Here's an odd one For you.

[charlie b]

This is just an FYI. I wanted to pass on an odd situation that I just discovered. The oddity is revealed at the end of this message.

A friend and the electrician he hired have been scratching their heads over a problem with a feeder to a 4-circuit sub-panel in his detached garage. I don't have all the details, but I gather that breakers have been tripping (in the house or in the garage or both, I'm not sure), and that the electrician suspects a short in the underground portion of the conduit run. There was a 2-pole, 20 amp breaker in the main panel to feed the garage, and the only action taken so far is to replace it with a 2-pole 30 amp breaker. I did not personally verify that the conductors were #10, but I trust the electrician did.

I told my friend that he would be better off supplying the sub-panel with a 50 amp breaker, just to ensure there is capacity for whatever he wants to put there. Then I told him I would look up the required sizes of conductors and conduit, since I didn't want to go by memory. So I check Table 310.16 this morning, and confirm my guess that a #8 would serve. Then I check Table 250.122, and see that a #10 EGC would serve.

Next, I think about the probable distance of the run. It might be close to, or even more than, 100 feet. So I then think of recommending an up-size, due to voltage drop considerations. That means a #6 conductor. It also means that the EGC would have to be upsized from a #10 to a #8. That is not because of Table 250.122, but rather because of article 250.122(B).

Now the plot sickens ? no I mean thickens. What if I had suggested that the sub-panel be supplied by a 60 amp breaker? (This presumes it is rated at 60 amps, or would be replaced by a panel that is rated at least that high). Now I need to use #6 conductors, but can still use a #10 EGC.

In summary, the choices are:

• #6 conductors, 50 amp breaker, #8 EGC, or
• #6 conductors, 60 amp breaker, #10 EGC

If a bigger breaker is used, you can get by with a smaller EGC. Go figure that one out! :?: :?: :?:

 

[Pierre C Belarge]

Charlie
You are very clever.


You are making this as clear as mud. :wink:


According to table 310.16, 6 AWG is permitted to be supplied by a 60 breaker, regardless of the length of the installation, as voltage drop for standard installations is referenced by an FPN and is not a requirement.

250.122(B) is a requirement though, if upsizing the conductor for any reason, the EGC also gets upsized.


The difference in your case is making sure the overcurrent device opens as quickly as possible.

The 60 amp breaker will have more current available than the 50, providing more "umph" to open the breaker, so.... we increase the size of the EGC to hopefully permit more current flow in the 50 amp circuit to open the overcurrent device.



I hope I explained that well enough??????

 

[fc]

Re: Here's an odd one For you.

Next, I think about the probable distance of the run. It might be close to, or even more than, 100 feet. So I then think of recommending an up-size, due to voltage drop considerations. That means a #6 conductor. It also means that the EGC would have to be upsized from a #10 to a #8. That is not because of Table 250.122, but rather because of article 250.122(B).

Charlie: The first problem I see is your thinking to much! :lol:
Maybe it's only 90' away?? 50amps #8's and #10 sound good.

Good point you bring up. I could not begin to figure that out so I would only hope it's 90' or less away.

 

[infinity]

I've looked at this from a different perspective. Your #10 EGC with the 50 OCPD and the #6 wire is compliant. The reason being that the #10 is already increased in size. 250.122 says that a #10 EGC can be used on a 60 amp circuit, therefore when using it on a 50 amp circuit it already is upsized. Any thoughts?

 

[Pierre C Belarge]

I personnally think that using 10 AWG for a 60 amp circuit may be stretching it to the max. I cannot say for sure, as I have not seen the test results, and I do not know the method of testing that was performed.

There are so many variables that occur in the field, I know it is hard for the people who do this to account for all of the potential situations.

Yet without knowing more than I do, I still think that unders certain circumstances, it is a stretch. Circumstances such as:
Proximity to the service
Length of the installation
Splices/terminations

 

[A/A Fuel GTX]

Re: Here's an odd one For you.

Next, I think about the probable distance of the run. It might be close to, or even more than, 100 feet. So I then think of recommending an up-size, due to voltage drop considerations. That means a #6 conductor. It also means that the EGC would have to be upsized from a #10 to a #8. That is not because of Table 250.122, but rather because of article 250.122(B).

According to my computations, A 50A, 240V feeder with a 5% voltage drop could extend 153' with a #8 and 244' with a #6 so I would think that 250.122 could be taken at face value without any over sizing on the feeders.

 

[charlie b]

250.122 says that a #10 EGC can be used on a 60 amp circuit, therefore when using it on a 50 amp circuit it already is upsized. Any thoughts?

I had considered that. But the rule about upsizing the EGC says to base the new size on the ratio of the "biggie size" conductor to the "normal size" conductor. You don't upsize based on the relative positions in Table 250.122.

 

[George Stolz]

The 60 amp breaker will have more current available than the 50, providing more "umph" to open the breaker, so.... we increase the size of the EGC to hopefully permit more current flow in the 50 amp circuit to open the overcurrent device.

Assuming a bolted fault, the 50A/60A aspect of the design becomes moot, doesn't it? I think their instantaneous trip values are roughly the same, aren't they?

I would go with the 50A (6 CU NM-B through the house, 4 AL URD underground) feeder not sized for voltage drop, as the run is close to the "concern line" (for me, 100' is the beginning of concern territory), and the #10 EGC. From the workshop I visualize (residential use) the load will probably be far lower than 50A anyway, so VD wouldn't scare me too bad in this case.

Depending on the whim of the moment, I might exercise "option (B)(2)" and forget the EGC altogether. :shock:

Back on topic: I have to say, these little oddities in the code are what tend to make some folks look at the NEC in disgust and begin referring to it as "That Stupid Book." :?

 

[dlhoule]

"That Stupid Book."

That is very simply not true.

After making references to a stupid tree, when I was a child and that small branch about 20' up broke and I fell, my grandfather informed me in great detail about how inanimate objects do not have the capacity to be stupid. Now that I am older and maybe wiser I agree with him.

Stupid people may write books and stupid people may try to interpret books but books are not stupid.