Apartment feeders
- Thread starter nvcape
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Re: Apartment feeders
The table specifically says 120/240 volt single phase. Not 120/208v three phase. I would say the table doesn't apply to your apartment complex.
The handbook also has a commentary that repeats "single phase". It also says the table applies for "a single set of 3 wire...." This also seems to exclude runs that branch out from the main service to each apartment.
Steve
The table specifically says 120/240 volt single phase. Not 120/208v three phase. I would say the table doesn't apply to your apartment complex.
The handbook also has a commentary that repeats "single phase". It also says the table applies for "a single set of 3 wire...." This also seems to exclude runs that branch out from the main service to each apartment.
Steve
BAHTAH
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- United States
Re: Apartment feeders
From your post I assume you are installing a multi-meter residential service, 208y/120 with the distribution for each dwelling unit being 3w single-phase with a utility company meter and main disconnect for each unit. In this case table 315-15(b)(6) applies to your installation. " and feeder conductors that serve as the main power feeder to a dwelling unit." It is in intent that when the conductors carry the total load of the dwelling that this table can be used and each apartment is considered an individual dwelling unit. You may want to check with your AHJ as the article references 120/240 and not 208/120 however the voltage really should have little to do with the application. This type of installation is very common since the utility is able to maintain a balanced three-phase system while serving single-phase loads and most utilities limit the service sizes on single-phase only services to less than the 208y/120 services.
From your post I assume you are installing a multi-meter residential service, 208y/120 with the distribution for each dwelling unit being 3w single-phase with a utility company meter and main disconnect for each unit. In this case table 315-15(b)(6) applies to your installation. " and feeder conductors that serve as the main power feeder to a dwelling unit." It is in intent that when the conductors carry the total load of the dwelling that this table can be used and each apartment is considered an individual dwelling unit. You may want to check with your AHJ as the article references 120/240 and not 208/120 however the voltage really should have little to do with the application. This type of installation is very common since the utility is able to maintain a balanced three-phase system while serving single-phase loads and most utilities limit the service sizes on single-phase only services to less than the 208y/120 services.
- Location
- Illinois
- Occupation
- retired electrician
Re: Apartment feeders
Grant,
I don't agree. If you are using two hots and the grounded conductor from a 208/120Y system you cannot use Table 310.15(B)(6). It can only be used with 120/240 volt single phase systems. One difference is that the grounded conductor is always a current carrying conductor in the 208/120Y system.
Don
I need to expand on my statement above. The grounded conductor of a system fed by 2 hots and the grounded conductor of a 208/120Y system is always a current carrying conductor. When all three hots are used, the grounded conductor is current carrying conductor only if a majority of the load is non-linear.
[ October 01, 2003, 10:38 PM: Message edited by: don_resqcapt19 ]
Grant,
I don't agree. If you are using two hots and the grounded conductor from a 208/120Y system you cannot use Table 310.15(B)(6). It can only be used with 120/240 volt single phase systems. One difference is that the grounded conductor is always a current carrying conductor in the 208/120Y system.
Don
I need to expand on my statement above. The grounded conductor of a system fed by 2 hots and the grounded conductor of a 208/120Y system is always a current carrying conductor. When all three hots are used, the grounded conductor is current carrying conductor only if a majority of the load is non-linear.
[ October 01, 2003, 10:38 PM: Message edited by: don_resqcapt19 ]
- Location
- Massachusetts
Re: Apartment feeders
I know all areas are different but isn't there engineered drawings for this project?
In this area a multi dwelling unit, at least any I have worked on have engineered drawings.
Don't you need these to get your permit?
I know all areas are different but isn't there engineered drawings for this project?
In this area a multi dwelling unit, at least any I have worked on have engineered drawings.
Don't you need these to get your permit?
charlie
Senior Member
- Location
- Indianapolis
Re: Apartment feeders
Don, I agree. Your interpretation is absolutely correct.
Don, I agree. Your interpretation is absolutely correct.
Re: Apartment feeders
Thanks for the comments. Agree that on 208V the neutral has current, but so can a 120/240V system. I am responsible for the plans which go through plan check; and don't reduce the neutral. I wrote the code committee on this but so far did not get a response. Wanted to know how other areas looked at this.
Thanks for the comments. Agree that on 208V the neutral has current, but so can a 120/240V system. I am responsible for the plans which go through plan check; and don't reduce the neutral. I wrote the code committee on this but so far did not get a response. Wanted to know how other areas looked at this.
