to neutral loads are connected to it.

250.102(C) is saying regardless what unbalanced load may be it can never be smaller than sizes presented there.

The two sizes smaller thing is sort of an unwritten rule that many inspectors will allow you to use without proving any calculations in most cases. There are times where you might have to use full sized neutral.
There are times when you have minimal line to neutral load and it doesn't take much to prove that 250.102(C) values are higher than your actual maximum neutral current.
Click to expand...
Ok so if I look at my circuit layout and show amperage on phase A and amperage on phase B and subtract one from the other and get very little amperage, would that usually be sufficient to just go 2 sizes down from 500mcm?
 
Doc13067 said:
Ok so if I look at my circuit layout and show amperage on phase A and amperage on phase B and subtract one from the other and get very little amperage, would that usually be sufficient to just go 2 sizes down from 500mcm?
Click to expand...
If you get very little you likely can use the value from 250.102(C).

If you have straight 240 volt loads, you kind of have an obvious amount of load on ungrounded conductors that will never end up loading the neutral.

If you have 208/120 single phase feeder though, you very well may have to go with full sized neutral if you have little to no straight 208 volt loads, as the neutral in that case will carry same current as the ungrounded conductors do.
 
Ok thank you. Almost all of my loads are 120 and very few are 240. But even with that being the case if those 120 volt loads are split evenly between the two phases you’d still wind up with nothing (relatively speaking) on the neutral, correct?
 
Doc13067 said:
Ok thank you. Almost all of my loads are 120 and very few are 240. But even with that being the case if those 120 volt loads are split evenly between the two phases you’d still wind up with nothing (relatively speaking) on the neutral, correct?
Click to expand...
if they all are relatively equal currents and all run at the same time - that is also a factor in how much the neutral will see at any given time. And in a dwelling there is a lot of diversity. Sometimes a lot is going at once and others hardly anything is going at all.

This diversity is a reason why it is somewhat acceptable to allow reduction of two sizes without proving the actual load though. Seldom will it be too small to handle that .
 
So now the poco is saying no cabinets or anything that can be opened ahead of the meter. They say just use two 2 gang meter sockets.and run 200 amp services out of them. They say 2 gang meter sockets are more available than 1 gang meter sockets. There is no way I am getting 2 gang meter sockets and these wires are way too large to fit in a 1 gang meter socket. Hell, even a meter/main has a section that can be opened before the meter so I don’t get what I’m supposed to do. My inspector said they’re out of their mind and I need to try to talk to someone else. Anybody ever have this issue?
 
Doc13067 said:
So now the poco is saying no cabinets or anything that can be opened ahead of the meter. They say just use two 2 gang meter sockets.and run 200 amp services out of them. They say 2 gang meter sockets are more available than 1 gang meter sockets. There is no way I am getting 2 gang meter sockets and these wires are way too large to fit in a 1 gang meter socket. Hell, even a meter/main has a section that can be opened before the meter so I don’t get what I’m supposed to do. My inspector said they’re out of their mind and I need to try to talk to someone else. Anybody ever have this issue?
Click to expand...
They can request anything they want for the most part. Though many will accept enclosures that have provisions for them to lock/seal ahead of meter(s). And like you said some metering centers do have such compartments that are sealable anyway on supply side of meters.
 
Doc13067 said:
I am doing my first multiunit job and was wondering if someone could direct me on how to size the mast wires. I will have three 100 amp meter/mains in a three gang meter socket. So out of each meter I will run number two aluminum to three sub panels inside the units. How do you size the wires that go from the 3 m to the powerlines? I hope it’s not 300 Amps but I’m sure someone will let me know.
Click to expand...
How did you end up sizing these? I have a similar problem I'm trying to work out except the house is a two family and will have a third panel as a house panel. I guess the 45% demand factor wouldnt apply to my situation.
 
DannyBoiii said:
How did you end up sizing these? I have a similar problem I'm trying to work out except the house is a two family and will have a third panel as a house panel. I guess the 45% demand factor wouldnt apply to my situation.
Click to expand...
I went overkill and sized the wires for 400 amps because I want future proofing. But i went painstakingly through every device and added up all of their wattage. I assigned 120 watts for each light and 180 watts for each receptacle with no assigned load. The rest of the appliances you should be able to get wattage from name plates. I did this for each unit individually and then did watts - 10,000 x .4 + 10,000 / 240 and that gave me the size of the main breaker for each unit.

Then I went to my overall wattage and did total watts - 30,000 (in your case it would be minus 20,000, 10,000 for each unit) x .4 + 30,000 / 240 and came up with 289 Amps required.

Do you have your load calculations?
 
Doc13067 said:
I went overkill and sized the wires for 400 amps because I want future proofing. But i went painstakingly through every device and added up all of their wattage. I assigned 120 watts for each light and 180 watts for each receptacle with no assigned load. The rest of the appliances you should be able to get wattage from name plates. I did this for each unit individually and then did watts - 10,000 x .4 + 10,000 / 240 and that gave me the size of the main breaker for each unit.

Then I went to my overall wattage and did total watts - 30,000 (in your case it would be minus 20,000, 10,000 for each unit) x .4 + 30,000 / 240 and came up with 289 Amps required.

Do you have your load calculations?
Click to expand...
You should not be adding for each receptacle or light. These loads are included in the 3 watt's per sqft general lighting load.
 
Status
Not open for further replies.
Top