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Old 11-02-2010, 09:30 AM   #29
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Ground wire size

FYI 2005 edition of the NEC. Table 250-122 minimum Size of Equipment Grounding Conductor for Grounding Raceways and Equipment. You new 50 amp RV outlet is considered equipment. These are the minimum requirements.
15 amp circuit breaker = #14 copper
20 Amp Circuit Breaker = #12 copper
30 Amp Circuit Breaker = #10 copper
40 Amp Circuit Breaker = #10 copper
60 Amp Circuit Breaker = #10 copper for some reason they skip a 50 amp in the table. But one can safely assume it's also #10.
100 Amp circuit breaker #8 copper Anything over 60 amp up to 100 amp requires #8.
200 Amp Circuit Breaker #6 copper
The chart goes all the way up to 6000 amp, which requires an 800 MCM about 1" in diameter.
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Old 11-02-2010, 10:16 AM   #30
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FYI 2005 edition of the NEC. Table 250-122 minimum Size of Equipment Grounding Conductor for Grounding Raceways and Equipment. You new 50 amp RV outlet is considered equipment. These are the minimum requirements.
I would not disagree that these are the minimums as set by the NEC.

Would you not agree that all the electrical cables in a circuit from the main circuit panel to the pedestal need to be the same gauge if the load conductors are larger. In the case of an RV the cord is comprised of 3/6 & 1/8 SOW cable. I would expect the bond to be the same gauge from the attached load back to the point where it is attached to the bonding bar in the distribution panel. All the conductors in the RV from the transfer switch to the distribution panel are comprised of 3/6 & 1/8.

I would expect that if the bond in the distribution panel in the RV is a #8, a number #8 wire needs to be carried back to the main distribution panel and not connected to a #10 at the pedestal.
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Old 11-02-2010, 11:26 AM   #31
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Since the purpose of the ground is to force the breaker to trip when there is a ground fault, the ground wire is sized so that its resistance is low enough to carry enough current to cause the trip. Although you can go with a larger size ground wire, a lot of research has been done to determine that the NEC sizes will do the job.
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Old 11-02-2010, 03:07 PM   #32
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Since the purpose of the ground is to force the breaker to trip when there is a ground fault, the ground wire is sized so that its resistance is low enough to carry enough current to cause the trip. Although you can go with a larger size ground wire, a lot of research has been done to determine that the NEC sizes will do the job.
Your very correct!!!!!!!
If OP wants to spend the extra money there is nothing wrong with upsizing. But with the cost of copper it's your call.
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Old 11-02-2010, 04:29 PM   #33
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OK .....

I think I have it after researching this for the past hour or so.

Here is the reason why you need a number 8 AWG.

According to 550.16 (C)2: The minimum wire size used for the ground is an 8 AWG minimum in the mobile home/recreational vehicle. If you look at your SOW cord you will see it has 1 number 8 AWG.

What we know about a 50A RV service is that it is NOT a 50 Amp service at all but it is a "100A Service." If you add L1 + L2 you will get the 100 Amps. It is totally likely that an RV will draw more current than the 60Amp rating that requires only a 10AWG ground in the 250.122 table.

According to NEC 2011 if you are grounding a 100 Ampere Service the minimum size ground conductor is a number 8 AWG.

Therefore: The minimum size conductor used as a bond (ground) on an RV pedestal providing power to a 50 Amp RV receptacle shall not be smaller than a 8 AWG.

I rest my case.
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Old 11-02-2010, 04:44 PM   #34
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The largest breaker you need to trip is a 50 amp. There is no "100" amp breaker in the system and no way a properly wired 50 amp pedestal connection can draw 100 amps in a short to ground, therefore the #10 wire meets NEC requirements.

The bonding conductor referred to in 550.16C2 is between the RV breaker panel & the RV chassis.
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Old 11-02-2010, 04:56 PM   #35
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The largest breaker you need to trip is a 50 amp. There is no "100" amp breaker in the system and no way a properly wired 50 amp pedestal connection can draw 100 amps in a short to ground, therefore the #10 wire meets NEC requirements.

The bonding conductor referred to in 550.16C2 is between the RV breaker panel & the RV chassis.
How many amps will the "Neutral Draw?"

I am thinking that you have to protect the current draw of the higest rated conductor. You can consume 9600 Watts pretty easily in an RV if you have 120 volts or less available, you could draw 80 amps or more over the neutral but not in the same phase.

That said a #8 won't safely handle 100 amps on its own so you would need a #4. This is very perplexing situation however I would not wire a 50A RV plug with any less copper than is going into the RV over the SOW cord.

Why have #8 in the RV and not carry it back to the main circuit panel?
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Old 11-02-2010, 06:03 PM   #36
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In a properly wired 50 amp receptacle, the 2 50 amp breakers are on different legs. The neutral will carry the difference between the two which can never be more than 50 amps. All bets are off if the receptacle is improperly wired (both breakers on the same leg) since the neutral would then carry the sum, or a possible 100 amps. Even then, a ground wire sized for 100 amps is not necessary because there will not be 100 amps between the neutral & the ground.

