I am trying to learn as much about electrical as I can and have what is probably a simple question for many electrians. Could someone explain to me how to interpret the NEC table 310-16 for sizing wire. My understanding is that there is also a separate table for service cable . Which table is it and how do you interpret that as well. Could you give some typical examples of both, for example, with a 200amp main panel what wire would be run to a 100amp sub panel.
thanks
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Replies
I could explain 310.16 to you. So that should answer your question. {0.0}
Actually, I'm just bumbing this post so I can answer it in the morning. Stay tuned.
Note: for 100 amp and above, you can use the 75º column.
~Peter
"Service cable" is ONLY the wire from the power company transformer to your meter .... and even then, that chart only applies for wires strung overhead.
For a sub-panel, use 310-16. Otherwise .... call a sparky. There's a reason it's called a SKILLED trade.
Edited 7/25/2007 9:44 am ET by renosteinke
""Service cable" is ONLY the wire from the power company transformer to your meter .... and even then, that chart only applies for wires strung overhead."That is wrong for section 310.15(B)(6).First the NEC has nothing to do with the POCO drop wires or under ground service.They only cover from the serive entrance in.Here is what the code says."(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."First it only includes single phase dwelling loads.It coverse above ground, below ground cables, and wires in racesways (conduit).And it covers service laterals and feeders from a disconnect to a separate (sub) panel.From the table heading."Conductor Types and Sizes for 120/240-
Volt, 3-Wire, Single-Phase Dwelling Services and Feeders.
Conductor Types RHH, RHW, RHW-2, THHN, THHW,
THW, THW-2, THWN, THWN-2, XHHW, XHHW-2, SE,
USE, USE-2"SE is above ground service cable.
USE is underground service cable.
And the others are wire that needs to be run in conduit.And from the NEC handbook there is some explanatory notes."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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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I think you're bringing in unrelated matters. I see nothing that allows the "service" ampacity table to be used for feeding a sub-panel. For that, the main table is uses (310.16). I believe the OP thinks the wires to the proposed sub-panel are "service" conductors. They are not. They are feeders. The effect of this is that he will need to use larger wire. He will also need a ground wire ... something absent from service conductors. On a more general note, questions such as this one indicate that someone is using the code book as a textbook, or a 'how to' manual, or a design manual ... something specifically proscribed in the introduction.
You're correct, I should be referring to the wire from the main panel to the sub panel as feeder cable instead of service cable (would the feeder not be SER cable though?). I am not an electrician but a G.C. who subs out the electrical. I usually ask the electrician questions but hate to use up all his time answering my questions (or even sounding like I'm questioning his work), As a G.C. I feel I should have a better understanding of the electrical system and use the code not so much as a text book but more of a reference. Even if I were to do the work myself I wouldn't guess which wire to use, that is why I'd like a better understanding of how to size the feeder cable whether it's by using the NEC table or by some other method.
As a G.C. I feel I should have a better understanding of the electrical system and use the code not so much as a text book but more of a reference. Even if I were to do the work myself I wouldn't guess which wire to use, that is why I'd like a better understanding of how to size the feeder cable whether it's by using the NEC table or by some other method.
Well that's a reasonable request. With 310.16 in homebuilding you mainly use the 75-degree column and slide down to the ampacity or wire size you want, copper or aluminum. That will get you either right on the mark or at least in the vicinity and then your electrician can tell you about any derating or other conditions that might apply. The most common is when romex or SER is run inside thermally-insulated walls or ceilings, then you use the 60-degree column.
To determine feeder sizes for subpanels is more of a calculation based on projected load . Once that's determined then you size your breaker and use 310.16 to figure your wire size.
Ed
Edited 7/26/2007 7:49 am ET by edlee
Thank you
Just confirm wether I'm getting the jist or not.
Most electricians I've seen on (your average sized residential) jobs don't sit down and do a load calc (or at least I haven't seen them), they seem to be able to look at the size of the project, ask questions and from experience will know what's needed.
Now, let's say, hypothetically, that there's are 2,000sqft house with an updated kitchen. Let's assume the calcs suggest that a 200amp main panel would be sufficient with room to spare (say for a future hot tub). So for a 200amp main panel there would be 2/0 SE cable from the utility splice to the meter base and the same from the meter base to the main panel. If the main panel feeds a 200amp sub panel the feeder cable will be SE type R 2/0. Or if it feeds a 100amp sub panel it would be SE type R #4 copper.
Am
So for a 200amp main panel there would be 2/0 SE cable from the utility splice to the meter base and the same from the meter base to the main panel. If the main panel feeds a 200amp sub panel the feeder cable will be SE type R 2/0. Or if it feeds a 100amp sub panel it would be SE type R #4 copper
I agree that on an average residential job there isn't usually much calculating going on to size a service, at least for a single-family: 100a, 200a, or 320a.