BAHTAH
Senior Member
- Location
- United States
Re: Apartment feeders
Don, I do not disagree that the neutral of a 208y/120 when associated with a 208/120 1ph feeder carries approximately the same current as the phase conductors. I do not disagree that article 310-15(b)(6) specifically applies to 120/240 1p3w systems. My though is this article allows for the reduced loads associated with residential demands and thus is allowing for the reduction of the conductor size in feeders that supply the total dwelling load. Even if the neutral were to carry the same load as the phase conductor, if that load reaches the max, the breaker trips since the neutral and phase loads are approximately the same. Is it possible to overload a neutral conductor that is the same size as the phase conductors? Residential loads are typically not where you would find harmonics being an issue on the neutral and if you had a 100amp feeder that actually eached 101 amps would'nt the neutral and phase conductor see the same current and the breaker trip? If you than had multiple feeders to dwellings and they were balanced across the 3ph4w main the service neutral would carry current based on a 3ph4w load. I guess the question I have is, can you unbalance the load enough to overload the neutral of either a feeder or service main without tripping a breaker?
[ October 02, 2003, 01:17 PM: Message edited by: grant ]
Don, I do not disagree that the neutral of a 208y/120 when associated with a 208/120 1ph feeder carries approximately the same current as the phase conductors. I do not disagree that article 310-15(b)(6) specifically applies to 120/240 1p3w systems. My though is this article allows for the reduced loads associated with residential demands and thus is allowing for the reduction of the conductor size in feeders that supply the total dwelling load. Even if the neutral were to carry the same load as the phase conductor, if that load reaches the max, the breaker trips since the neutral and phase loads are approximately the same. Is it possible to overload a neutral conductor that is the same size as the phase conductors? Residential loads are typically not where you would find harmonics being an issue on the neutral and if you had a 100amp feeder that actually eached 101 amps would'nt the neutral and phase conductor see the same current and the breaker trip? If you than had multiple feeders to dwellings and they were balanced across the 3ph4w main the service neutral would carry current based on a 3ph4w load. I guess the question I have is, can you unbalance the load enough to overload the neutral of either a feeder or service main without tripping a breaker?
[ October 02, 2003, 01:17 PM: Message edited by: grant ]
- Location
- Illinois
- Occupation
- retired electrician
Re: Apartment feeders
Grant,
I don't think it really has to do with overloading the grounded conductor. I think that is based on the heat in the conductors. In the system that is served with 2 phases of a wye system, all three conductors could be producing the same amount of heat. In the single phase system the maximum heat is when 2 conductors are fully loaded. Any other loading will produce less heat. The wire sizes permitted by the section are smaller than permitted for other occupancies, and the additional heat produced by have all three conductors heat producing may push the conductor past the temperature limit without tripping the breaker.
Don
Grant,
I don't think it really has to do with overloading the grounded conductor. I think that is based on the heat in the conductors. In the system that is served with 2 phases of a wye system, all three conductors could be producing the same amount of heat. In the single phase system the maximum heat is when 2 conductors are fully loaded. Any other loading will produce less heat. The wire sizes permitted by the section are smaller than permitted for other occupancies, and the additional heat produced by have all three conductors heat producing may push the conductor past the temperature limit without tripping the breaker.
Don
BAHTAH
Senior Member
- Location
- United States
Re: Apartment feeders
Don, I appreciate your insite. What you say may be correct. I think there may be room for some testing in this area. I have to wonder if the 120/240 is like when the RV parks were only allowed to be 120/240 1ph3w but are now allowed to use 208y/120 for 20 and 30 amp pedestals. Maybe with some testing we will see a change in this article. Again I appreciate your point of view as you are a great resource to this site.
Grant
Don, I appreciate your insite. What you say may be correct. I think there may be room for some testing in this area. I have to wonder if the 120/240 is like when the RV parks were only allowed to be 120/240 1ph3w but are now allowed to use 208y/120 for 20 and 30 amp pedestals. Maybe with some testing we will see a change in this article. Again I appreciate your point of view as you are a great resource to this site.
Grant
Re: Apartment feeders
I should have read a little farther before posting that the reduced sizes were allowed only for a single set of conductors serving a dwelling.
The handbook does have examples of feeding each apartment with the reduced sizes (for 120/240 volts).
It seems like "dwelling" can define either the entire apartment building, or it can mean a single apartment.
Steve
I should have read a little farther before posting that the reduced sizes were allowed only for a single set of conductors serving a dwelling.
The handbook does have examples of feeding each apartment with the reduced sizes (for 120/240 volts).
It seems like "dwelling" can define either the entire apartment building, or it can mean a single apartment.
Steve
- Location
- Illinois
- Occupation
- retired electrician
Re: Apartment feeders
Grant,
The issue with RV parks is not the same. The issue there is supplying the correct voltage. The 20 and 30 amps receptacles are 125 volt devices and either the single phase 120/240 or the three phase 208/120Y system will supply 120 volts line to grounded conductor. When you move up to the 50 amp receptacles you are changing the voltage to 120/240 and Article 551 does not permit the 120/240 volt receptacles to be fed from the wye system because the line to line voltage is only 208, not 240. This is another example of the code rules in 550 and 551 being different from the rules in Chapters 1 through 4 for similar applications.
