upsizing ground wires

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Grouch

Grouch

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New York, NY
Hey gang.

I have a question on upsizing ground wires due to voltage drop. If you upsize the phase wires due to voltage drop, the NEC says you have to upsize the ground wires based on the proportion of how much the phase wires were upsized... that part makes sense.

Let's say we have a 60 amp breaker. We can use #8 phase wires (rated for 50 amps, which we can use since we're at less than 800 amps), in which case the ground wire will be a #10. I know a lot of people would use a #6 phase wire (rated for 65 amps), which i would use also. Since a #6 wire, which is rated for 65 amps, exceeds the breaker size by 5 amps, does that mean the #6 is considered to be upsized?... so should the ground wire be a #10 or should it be upsized to a #8?

Thanks.
 
You cannot use a #8 on a 60 amp OCPD so in your example you are not increasing the size of the ungrounded conductors. 50 amps is a standard size so increasing to the next standard size OCPD is not permitted.
 
To answer your question I would say no. The upsize rule has a lot to be desired but you basically start at the wire size that is appropriate for the circuit. So in your case #6 is the proper size so no upsizing is required, IMO.
 
They tried to clean it up a bit in the 2014 by adding the wording "from the minimum size that has sufficient ampacity for the intended installation, wire-type" which would seem to include voltage drop.

(B) Increased in Size. Where ungrounded conductors are increased in size from the minimum size that has sufficient
ampacity for the intended installation, wire-type equipment grounding conductors, where installed, shall be increased in
size proportionately according to the circular mil area of the ungrounded conductors.
Click to expand...
 
If the circuit is for a motor, air conditioner, motor operated appliance... where higher then usual branch circuit overcurrent device is permitted, you often will find a 8 AWG is allowed on a 60, 70 or even 80 amp device, but that is the "normal" selection and not an increase. Of course if 70 or 80 amp device is what is used then the minimum EGC is 8 AWG according to table 250.122.
 
infinity said:
They tried to clean it up a bit in the 2014 by adding the wording "from the minimum size that has sufficient ampacity for the intended installation, wire-type" which would seem to include voltage drop.
Click to expand...
I would not think so. Maximum voltage drop is not mandated by the NEC, so necessary ampacity alone is the number I use as a starting point. If I size up from there for Vd, I upsize the EGC by the same or greater proportion from the minimum size dictated by 250.122.
 
ggunn said:
I would not think so. Maximum voltage drop is not mandated by the NEC, so necessary ampacity alone is the number I use as a starting point. If I size up from there for Vd, I upsize the EGC by the same or greater proportion from the minimum size dictated by 250.122.
Click to expand...


"sufficient ampacity" would seem to include voltage drop compensation.
 
infinity said:
"sufficient ampacity" would seem to include voltage drop compensation.
Click to expand...

To me, "ampacity" means the capacity to carry amperes, without the wire insulation or termination connections exceeding the NEC allowable limits to prevent failure. This is a local phenomena that is independent of the grand total length of the circuit. I will often refer to "the minimum size that has sufficient ampacity for the intended installation" as "the minimum local size". Meaning the minimum wire size that could be NEC compliant in the circuit, if the circuit length weren't significant enough to matter. In otherwords, we don't care if it is 10 ft or 20 ft, it has to be at least this size. But if it is 200 ft, it might need to be larger.

If you have a circuit that has significant voltage drop, possibly 10%, the wire and terminations isn't going to fail because of this. Instead, you will deliver power to the load, much less efficiently. If it is a simple incandescent lighting or resistive heater, there is no safety issue with operating at insufficient voltage. It will simply glow dimmer as an incandescent bulb, or as a heater, it would generate less heat and have to cycle with a higher percentage of on-time.

Some loads, particularly motors, are much more sensitive to voltage being within a manufacturer's tolerance from nominal, and for these loads, it is important to design with proper voltage drop limits. But the failure of the components inside the load equipment, has nothing to do with failure of the wire or terminations. If you were to replace the motor with an incandescent bulb that has the same full load amperes, the circuit will still operate safely, even though the motor otherwise connected to it would fail.
 