Again, the ground does not need to carry current continuously - only long enough to trip the breaker. It can be under sized compared to a current carrying wire because it only carrys the current for a short time - not long enough to overheat.
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Old 11-02-2010, 06:11 PM   #37
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.... about that Neutral .. the more I think about this the more I see this load situation for what it is, The Neutral "never" exceeds the capacity of either L1 or L2 in which each are rated for 50A. Although the loads can be cumulative on an electric bill they are not cumulative in an electric circuit.

If the same RV in the example above is consuming 9600 watts (not likely) the Neutral would be averaging the load because of the phase difference. So although the meter is spinning over 9600 Watts the Neutral is only being subject to 4800 watts at any one time OR 40 Amps.

I am holding the line on using a #8 AWG though for a bond. I am not fond of changing horses in mid-stream. There has got to be something written in the NEC that supports the use of an 8 AWG in the SOW cord other than referring back to Section 250 Part I to V and why it does not be carried back to the main distribution panel.
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Old 11-02-2010, 06:16 PM   #38
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As to the use of the #8 ground in your SO cord feeding your RV, since most flexible cable is manufactured with all the wires the same size, it would probably cost more for the manufacturers to have specially made cable for their RV power cords.
In this day and age you would expect that if they could get away with less copper it would yield bigger profits. The last time I did a 50A plug I went to Home Depot and the cable if I recall properly was a 4 conductor cable that had 3 - 6 and 1 - 8 Green. ( <-- That could have been a #10 )

Cerro 6/3 wire comes with a #10 bond:
Pn# 147-4203 6-3 7 Strands #10 Bond.
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Old 11-02-2010, 06:28 PM   #39
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Sorry - I cut the section on cord out when I edited my post.

As to rubber covered cable - in most cases the cost of having specially made cable is pretty expensive. A large manufacturer might save enough to make it worth it, but in most cases, power cords have the same size wire for all conductors.

If you are running to a pedestal, particularly with conduit, it makes sense to run individual wires, for example THHN. In that case you can size the wires to code & save yourself some copper costs. Even direct burial cables can be sized with different size individual wires.
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Old 11-02-2010, 06:38 PM   #40
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If you are running to a pedestal, particularly with conduit, it makes sense to run individual wires, for example THHN. In that case you can size the wires to code & save yourself some copper costs. Even direct burial cables can be sized with different size individual wires.
Yes, You could do a really nice job with THHN and get more than 50A out of a #6 to boot BUT that would be limited by the breaker. You can in that case run a #10 stand alone - no problem. PVC is easy to work with.

You might not believe this but I had to relocate the 120V WP Box on my pedestal and I actually had the guy cut 3 pieces - 24" of #12 THHN. Just had enough wire for the move - didn't throw any wire away either.

After all this re-think and look see at this and that, I don't think you can buy a cable that has 3-6 and 1-8 in the same jacket as NMB, SER, UF or BX or any other type. So it would appear that a #10 bond is going to win this argument if only from the stand point that getting a #8 out to the pedestal economically isn't going to happen.



I had to install a new WP box stand alone because my builder didn't do me any favor by stopping short with the 20A plug in the blanked out box just below the 50A WP box. The SOW cord would hang on one side or the other and in time that would have damaged the jaws in the receptacle. Now when I plug in the cord it sits nice and straight with no twisting on the jaws.

On the back side of the post (picture #2) there's a 2nd box box with a Telco RJ11 and a coax combo receptacle. 5 way hookup just like our favorite campground.

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Old 11-02-2010, 07:21 PM   #41
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.... about that Neutral .. .....

If the same RV in the example above is consuming 9600 watts (not likely) the Neutral would be averaging the load because of the phase difference. So although the meter is spinning over 9600 Watts the Neutral is only being subject to 4800 watts at any one time OR 40 Amps.
Well ... not exactly. If the outlet is wired on opposite phases/legs then the neutral is only carrying the IMBALANCE between the two phases NOT the AVERAGE of the two.

In your example above, the RV is consuming 9600 watts with them perfectly balanced on each phase, ie 4800 watts per phase. The neutral would be carrying zero amps, nada.

If one phase is carrying 4800 watts and the other nothing, then the neutral would be carrying 4800 watts or 40 amps.

If one leg is at 40 amps and the other at 15 amps, the neutral would be at 25 amps, ie 40-15=25.

It's counter-intuitive.
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Old 11-02-2010, 07:33 PM   #42
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In your example above, the RV is consuming 9600 watts with them perfectly balanced on each phase, ie 4800 watts per phase. The neutral would be carrying zero amps, nada..
I just can't see it from here ...

RMS is a positive value and if one phase is peaking the other is coming out of it's negative side and then the whole process starts all over. In a 120 volt circuit I believe that the neutral must be carrying whatever the real load is on L1 or L2 and not zero. If you meter (induction coil / amp probe) from either side of the 2P breaker you will directly read the amps flowing though that side of the breaker. Now what you are telling me is that if I clamp the neutral the meter will read "0" (if perfectly balanced). Very weird indeed.

I would not argue the point in a 220V circuit - that's the way it's supposed to work but without a Neutral.

I don't know about you, but this topic is a lot of fun to me. (might just learn something)
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