Why was table 310.15 B 7 moved out of the NEC IN THE 2014 addition? Ive always used that table for doing residential services and now I'm just noticing its gone? In the 2014 addition that section is highlighted gray now. Does that mean that it was removed?
310.15 b7
- Thread starter fireryan
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jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
You use a .83 multiplier now. The numbers work out the same.
I get lazy and still use the old chart for convenience at times.
I get lazy and still use the old chart for convenience at times.
Yeah I now see in the highlighted gray area where it say 83 percent. They just can't leave anything alone can they:rant:
Carultch
Senior Member
- Location
- Massachusetts
Which in my opinion, makes a lot more sense than having an extra table to look it up. If you prefer having the table, just snip it out of your old code book, and glue it in your new one.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Feel free to yell at George Stoltlz, it was his proposal that got it changed.
Haha maybe I will
Table is memorized:thumbsup:
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I think it was a great change and , in fact, 83% is the same as the table. What the change has done was taken away any question about having to use de-rating.
The table would allow 4/0 aluminum for 200 amps however if the 4/0 went thru areas of high temp or was pulled thru insulation without having to de-rate
The table would allow 4/0 aluminum for 200 amps however if the 4/0 went thru areas of high temp or was pulled thru insulation without having to de-rate
- Occupation
- Licensed Electrician
Yeah, we better all get out there and start ripping out old SER now before summer gets into full swing and something bad happens
.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Yeah, we better all get out there and start ripping out old SER now before summer gets into full swing and something bad happens
I understand that there hasn't been an issue with the wiring but what we are saying is that a service that calculates to 190 amps or so is allowed to have a conductor such as 4/0 Al. even if it is rated 60C-- that is 150 amps.... Where do you draw the line. Also with that logic we can say we should allow nm in the ground because I have seen it work for 20 years without issue or why not allow parallel #12 and fuse at 30 amps-- I bet it has been done without an issue also.
Carultch
Senior Member
- Location
- Massachusetts
I thought about that as well. That the general procedure for conductor sizing can be so nuanced with derate factors and terminal temperature ratings. I would intuitively think that the same general algorithm should apply, no matter whether it is a service conductor, feeder, or branch circuit.
However, now it all makes sense that we have one master 83% factor to account for the status of a conductor as a service conductor. And the entire rest of the wire sizing algorithm still applies, in what you calculate after taking the 83% factor into consideration.
It is a mystery to me why we even have service conductors sized less than a feeder of the same ampere rating in the first place. I know the explanation is usually load diversity (that you aren't going to turn everything on all at once), but I would expect that to be already accounted for, when using either the standard or optional load calculation.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
- Occupation
- Electrician
I think the change is better than the old way because it does clear things up, allow for parallel runs, etc, but I cant help but think the whole concept is bit silly: Why not just size residential services closer to what the actual load will be and size the conductors like every other conductor? Its like saying, "we will raise the speed limit 20%, but we will also make the speedometer read 20% high :?:
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Both standard and optional calculations on the NEC are very conservative and the POCOs know it. They also know that a lot of high load combinations are only present for a few minutes and the wires are good for many minutes at a low overload.
- Location
- Massachusetts
I agree 100% and I think that if we asked George that would be his choice as well.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I don't necessarily know how to determine how to come up with 83% or what level of load diversity truly is on average for a dwelling, but can definitely tell you from experiences that dwelling loads seldom max out the service/feeder as calculated per art 220. Sure there are times of peak demand but often they are short enough in duration that I would think we could even go with 50% instead of 83% and never have any overheating issues. HVAC is the one thing that does raise the diversity factor, outside of that it is low.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
Indeed. My proposals to eliminate 310.15(B)(7) and move it to Article 220 where the concept belongs were met with resistance, to say the least. I can't understand what part is so hard for them to understand any more than they can understand it. Except Mr. Mellow, as I recall.
If the load calc is wrong, fix it; don't slap too large a breaker on too small a wire and then say "Well, the wire will probably be fine anyway." That is exactly what we do, only in this instance, and it is justified with "we've done it that way for years."
Meh.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Maybe they didn't like you moving content from one of their sections to one covered by another code making panel.
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