A few questions about a multi-meter service

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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Hey, all y'all! (That's plural for y'all.)

I have a 2-gang meter, each feeding an upstairs apartment in a mixed-use building. I calculate a 155a demand for each, with 69a on the neutral.

1a. Do I need 350mcm copper (310a @75 deg), or can I use 300mcm copper (320a @90 deg), or can I combine the two loads and re-calculate to a lesser total demand?

1b. Also, can I really take the neutral all the way to #1 (150a @ 90 deg), or must I use #1/0 (150a @75 deg)

1c. It looks like I'll need 2.5" conduit unless I can make the shared entrance conductors smaller than my above calculations, right?


2. There will be two 3R 200a disco's beneath the 2-gang meter base for tenant access. Since we're not allowed to land any grounding conductors in the meter, I have to land them in the disco's.

If the water pipe is near the service, I can use either a single #2 conductor with taps, or a separate #4 cu for each, right?


3. The downstairs retail space will have its own 200a service (separate service entrance due to high-leg for existing 3ph AC unit) sharing the same service drop, separate weatherhead and entrance.

This service's disco will be in its panel, inside the building (back-to-back with the separate 3ph meter), which does not require grouping due to differing characteristics. Is it permissable to run a single, unbroken #6 from disco to disco to disco and then to the driven grounds?


Here's a click-to-enlarge drawing illustrating this installation:

 

Davis9

Senior Member
Location
MA,NH
You can add the two apartments together, then recalculate the feeder as far as I can tell. They(apartments) must be large for a 155A load. Electric heat, I'm guessing?

Tom
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Larry, you cheat. It looks like a couple questions, but even the subdivided questions have different questions concealed in them! :D

LarryFine said:
1a. Do I need 350mcm copper (310a @75 deg), or can I use 300mcm copper (320a @90 deg)
You can't use the 90 degree rating unless the lugs are 90, and I doubt that is so. 310.15(B)(6) is for the conductors serving an individual dwelling unit, not a pair, so I don't believe you can use that (if that's what the 90 was getting you).

or can I combine the two loads and re-calculate to a lesser total demand?
Absolutely - you can combine the floor area, appliances, and so forth. Why else do you think Table 220.55 has a "61 & over" option? ;)

1b. Also, can I really take the neutral all the way to #1 (150a @ 90 deg), or must I use #1/0 (150a @75 deg)
Same lug issue as above. Why so skimpy? It seems kinda odd to have 350 MCM copper hots and a 1/0 neutral. I guess it would be okay by the math, just sounds a little odd to me.

1c. It looks like I'll need 2.5" conduit unless I can make the shared entrance conductors smaller than my above calculations, right?
I'll trust your math, I'm too lazy for pipe fill tonight. :)

If the water pipe is near the service, I can use either a single #2 conductor with taps, or a separate #4 cu for each, right?
With 350's, we're looking at 1/0 CU for a single GEC.

I've heard of each disconnect getting it's own size GEC run to the grounding electrode. I don't believe it's entirely legal, and if I weren't in a lazy mood I'd check out the 2008 ROPs, in the neighborhood of 250.64. There was a proposal (that I think passed) to add language that seperate GECs for the smaller disconnect size would be acceptable.

The way it's worded now, I think the problem is that a GEC tap must connect on one end to a GEC. Most diagrams I've seen take display it that way too. But, that's neither here nor there, I reckon.

Is it permissable to run a single, unbroken #6 from disco to disco to disco and then to the driven grounds?
Would it be easier to just connect the ground rod to the water pipe? Another loop is being created in the method you describe. That said, I'd say it's legal, as far as I know.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Davis9 said:
You can add the two apartments together, then recalculate the feeder as far as I can tell. They(apartments) must be large for a 155A load. Electric heat, I'm guessing?
Good guess: heat pumps with 10Kw electric-strip backup.


georgestolz said:
Larry, you cheat. It looks like a couple questions, but even the subdivided questions have different questions concealed in them! :D
Well, yeah.

