I have a code question about derating...
If I use #10 on a 20A branch ckt for volt drop purposes, do I have to pull a #10 grounding conductor as well? Or can I still use a #12?
Thanks guys,
H20
think 250-122(B) requires me to pull a #10 as well, if I am interpreting this correctly... increase size(no reason given I.E. derating for # of cond. in raceway or volt drop.
Is this correct interpretation?
That is the way I read it too.
The only time I see you getting a possible break is if you went from a #10 to a #8 since a #10 is the standard EGC for a #8 (up to 60a)
I have to agree with Greg.
The Canadian code recently changed to require this. Bonding wires used to be sized according to the breaker or fuse size but this created a problem for long runs where the bond wire would be say #14 but the circuit conductors had to be #6 for VD reasons IE still a 15 amp circuit.
Current rules now size the bonding wire (equipment ground) to the circuit conductor. A very good rule change which the NEC already had that lowered the impedance of the return wire so the breakers or fuses would trip faster.
Hey Mikesh, I think the Canadian Code is the same as the NEC on this but I don't have a copy of the Canadian Code. The NEC requires us to increase the EGC when we increase the ungrounded conductors and they use the word "proportionately". There's an easy way to do that using Table 8 in the back of the NEC.
250.122(B)
George
Yes the current rules in the 2009 Canadian code does require the bond (equipment ground) wire to be sized according to the circuit wire size and use table 16.
So if the load was 12 amps but it was far enough away to need a bigger wire than #14 then the bond wire must get bigger too.
In the CEC before 2009 the bonding wire never had to get any bigger than #14 because the circuit was connected to a 15 amp breaker and it was the breaker or fuse size that used to determine the bond wire size.
The new way compensates for voltage drop on the bond wire and the old rule did not. So the old way was like adding a resistor to a fault a long way out from the source and did cause longer trip or longer fusing.
I think the NEC has always had this one right and the CEC only just got it right in 2009.
Greg,
Going up in wire size without changing the size of the OCPD will not change anything. If you have a 20 amp breaker, the EGC must be the same size as the ungrounded conductor, no matter how large that conductor is.
That is where 250.122 becomes ambiguous. If that is true, you can't upsize using a cable wiring method greater than #8 since once you get to #8, cables have smaller EGCs.
Greg,
I don't think the rule is one bit ambiguous. It is clear that when you are using OCPDs rated 30 amps or less that the EGC must be of the same size as the ungrounded conductors. If you are using a cable wiring method, you will have to get custom cable or use a cable 4 conductor cable in place of a 3 or 5 in place of a 4 to get a full size condutor to use as the EGC.
It is my opinion that the size of the EGC should be based on the size of the ungrounded conductor and not on the size of the OCPD. That would make table 250.122 work like 250.66, but that is not what the code says and a proposal to that effect was rejected by CMP 5.
When you step back and look at the rules in general, it appears this is a "small conductor" rule and once you get to #8 it is not a small conductor anymore but I agree that is what the rules say.
250.122 (B) is quite clear Up the ungrounded conductor because of Voltage drop Upsize the Equipment ground Proportionately according to circular mill area of the conductor. Does not note size Small or large.
Yoopersup
Note: but in no case does the EGC have to be larger then conductor: (250.122 (A)
That does bring up the interesting conundrum that you can have an 8 ga circuit with a 10ga EGC on a 50a breaker but if you put a 30a breaker on it you need an 8ga EGC.
Things that make you say huh?
Ambiguouis? Maybe not. Logical? Probably. Consistant? Not a chance.
I woud submit that if a #10 is adequate for 30 amp fault current when voltage drop is not a concern, then that same #10 ought to be adequate for 20 amps when you are worried about voltage drop.
Yet, strictly speaking, that's not how the code reads. This, in turn, leads to the conclusion that many cable wiring methods are not suitable for long runs.
This brings to mind the 'insulated ground' requirement for some pool panels.
These little glitches area direct consequence of the way the NEC is written. Frequent revisions, isolated panels, lobbying interests - and the latest twist, an 'appeals' element. I suppose the real miracle is that there are not more such unicorns in the code forest.
It's something to think about as we prepare to receive a new edition. From the crystal clarity of the Constitution, the NEC is more beginning to resemble the tax code.
Ultimately, it's not about wire sizing.
Like you said John, not too hard to get inconsistancies when the CMPs are focused on one area and don't see how their change can affect other Articles. Sometimes you need to step back and look at the NEC as a whole.
There are also the ones like you and I have discussed like the cord and plug range hood that needs a dedicated circuit, but if you hardwire it you do not. If you are doing this to allow a possible future change to a microhood you should have the circuit dedicated regardless of connection method. But wait, isn't this a design issue?
I really think its Quite Clear (250.122b )
Voltage drop must be taken in account in the GEC as well as phase conductors. Whats unclear about that???
You start getting answers like "No I didn't upsize for voltage drop, it was just what I had on the truck".
Youpersup,
There is no code requirement to take voltage drop into account other than a couple of specific circuits.
250.122(B) no longer mentions voltage drop(that wording was last used in the 1999 code). If you use an ungrounded conductor that is larger than the minimum size permitted by Table 310.16, the you must increase the size of the EGC in direct proportion to the increase in size of the ungrounded conductor.
So if you increase the wire size , Example street light outlets on poles to 1/0 . Equipment ground size does what????
RV parks same as a cpl examples where Much larger phase conductors are used because of VD/