Don
Grant,
The issue with RV parks is not the same. The issue there is supplying the correct voltage. The 20 and 30 amps receptacles are 125 volt devices and either the single phase 120/240 or the three phase 208/120Y system will supply 120 volts line to grounded conductor. When you move up to the 50 amp receptacles you are changing the voltage to 120/240 and Article 551 does not permit the 120/240 volt receptacles to be fed from the wye system because the line to line voltage is only 208, not 240. This is another example of the code rules in 550 and 551 being different from the rules in Chapters 1 through 4 for similar applications.
Don
charlie
Senior Member
- Location
- Indianapolis
Re: Apartment feeders
The following is from memory so take it with a bit of salt. However, I believe it to be true.
The following is from memory so take it with a bit of salt. However, I believe it to be true.
BAHTAH
Senior Member
- Location
- United States
Re: Apartment feeders
Don,
I think you may have taken my responce out of context in reference to the 208/120 in RV parks compaired to the 120/208 for dwellings and the code allowing 208/120 in rv parks but not yet allowing 120/208 for reduced feeders in dwellings. I know the 208y/120 for Rv parks can not be used on the 50amp spaces, that is why my post only list the 20 and 30 amp for that system. What I was trying to convey (maybe not so well) was RV parks at one time were only 120/240 for all spaces and over time the NEC came to allow the 208y/120 system to be used in a park and that the feeders to apartments that are 120/240 are allowed to be reduced in size but someday with testing maybe that will be extended to 120/208V feeders for aparments since we see so many derived from that system. Since 310-16 does not require derating for three current carrying conductors and the loads in dwellings are low demand (except on thanksgiving) I would think some testing may show that the 120/208 could be included in the same chart with the 120/240 size reduction.
Don,
I think you may have taken my responce out of context in reference to the 208/120 in RV parks compaired to the 120/208 for dwellings and the code allowing 208/120 in rv parks but not yet allowing 120/208 for reduced feeders in dwellings. I know the 208y/120 for Rv parks can not be used on the 50amp spaces, that is why my post only list the 20 and 30 amp for that system. What I was trying to convey (maybe not so well) was RV parks at one time were only 120/240 for all spaces and over time the NEC came to allow the 208y/120 system to be used in a park and that the feeders to apartments that are 120/240 are allowed to be reduced in size but someday with testing maybe that will be extended to 120/208V feeders for aparments since we see so many derived from that system. Since 310-16 does not require derating for three current carrying conductors and the loads in dwellings are low demand (except on thanksgiving) I would think some testing may show that the 120/208 could be included in the same chart with the 120/240 size reduction.
- Location
- Massachusetts
Re: Apartment feeders
Not having wired any RV parks I would have to ask how you could reduce the size of the conductors, considering the length of the runs from the supply to the pedestals must be (I imagine) fairly long as compared to apartment feeders.
Wouldn't voltage drop force the use of larger conductors than required by 310.16?
Not having wired any RV parks I would have to ask how you could reduce the size of the conductors, considering the length of the runs from the supply to the pedestals must be (I imagine) fairly long as compared to apartment feeders.
Wouldn't voltage drop force the use of larger conductors than required by 310.16?
BAHTAH
Senior Member
- Location
- United States
Re: Apartment feeders
iwire,
The reference to RV parks only has to do with the NEC recognizing the use of more than one voltage over time. Like RV Parks that were once only 120/240 and are now 208y/120 and 120/240v, Apartments may someday have the same voltage recogintion where they are now allowed to reduce feeder sizes for only 120/240, someday maybe the table will include 120/208. No Rv parks getting wired in this thread.
iwire,
The reference to RV parks only has to do with the NEC recognizing the use of more than one voltage over time. Like RV Parks that were once only 120/240 and are now 208y/120 and 120/240v, Apartments may someday have the same voltage recogintion where they are now allowed to reduce feeder sizes for only 120/240, someday maybe the table will include 120/208. No Rv parks getting wired in this thread.
- Location
- Massachusetts
Re: Apartment feeders
I'm sorry Grant but this threads topic was using reduced size feeders and when reading your post I got the impression that you thought with further testing both 208 volt apartment feeders and 208 volt RV feeders should be allowed to be reduced in size.
I am just a stickler for not using the absolute code minimum conductor size for anything.
Most times if you try to maintain less than a total of 5% drop the code minimums do not come close.
I'm sorry Grant but this threads topic was using reduced size feeders and when reading your post I got the impression that you thought with further testing both 208 volt apartment feeders and 208 volt RV feeders should be allowed to be reduced in size.
I am just a stickler for not using the absolute code minimum conductor size for anything.
Most times if you try to maintain less than a total of 5% drop the code minimums do not come close.
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