Multi unit service sizing

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Doc13067

Doc13067

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Bucks County/Philadelphia, PA USA
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.
 
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.
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See Table 220.84 (Multi-dwelling units demand factors) if applicable to your problem of yours. 3 units will have a demand factor of 45% of the total ~135A.
2/0 Al? Wait for others to chime in.
 
topgone said:
See Table 220.84 (Multi-dwelling units demand factors) if applicable to your problem of yours. 3 units will have a demand factor of 45% of the total ~135A.
2/0 Al? Wait for others to chime in.
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Embarrassed I never thought about that as a "shortcut". i always did a load calculation but assuming the 100 amp individual services are adequate the simple application of 220.84 to those service sizes is a simple "quick fix". :)
 
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.
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Art 220 load calculations can be used to determine minimum ampacity of the "common conductor" here.

You could go with a 300 amp conductor like you mentioned, but that usually will be more than you will get doing proper load calculations.

There is some optional calculations in art 220 that can be used in different situations. You can use whatever option gives you the lowest value if you want.

Keep in mind when figuring the service vs individual unit feeders you have more demand factor that may be able to be applied than for each individual unit. For example when determining electric range load from T220.55 for the individual unit feeder you only can use value for 1 appliance from the table which in column C would be 8 kW. But for the service to three units you could take the value in the table for 3 appliances, which is 14 kW, which is significantly less than going with 8 kW for each range.
 
I have 116,280 watts so what i did was :
116,280VA - 30,000VA = 86,280VA x .4 = 34,512 + 30,000VA = 64,512VA / 240V = 268.8 A or 300A service

Upsize to 400A for future work (adding a garage that a business will use later) so we can just add another meter, tap the 400A conductors, and trench from the meter to said garage.

Now the next issue is multi gang meter sockets have been willed out of existence by the electrical demons so I have to build my own contraption. Maybe I’ll build a box around the wires with 2x4s. But it sounds like I’m gonna have to run to run 400 amp wires to a gutter and come out of the gutter with 3 meter sockets and come out of the 3 meter sockets to 3 100A breaker enclosures and come out of each of them with SER.

My question with this now is going to be how do I run 500 mcm cu x 3 into a gutter and then connect each of them to 4 #2s that can then go down to the meters (I would keep the 4th set of wires in the gutter for future attachment). Could I just run each of the wires the length of the gutter and just grab onto each at various locations with Ilsco insul—eaters? And also, the poco requires bonding bushings at all of these locations but the GEC is 1/0 cu. Surely I don’t have to run that around to all the bushings. Anyone know if I can treat it like ground rods and run #6 cu to the bonding bushings?
 
Doc13067 said:
I have 116,280 watts so what i did was :
116,280VA - 30,000VA = 86,280VA x .4 = 34,512 + 30,000VA = 64,512VA / 240V = 268.8 A or 300A service

Upsize to 400A for future work (adding a garage that a business will use later) so we can just add another meter, tap the 400A conductors, and trench from the meter to said garage.

Now the next issue is multi gang meter sockets have been willed out of existence by the electrical demons so I have to build my own contraption. Maybe I’ll build a box around the wires with 2x4s. But it sounds like I’m gonna have to run to run 400 amp wires to a gutter and come out of the gutter with 3 meter sockets and come out of the 3 meter sockets to 3 100A breaker enclosures and come out of each of them with SER.

My question with this now is going to be how do I run 500 mcm cu x 3 into a gutter and then connect each of them to 4 #2s that can then go down to the meters (I would keep the 4th set of wires in the gutter for future attachment). Could I just run each of the wires the length of the gutter and just grab onto each at various locations with Ilsco insul—eaters? And also, the poco requires bonding bushings at all of these locations but the GEC is 1/0 cu. Surely I don’t have to run that around to all the bushings. Anyone know if I can treat it like ground rods and run #6 cu to the bonding bushings?
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Insulation piercing connectors is one option, so is Polaris type connectors, terminal blocks, even split bolts, compression taps.