You can't use the 90 degree rating unless the lugs are 90, and I doubt that is so. 310.15(B)(6) is for the conductors serving an individual dwelling unit, not a pair, so I don't believe you can use that (if that's what the 90 was getting you).
It was, so I get I need the 60 deg numbers.


Absolutely - you can combine the floor area, appliances, and so forth. Why else do you think Table 220.55 has a "61 & over" option? ;)
But that only counts for three dwellings and up; I have two.


DaveBowden said:
Wouldn't 220.85 be applicable?
You guys keep quoting the '05 NEC, while we're on the '02.

Thanx for the responses. It's tough doing load calcs when appliances and equipment aren't even on-site yet.


So, can or cannot I combine the two apartments for the single entrance, and how?

"I'm so confused!" ~ Vinny Barbarino
 
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George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
LarryFine said:
It was, so I get I need the 60 deg numbers.
If you're using 75 degree conductors and lugs, you can use 75 degree numbers.

Absolutely - you can combine the floor area, appliances, and so forth. Why else do you think Table 220.55 has a "61 & over" option?

You guys keep quoting the '05 NEC, while we're on the '02.
Sorry, Table 220.55 is the range table, it was Table 220.19 in the '02.

So, can or cannot I combine the two apartments for the single entrance, and how?
Treat the two apartments as one unit and do a standard load calculation (pre-Part III of Article 220).
Then do an optional calculation for the pair seperately.

Then apply Dave's suggestion:
NEC-2002 220.33 Optional Calculation ? Two Dwelling Units. Where two dwelling units are supplied by a single feeder and the computed load under Part II of this article exceeds that for three identical units computed under 220.32, the lesser of the two loads shall be permitted to be used.

Confusing? I hope so, I just stared at this for 20 minutes trying to understand it. Basically, I read this as "If a standard calculation results in a number that is bigger than 1.3 times an optional calculation for the same units, then use the smaller number." This is nonsensical.

In the 2005, without even a proposal or a comment that I could find (suggesting it was a typo that spurred the change?), this became:

NEC-2005 220.85 Two Dwelling Units. Where two dwelling units are supplied by a single feeder and the calculated load under Part III of this article exceeds that for three identical units calculated under 220.84, the lesser of the two loads shall be permitted to be used.

"Where an optional method for the pair exceeds 3 units taken under the multifamily method, then you can use the fictional 3-plex's numbers."

This makes sense, and I would use the 2005's text if I could.

"I'm so confused!" ~ Vinny Barbarino
Amen to that. :D
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Okay, so far, so good. You guys are the best. I know how it feels when I help answer questions.

About combining load calcs for two apartments sharing the single entrance conductor set, do I combine the total floor area, appliances, etc. of each as if it were all a single load, then do the usual calc, or do I take the results of the two separate calcs, add them up, and apply the calcs again?

It seems that the second method would allow me to use the "35% for over 3Kva" demand reduction twice. Or would they work out to the same end results either way? :confused:
 
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radiopet

Senior Member
Location
Spotsylvania, VA
He would certainly be able to apply 310.15(B)(6) here but it ends up being the same basically as the 75 Degree listing anyway.