If you're using SE cable then you're talking aluminum, so according to 310.15(B)(6) you'd need 4/0, not 2/0, for the service entrance.
For a 200a SUBPANEL off of a larger 300a or 400a service then I guess you'd need to go to a larger size, 3/0 copper or 250mcm aluminum. If you're talking about a 200a panel downstream of the 200a main switch, and that panel carries the entire load of the service, then you can stay with the 4/0 aluminum.
For the 100a subpanel you'd use 310.16, 75-degree column, and you will see that you'd need #3 copper or #1 aluminum. Since you said SER then you'd be using the aluminum.
Ed
"If you're using SE cable then you're talking aluminum, so according to 310.15(B)(6) you'd need 4/0, not 2/0, for the service entrance."Unless you are talking about CU. It is available, even it might not be the most commonly used.http://tinyurl.com/277qws"If you're talking about a 200a panel downstream of the 200a main switch, and that panel carries the entire load of the service, then you can stay with the 4/0 aluminum."Even if it does not carry the full load you can still use the smaller load.The downtream cable does not have to be bigger than the servce entrace size.For example you can use one of the combo meter/panesl with 6-8 slots and a main.Feed the AC and hot tube off breakers in the panel and use feed through to a 200 amp panel for the main house loads..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill,
Sure copper SE is available, but not readily. I imagine it would be a special order, cut to length, with a shipping charge. Not really the point of OP's queries.
"Even if it does not carry the full load you can still use the smaller load."
Did you mean "you can use the smaller wire?" If so, then I disagree. This is a common debate about Section 310.15(B)(6) and you and I are apparantly in opposite camps.
Ed
Yes, I meant the smaller wire.Here is what the NEC Handbooks says."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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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
" I see nothing that allows the "service" ampacity table to be used for feeding a sub-panel."It is mentioned twice."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."But those are special cases and not does not apply to his example.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
In their effort to make the NEC as incomprehenisble as possible, the Code making committee created Table 301-16. They could have produced a simple list but that would be too easy. To start with, you have to know what type of wire you're dealing with.
You should also do a load calculation. [See Article 220.] Just because you have a 100 amp panel does not mean that you necessarily ever be drawing anywhere near 100 amps from it. For instance, if your max is 55 amps, then you can feed it with 55 amp wire [#6] and protect that with a 60 amp breaker. [240-3(b)]
One thing to note is Article 110-14c. This essentially says that above 100 amps, the terminals are rated for 75*centigrade. Below 100 they assume 60*C. However you can search the device with your Edmund Scientific 1000X microscope and it might say otherwise. So we will assume that you want 100 amps and the breakers are rated for 75*. This means you can use the 75*C column. Except Romex, TW and UF must be from the 60* column. You look down the list and find 100 amps and then to the left and the answer is #3 wire.
But note the asterisk next to 14, 12 and 10. This refers you to 240-3 which says they don't really mean it when they say in the 60* column that these sizes are rated for 20, 25 and 30 amps respectively. Those double-crossers really meant 15, 20 and 30 amps but its a little more confusing this way.
But wait... there's more. And that is the wonderland of de-rating. You will notice at the bottom of the table a sub-table labeled "Correction Factors". Since the Code is pseudo-scientific, they realize that as temperature goes up, thhe resistance of the wire goes up so the carrying capacity goes down. So you have to multiply the amperage of your chosen wire by the appropriate factor. For instance, if your wire runs through the attic and the ambiant temperature there is 141*F, then you multiply your 100 amp by .33 and get 33 amps which is no longer good enough so you have to try larger sizes. This is good to know if you are taking an exam. In the field, you would measure the ambient attic temperature in the middle of winter.
But that's not all. If you carefully read the entire Article 310, you will stumble across Table 310-15(b)(2)(a). This is the famous de-rating mechanism for conduits carrying more than 3 [current carrying] conductors. This trips up many pretend electricians who are really only installers and don't know their own Code or even own a Code book.
On the surface, it seems simple. You have say nine #14 THHN wires. The de-rating factor for 9 wires is 70%. So you multiply 15 amps X .70 = 10.50 amps. Using 240-3b you could protect this with 15 amp breakers but you can't exceed 10 1/2 amps on any of those circuits.
But there is hope. Since the wire is THHN, you can start your de-rating with the value in the 90* column. So your calculation is 25 amps X .70 = 17.50. This is okey-dokey, hunky dory and acceptable.
~Peter
"Let me be clear about this. I attached the two black wires to one side of the switch and the two white wires to the other screw." --DIY forum.