SSBJ is what size conductor the bonding bushings will require which in your case should also be same size as GEC.

Most the bonding bushings over 1.25 or 1.5 trade size I have used should accept a 1/0 conductor. If anything you might need to use larger nipple to a meter even if you have smaller conductors to that meter
 
If the 3 combined units work out to 269 amps I doubt 100 amps is sufficient for each unit. The additional demand factors should lower the total much more than that. What is your calculation for each unit?
 
curt swartz said:
If the 3 combined units work out to 269 amps I doubt 100 amps is sufficient for each unit. The additional demand factors should lower the total much more than that. What is your calculation for each unit?
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You can have 200 amp feeds to each unit and still have a common supply conductor with an ampacity of 269 in such situation and is code compliant.
 
kwired said:
You can have 200 amp feeds to each unit and still have a common supply conductor with an ampacity of 269 in such situation and is code compliant.
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Yes, the minimum service conductors get sized by the total calculated load. The OP stated there was going to be 100 amp panel for each unit. If each unit requires less than 100 amps the total for all 3 should be less than the 269 amps they came up with. You can always use larger conductors than required.
 
curt swartz said:
Yes, the minimum service conductors get sized by the total calculated load. The OP stated there was going to be 100 amp panel for each unit. If each unit requires less than 100 amps the total for all 3 should be less than the 269 amps they came up with. You can always use larger conductors than required.
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You are likely right that total for all three is likely less than 269, or at least can be less than 269.

A single dwelling unit likely may have different demand factors that can apply to it's individual feeder than what possibly can be applied to a common supply conductor for all three units.

First common example would be taking demand for one range on individual unit feeder vs taking being able to take a demand for three ranges on a common supply conductor.
 
I went way heavy on the load calc and included things that are not even in all units. I included loads for electric water heaters in all even though only one has one. Same with central AC and I’ve got 1.5 amps for every receptacle that will mostly have nothing on them. Another thing is the formula usually calls for subtracting 10,000 before dividing and then adding 10,000 back - I put aside 30,000 VA instead of 10,000. So my number is definitely high and I’m going even higher with the mast wires. But I’d be quite safe with 100 amps at each individual unit.
 
kwired said:
Insulation piercing connectors is one option, so is Polaris type connectors, terminal blocks, even split bolts, compression taps.

SSBJ is what size conductor the bonding bushings will require which in your case should also be same size as GEC.

Most the bonding bushings over 1.25 or 1.5 trade size I have used should accept a 1/0 conductor. If anything you might need to use larger nipple to a meter even if you have smaller conductors to that meter
Click to expand...
Thank you this is great information, although it sucks running 1/0 around to 3 points in the gutter or cabinet and to 3 meter sockets and 3 discos. Seems like overkill so maybe I’ll double check with my poco. The thought just occurred to me that the terminal on the bonding bushings screw’s off right? If I really need to do this maybe I could keep my 1-1/4 trade size and sub larger bonding terminals on the bushings.
 
Also, table 250.102 (c) states that if I’m using 500 mcm cu for my hots, then my neutral must only be 1/0 cu. Am I reading that right for a 400 amp service? Sorry for all the questions, like I said it’s my first time taking on a project of this size and I just want to get it right.
 
Doc13067 said:
Also, table 250.102 (c) states that if I’m using 500 mcm cu for my hots, then my neutral must only be 1/0 cu. Am I reading that right for a 400 amp service? Sorry for all the questions, like I said it’s my first time taking on a project of this size and I just want to get it right.
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Your neutral needs to be sized to carry all of the unbalanced load. 250.102C is just a minimum to provide a low impedance fault path.
 
mbednarik said:
Your neutral needs to be sized to carry all of the unbalanced load. 250.102C is just a minimum to provide a low impedance fault path.
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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
 
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