Here is the 2008 you asked about on 250.64

(D) Service with Multiple Disconnecting Means Enclosures.
Where a service consists of more than a single enclosure
as permitted in 230.71(A), grounding electrode
connections shall be made in accordance with (1), (2) or
(3). [ROP 5?192]
(1) Grounding Electrode Conductor Taps. Where the
service is installed as permitted by 230.40, Exception No.
2, a common grounding electrode conductor and grounding
electrode conductor taps shall be installed. The common
grounding electrode conductor shall be sized in accordance
with 250.66, based on the sum of the circular mil area of
the largest ungrounded service-entrance conductor(s).
Where the service-entrance conductors connect directly to a
service drop or service lateral, the common grounding electrode
conductor shall be sized in accordance with Table
250.66, Note 1. A tap conductor shall extend to the inside
of each service disconnecting means enclosure. The
grounding electrode conductor taps shall be sized in accordance
with 250.66 for the largest conductor serving the
individual enclosure. The tap conductors shall be connected
to the common grounding electrode conductor by exothermic
welding or with connectors listed as grounding and
bonding equipment in such a manner that the common
grounding electrode conductor remains without a splice or
joint. [ROP 5?192]
(2) Individual Grounding Electrode Conductors. A
grounding electrode conductor shall be connected between
the grounded conductor in each service equipment disconnecting
means enclosure and the grounding electrode system.
Each grounding electrode conductor shall be sized in
accordance with 250.66 based on the service-entrance conductor(
s) supplying the individual service disconnecting
means. [ROP 5?192]
(3) Common Location. A grounding electrode conductor
shall be connected to the grounded service conductor(s) in
a wireway or other accessible enclosure on the supply side
of the service disconnecting means. The connection shall
be made with exothermic welding or a connector listed as
grounding and bonding equipment. The grounding electrode
conductor shall be sized in accordance with 250.66
based on the service-entrance conductor(s) at the common
location where the connection is made. [ROP 5?192]

Lary you are going to size each unit first to determine the feeder size to each unit and so on and then you will do the combined after to determine the service size to the building itself.
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
LarryFine said:
About combining load calcs for two apartments sharing the single entrance conductor set, do I combine the total floor area, appliances, etc. of each as if it were all a single load, then do the usual calc, or do I take the results of the two separate calcs, add them up, and apply the calcs again?
You treat the pair as though they were one dwelling unit. So, two 500 square foot apartments would be 1000 square feet, two dishwashers, two laundries, four SABCs, etc.

Before you get too far (heh heh) I wanted to make a correction:
georgestolz said:
Treat the two apartments as one unit and do a standard load calculation (pre-Part III of Article 220).
Then do an optional calculation for the pair seperately.

Then apply Dave's suggestion:
If you were to use the language in the 2005 to try to make use of Dave's suggestion, doing a standard load calc is unnecessary. I was still looking at the 2002 when I wrote what I quoted above, and wrote before I read the sections and compared them cycle to cycle.

An optional load calc will almost always (maybe always) give you a lower number, so if you're shooting for the minimum, I'd just skip the standard calc unless you're as curious as I am about how different the two calculations are in each situation.

The 2005 language of 220.85 is basically the optional method on a diet plan.

radiopet said:
He would certainly be able to apply 310.15(B)(6) here...
In the 2002, I would agree. Under the 2005 I would not. The 2002 language is missing the word "individual" that is present for the 2005, so I would lose my grounds for saying that the service conductors that feed a pair of dwelling units could not make use of 310.16(B)(6). :cool:

NEC-2002 310.15(B)(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For dwelling units, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s). The feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors.
The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.22, and 230.42 are met.
Larry, the voltage for the apartments is 120/240, correct? Or is it 120/208?

Edit to add: Paul, thanks for posting that text from the 2008 draft! It was number "2" in that I was remembering. :cool:
 
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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Well, I just ran an optional-method calc for one apartment, and I got the same result: 155a.

When I run the total for both apartments as if they were one, using total square footage, appliances, etc., I get a demand of 260a, not (2 x 155) 310a.

However, if I add the two adjusted lighting, SA, and laundry numbers, apply another 35%-for-over-3Kva factor, and then add 2 x 5Kw for dryers and 2 x 11Kw for heat, and 11Kw for two ranges, I get 201a.

Why isn't this easy? :confused:
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
George, its 240/120 for the apartments, and we're using the '02 NEC. Yes, I'm aiming for lowest calc. for the shared service conductors from the point-of-attachment to the 2-gang meter base.