Grouch1980 said:
Hey gang.

I have a question on upsizing ground wires due to voltage drop. If you upsize the phase wires due to voltage drop, the NEC says you have to upsize the ground wires based on the proportion of how much the phase wires were upsized... that part makes sense.

Let's say we have a 60 amp breaker. We can use #8 phase wires (rated for 50 amps, which we can use since we're at less than 800 amps), in which case the ground wire will be a #10. I know a lot of people would use a #6 phase wire (rated for 65 amps), which i would use also. Since a #6 wire, which is rated for 65 amps, exceeds the breaker size by 5 amps, does that mean the #6 is considered to be upsized?... so should the ground wire be a #10 or should it be upsized to a #8?

Thanks.
Click to expand...

Unless the complete system is rated for 90C (wire, terminations, and manufactured equipment), which is VERY RARE, then a #8 wire is not sufficient for an application where a 60A OCPD is required. The "next size up rule", 240.4(B), doesn't allow you to use the next standard size breaker, when the ampacity of your wire is an exact match to a standard size breaker.

You would need to exceed 50A in the wire's final ampacity to connect to a 60A breaker. Exactly how much...? I don't really know. If you round to the nearest ampere, it would have to at least be 50.5A worth of wire ampacity. So in your case, a #6 wire is the minimum local size, and therefore not considered upsized.

A more practical example would be as follows, to demonstrate the issue you are probably asking about:

Consider a fully utilized circuit with an 80A breaker. If we didn't know the termination ratings, we'd have to default to 60C ampacity prior to derates. So the default size with our ignorance, is #3 Cu, and #8 Cu would be the EGC. If we did know that both ends of the circuit are rated for 75C, we could use #4 Cu wire with a #8 Cu EGC.

Now suppose that you've already committed to #3 wire, and you show up to the jobsite to see that both ends of the circuit are listed and marked otherwise for 75C. Most modern equipment is listed and labeled otherwise for 75C, so t is a more academic rather than practical rule to default to 60C. Is it now considered "upsized" to connect #3Cu for this circuit, such that we'd need to increase the EGC from #8 to #6?
 
infinity said:
Then what does it address? If I have a motor that won't start because the voltage is so low how does that not factor in voltage drop?
Click to expand...
You said it yourself: voltage and voltage. Ampacity has to do with current, not voltage. Ampacity numbers are there to protect the conductors from overheating when too much current is drawn through them. If your motor is rated at 30A, then when you design your conductors they must be able to safely carry 30A. If they need to be bigger to give the motor the voltage it needs, then you need to make the EGC bigger as well because since the CCC's can now provide more current to a fault, the EGC has to handle more current to clear it.

Interpret it however you like, but be prepared to have a fight on your hands when your inspector sees it differently.
 
infinity said:
Then what does it address? If I have a motor that won't start because the voltage is so low how does that not factor in voltage drop?
Click to expand...
I don't see it as having anything to do with the voltage drop. If you are using a conductor larger than the minimum size that would be permitted by the code for that application, you have increased the conductor size and have triggered the requirements of 250.122(B).
 
Confession

Confession

Taking this all a bit further, I need to confess. A couple of years ago I had a crew out at a remote site running a 20 amp 120 volt circuit a long distance to a small restroom building for a couple of lights and a convenience receptacle. I sent them to run 3 #10 CU but when they were at the site a manager there said that in the winter they would want to operate a space heater as well. Game changer; so I did some math and sent out some #8 to replace the #10. The crew pulled in 2x#8 and 1x#10. Sharp inspector said no, the equipment ground needed to be #8 as well. I did some more math and had them install a 2 circuit load center at the building and changed the 20 amp CB out to a 40 which complied with the NEC, made the inspector happy and saved quite a bit of $$ compared with the cost of re-pulling this particularly complex run of conduits. My point is, the circuit load and circuit dynamics did not change and, one might argue, we made an installation potentially worse with plan B than the non-compliant plan A. Please don't beat me up too badly, I just got started in the trade in 1974 and haven't missed a day on the job since. That must mean I am a "special needs" contractor and deserve a little slack maybe?
 
