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Old 08-09-2017, 10:02 PM   #1
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Default Residential 200 amp service with two 100 amp feeders

i am working on a DADU (detached additional dwelling unit) in Seattle. I have to upgrade the main service at the house to 200 amps and put in a double socket meter base. The main house needs a 100 sub panel and the DADU also needs a 100 amp sub panel. Table 310.15b6 used to let me use #4 wire for a 100 amp panels whether it was the main service or feeder as long as its intended for residential. The table no longer exists. my question, is #4 still acceptable for 100 amp feeders to sub panels?
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Old 08-10-2017, 07:55 AM   #2
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What year was that from?

Were those open air conductors?

Haven't seen anyone use less than #2 copper for 100 amps.
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Old 08-10-2017, 08:18 AM   #3
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The table has become a percentage. You can find the table in one of the annexes. It is still valid, it's just changed to a percentage instead of a table.
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Old 08-10-2017, 11:15 AM   #4
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Originally Posted by MechanicalDVR View Post
What year was that from?

Were those open air conductors?

Haven't seen anyone use less than #2 copper for 100 amps.

It is in pvc conduit. ive seen other panels where electricians used #2 copper as well. however 310.15b6 did allow #4 copper for 100 amps. I am so confused.
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Old 08-10-2017, 12:39 PM   #5
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It is in pvc conduit. ive seen other panels where electricians used #2 copper as well. however 310.15b6 did allow #4 copper for 100 amps. I am so confused.
#4 in conduit to me is good for 85 amps.
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Old 08-10-2017, 12:42 PM   #6
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Originally Posted by MechanicalDVR View Post
#4 in conduit to me is good for 85 amps.
But the table he is talking about allowed us to use #4 copper for a dwelling unit.

They removed the table and now make you do a calculation or some such nonsense, but he should still be fine with #4 copper. He would be even better with #2 aluminum.
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Old 08-10-2017, 02:01 PM   #7
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Here's a post from another thread:
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Originally Posted by HackWork View Post
310.15(B)(7) is a revision to the code. Remember the Table 310.15(B)(7) in the 2011 code that allowed you to use, for example, 4/0 aluminum for a 200A dwelling service, when 4/0 could only carry 180A according to the normal ampacity? Likewise, you could use #2 aluminum for a 100A service, while normally #2 was only good for 90A?

In the 2014 that table was deleted and 310.15(B)(7) was added instead. The end result of 310.15(B)(7) in most cases works out to be the same as the old Table from the 2011 code.

That's all it's doing, allowing you to use a little bit smaller wire (83% of service size) when it's a single family dwelling unit. It's the same as it always was.
dham206, you should be fine.
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Old 08-10-2017, 02:10 PM   #8
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Dham206 .,, Hackwork did bring up very good point on the service conductor size for 100 amp with #4 copper conductors and yes it still legit only from the meter to first OCPD typically main breaker.

I am aware that is rated for 85 amp in other appactions but make sure you hang on that art number as Hack posted in case the inspector want to ruffle the feathers on ya.
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Old 08-10-2017, 02:34 PM   #9
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Dham206 .,, Hackwork did bring up very good point on the service conductor size for 100 amp with #4 copper conductors and yes it still legit only from the meter to first OCPD typically main breaker.
It goes further than that. It covers service or feeder that powers an entire dwelling unit.
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Old 08-16-2017, 07:10 PM   #10
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Here's a post from another thread: dham206, you should be fine.
Thx Hackwork. I've been losing sleep over this one.
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Old 08-16-2017, 08:42 PM   #11
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The way it was worded was service entrance conductors for a single family dwelling or feeders to a multi-family could be reduced. So if the service entrance conductors fed a meter stack or group of disconnects, you had to go full size.

It is probably the same way. But you could go reduced if you ran two service entrance conductors up the side of the house.

I could be wrong, but I think you have to read between the lines.
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Old 08-16-2017, 08:48 PM   #12
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@mech you showed your commercial experience right here. I feel resi guys could go to commercial, I am not so sure commercial guys could go to resi.
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Old 08-16-2017, 08:50 PM   #13
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Originally Posted by cabletie View Post
The way it was worded was service entrance conductors for a single family dwelling or feeders to a multi-family could be reduced. So if the service entrance conductors fed a meter stack or group of disconnects, you had to go full size.
I don't agree. That's not what the code said. You were allowed to use the table for multi-family house services, and we always did (4/0 AL feeding a 200A service and #2 AL feeding 2 100A panels).


The 2011 code section 310.15(B)(7) said the following:

Quote:
(7) 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)(7), 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 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 between the
main disconnect and the panelboard that supplies, either by
branch circuits or by feeders, or both, all loads that are part
or associated with the dwelling unit. 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.
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Old 08-16-2017, 09:02 PM   #14
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Quote:
Originally Posted by cabletie View Post
The way it was worded was service entrance conductors for a single family dwelling or feeders to a multi-family could be reduced. So if the service entrance conductors fed a meter stack or group of disconnects, you had to go full size.

It is probably the same way. But you could go reduced if you ran two service entrance conductors up the side of the house.

I could be wrong, but I think you have to read between the lines.
The conductors to a meter stack are only based on the calculated load. If the feeders do not feed anything other than the whole residence you can use the 83% percent thing but in real life just go to the table we have used for 40 years.
In real life it is a really stupid code because if you take the ac unit off and feed it from the main panel(which is common) you would need to run larger conductors.
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Old 08-16-2017, 09:07 PM   #15
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Quote:
(7)Single-Phase Dwelling Services and Feeders
For one-family dwellings and the individual dwelling units of two-family
and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted to be sized in accordance with 310.15(B)(7)(1) through (4).
For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, single-phase feeder conductors consisting of 2 ungrounded conductors and the neutral conductor from a 208Y/120 volt system shall be permitted to be sized in accordance with 310.15(B)(7)(1) through (3).
(1)For a service rated 100 through 400 amperes, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating.
When it feeds a meter stack, they cannot be reduced, but the feeders to the individual units can. The service entrance conductors in this case are already calculated by the load and are small enough.

If you run two seperate service entrance rises up the house, they can be reduced.
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Old 08-16-2017, 09:15 PM   #16
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Quote:
Originally Posted by dham206 View Post
i am working on a DADU (detached additional dwelling unit) in Seattle. I have to upgrade the main service at the house to 200 amps and put in a double socket meter base. The main house needs a 100 sub panel and the DADU also needs a 100 amp sub panel. Table 310.15b6 used to let me use #4 wire for a 100 amp panels whether it was the main service or feeder as long as its intended for residential. The table no longer exists. my question, is #4 still acceptable for 100 amp feeders to sub panels?
You are right. Plus you get to use 4/0 al for the riser.
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Old 08-16-2017, 09:56 PM   #17
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Originally Posted by sbrn33 View Post
@mech you showed your commercial experience right here. I feel resi guys could go to commercial, I am not so sure commercial guys could go to resi.
If a resi guy tried to run a 100amp circuit on #4awg in a commercial app he wouldn't last long.
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Old 08-16-2017, 10:50 PM   #18
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Originally Posted by MechanicalDVR View Post
If a resi guy tried to run a 100amp circuit on #4awg in a commercial app he wouldn't last long.
If a commercial guy tried to run #2 copper for a 100A feed without showing me a voltage drop calc, I would tell him to stop wasting my money and run #3 copper.
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Old 08-16-2017, 11:35 PM   #19
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100 amps and under I always size by the 60 degree column. So #2 is good for 95 amps and I would use the next size higher rule. I would never think of running #3.
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