Note that the two units do not share a feeder. The 2-gang meters share service conductors, which is what I'm trying to find the lowest demand for. Each unit will have 4/0 SER from its disco under the meters to the M/L panels. (see prior drawing)

Here are the real numbers:

Apt. A is 1295 sq.ft. = 4885 va demand.
Apt. B is 880 sq.ft. = 4449 va demand.
2 S/A & 1 laundry ckts = 4500 Kw.
3 ton HP w/10 Kw heat strips = 11 Kw.
Water heater = 4.5 Kw
Fridge, D/W, & disp = 3.5 Kva
Dryer = 5 Kva demand
Range = 8 Kva demand

That's how I came up with the 155a per unit.

I'm still having trouble with the best (read cheapest) method for determining the total demand on the shared entrance, so I can minimize the conduit size, also. I plan on using copper in PVC, although EMT might be necessary.

Help!
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
When I run the total for both apartments as if they were one, using total square footage, appliances, etc., I get a demand of 260a, not (2 x 155) 310a.
That sounds right. This sounds like bottom dollar to me.

However, if I add the two adjusted lighting, SA, and laundry numbers, apply another 35%-for-over-3Kva factor, and then add 2 x 5Kw for dryers and 2 x 11Kw for heat, and 11Kw for two ranges, I get 201a.
I'm not getting this.
------------------------
So, how does a single 250 MCM CU run on a 300A service grab ya, using 310.15(B)(6)?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
georgestolz said:
I'm not getting this.
It was explained to me that it's possible to add the lighting/SA/laundry demands together, again apply 100% demand for the first 3Kva and 35% of the remainder, and add the two dryers, ranges, and heat loads. for a final demand. No?
------------------------
So, how does a single 250 MCM CU run on a 300A service grab ya, using 310.15(B)(6)?
I can certainly live with that. With a neutral demand of 114a, the same table allows me to use #2 cu, correct?

Okay, we're cool on this one.

Okay, for the next question, for the 3-phase separate service (see drawing again), the high-leg will only have a single load, the existing roof-top AC unit, fed with #10 if I'm not mistaken.

Since this panel will have a 3-pole main breaker, how do I size the high-leg entance and feeder conductor size? Without a properly-sized breaker at the load end, I can't use a tap rule, right?
 

radiopet

Senior Member
Location
Spotsylvania, VA
George,

Thanks...yeah I posted 2002 because I knew Larry was in the same state I was in and we are in the 2002 cycle.

Do have a question for you then......even if it was the 2005 cycle the individual to me would still apply for his serve latteral....would it not.

(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders For individual
dwelling units of one family, two-family, and multifamily dwellings, conductors, as listed
in Table 310.15(B)(6), shall be permitted
as 120/240-volt, 3-wire, single-phase serviceentrance
conductors, service lateral conductors, and feeder conductors that serve as the
main power feeder to each dwelling unit and are installed in raceway or cable with or
without an equipment grounding conductor. For application of this section, the main
power feeder shall be the feeder(s) between the main disconnect and the lighting and
appliance branch-circuit panelboards(s). The feeder conductors to a dwelling unit shall
not be required to have an allowable ampacity rating greater than their service-entrance
conductors. The grounded conductor shall be permitted to be smaller than the
ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.

intent:
For the 2005 Code, only editorial changes were made to 310.15(B)(6). If a single set of
3-wire, single-phase, service-entrance conductors in raceway or cable supplies a onefamily,
two-family, or multifamily dwelling, the reduced conductor size permitted by
310.15(B)(6) is applicable to the service-entrance conductors, service-lateral
conductors, or any feeder conductors that supply the main power feeder to a dwelling
unit.

Section 310.15(B)(6) permits the main feeder to a dwelling unit to be sized according
to the conductor sizes in Table 310.15(B)(6) even if other loads, such as ac units and
pool loads, are fed from the same service. The feeder conductors to a dwelling unit are
not required to be larger than its service-entrance conductors.
Exhibits 310.7 and 310.8 illustrate the application of 310.15(B)(6). In Exhibit 310.7,
the reduced conductor size permitted is applicable to the service-entrance conductors
run to each apartment from the meters. In Exhibit 310.8, the reduced conductor size
permitted is also applicable to the feeder conductors run to each apartment from the
service disconnecting means, because these feeders carry the entire load to each
apartment.
 
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