Carultch said:
Unless the complete system is rated for 90C (wire, terminations, and manufactured equipment), which is VERY RARE, then a #8 wire is not sufficient for an application where a 60A OCPD is required. The "next size up rule", 240.4(B), doesn't allow you to use the next standard size breaker, when the ampacity of your wire is an exact match to a standard size breaker.

You would need to exceed 50A in the wire's final ampacity to connect to a 60A breaker. Exactly how much...? I don't really know. If you round to the nearest ampere, it would have to at least be 50.5A worth of wire ampacity. So in your case, a #6 wire is the minimum local size, and therefore not considered upsized.

A more practical example would be as follows, to demonstrate the issue you are probably asking about:

Consider a fully utilized circuit with an 80A breaker. If we didn't know the termination ratings, we'd have to default to 60C ampacity prior to derates. So the default size with our ignorance, is #3 Cu, and #8 Cu would be the EGC. If we did know that both ends of the circuit are rated for 75C, we could use #4 Cu wire with a #8 Cu EGC.

Now suppose that you've already committed to #3 wire, and you show up to the jobsite to see that both ends of the circuit are listed and marked otherwise for 75C. Most modern equipment is listed and labeled otherwise for 75C, so t is a more academic rather than practical rule to default to 60C. Is it now considered "upsized" to connect #3Cu for this circuit, such that we'd need to increase the EGC from #8 to #6?
Click to expand...


Wow... thanks everyone for the responses. I saw the first 5 responses and meant to fix my question, just didn't have time to sit down... yes, if you have a #8 wire (at 75 deg C), you have to use the standard size of a 50 amp breaker. A 60 amp breaker is not allowed. Sorry about that! Let me use different numbers, one's that work: (ahem, allow me to try again :angel:)

I can use a #6 wire (copper, at 75 deg C), which is rated for 65 amps, on a 70 amp breaker... my ground wire would be a #8.
Most engineers i know, including me, would use a #4 wire (at 75 deg C), which is rated for 85 amps, on this 70 amp breaker. Now.. can my ground wire still be a #8, or does it have to be upsized to a #6? (since the 85 amp rating of the wire is much greater than the 70 amp breaker)

I saw infinity's comment that mentioned "Where ungrounded conductors are increased in size from the minimum size that has sufficient
ampacity for the intended installation, wire-type equipment grounding conductors, where installed, shall be increased in
size proportionately according to the circular mil area of the ungrounded conductors."

So for my example above, is a #6 wire the minimum size that has sufficient ampacity? (since the code allows this for anything less than 800 amps), or does the #4 have sufficient ampacity?

Thanks all again.
 
don_resqcapt19 said:
I don't see it as having anything to do with the voltage drop. If you are using a conductor larger than the minimum size that would be permitted by the code for that application, you have increased the conductor size and have triggered the requirements of 250.122(B).
Click to expand...

I've been rethinking this and you're correct. I was thinking of the word capacity instead of ampacity. In the example of a motor and a large voltage drop you could still have a code complaint conductor size from an ampacity perspective but it may not have the capacity to allow the motor to start.

So from this list which ones would require an increase in size of the EGC according to 250.122(B)?

1-Voltage drop compensation
2-Derating compensation
3-Just had larger conductors one the truck
 
NEC still needs work in this area. Last change is better then what was there before but still leaves us with situations that don't really make much sense.

The real issue is making sure the EGC has low enough impedance to allow enough current to flow to facilitate operation of OCPD. The other reality is different OCPD's have different time/current trip curves. This makes it a little more difficult to make a one size fits all requirement.
 
infinity said:
I've been rethinking this and you're correct. I was thinking of the word capacity instead of ampacity. In the example of a motor and a large voltage drop you could still have a code complaint conductor size from an ampacity perspective but it may not have the capacity to allow the motor to start.

So from this list which ones would require an increase in size of the EGC according to 250.122(B)?

1-Voltage drop compensation
2-Derating compensation
3-Just had larger conductors on the truck
Click to expand...

IMO:
1 - yes
2 - no (derating does affect CCC ampacity)
3- yes
 
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