AC and Motor EGC

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mbrooke

mbrooke

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When I increase the breaker size for short circuit and ground fault protection in AC unit and motor wiring, do I need to increase the EGC size as well to correspond with table 250.122?
 
mbrooke said:
When I increase the breaker size for short circuit and ground fault protection in AC unit and motor wiring, do I need to increase the EGC size as well to correspond with table 250.122?
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Yes, the EGC needs to be sized per 250.122, though it does not need to be larger than the circuit conductors. See 250.122(D).
 
david luchini said:
Yes, the EGC needs to be sized per 250.122, though it does not need to be larger than the circuit conductors. See 250.122(D).
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Btw, just confirming I got it right, table 250.122 reads:


If I have an OCPD:

15 amps and smaller 14 gauge.
16-20 amps 12 gauge
21-60 amps 10 gauge
61-100amps 8 gauge
101-200amps 6 gauge
201-300amps 4 gauge
301-400amps 3 gauge
401-500amps 2 gauge

I am reading this correct?
 
mbrooke said:
Btw, just confirming I got it right, table 250.122 reads:


If I have an OCPD:

15 amps and smaller 14 gauge.
16-20 amps 12 gauge
21-60 amps 10 gauge
61-100amps 8 gauge
101-200amps 6 gauge
201-300amps 4 gauge
301-400amps 3 gauge
401-500amps 2 gauge

I am reading this correct?
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Yes
 
Where there is more confusion is when you increase conductor size for reasons like voltage drop, then the EGC needs increased in a similar ratio from what it would otherwise have been required to be. This isn't so much a problem for most to figure out with 30 amp and less circuits, but take a conductor with a required minimum ampacity of 34 amps and max overcurrent protection of 60 (could happen with AC units or motor circuits) and increase conductor size because of voltage drop - you may need to increase the EGC size to something larger then 10 AWG even though overcurrent protection is still only 60 amps.
 
kwired said:
Where there is more confusion is when you increase conductor size for reasons like voltage drop, then the EGC needs increased in a similar ratio from what it would otherwise have been required to be. This isn't so much a problem for most to figure out with 30 amp and less circuits, but take a conductor with a required minimum ampacity of 34 amps and max overcurrent protection of 60 (could happen with AC units or motor circuits) and increase conductor size because of voltage drop - you may need to increase the EGC size to something larger then 10 AWG even though overcurrent protection is still only 60 amps.
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I agree, voltage drop is a big one. That use to confuse me all the time until I thought about it from a fault clearing perspective, if the impedance is enough to cause excessive voltage drop then it will limit fault current substantially, not the goal of grounding/bonding.


So with in a scenario with a 51 MCA, and an 80amp breaker, the EGC can remain a #10?
 
mbrooke said:
So with in a scenario with a 51 MCA, and an 80amp breaker, the EGC can remain a #10?
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No, based on T250.122 the minimum EGC on that one will be at least a 8 AWG . Only way it is smaller is if somehow the ungrounded conductors are allowed to be smaller then 8AWG (not happening with a MCA of 51 in this case though) then the EGC never needs to be larger then the ungrounded conductors.
 
mbrooke said:
Ok, but if 6-2 is used? The breaker is over 60amps so in theory 250.122 is violated.
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Cables with larger then 10 AWG can get you into this kind of trouble sometimes if they have reduced EGC within them, so you need to watch out. Even a 6-2 with 10 AWG where size was selected because of voltage drop can get you into trouble even if only on a 20 amp overcurrent device - as the EGC needs to be 6 AWG in that situation.
 
kwired said:
Cables with larger then 10 AWG can get you into this kind of trouble sometimes if they have reduced EGC within them, so you need to watch out. Even a 6-2 with 10 AWG where size was selected because of voltage drop can get you into trouble even if only on a 20 amp overcurrent device - as the EGC needs to be 6 AWG in that situation.
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Correct, but for an AC unit? Let me re-read the section, maybe im just not reading it right.
 
mbrooke said:
Correct, but for an AC unit? Let me re-read the section, maybe im just not reading it right.
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Your typical 6-2 NM cable with ground has a 10 AWG grounding conductor.

I have never run into a single phase AC unit that needed more then a 60 amp breaker, 5 tons seems to be about the largest unit made, if more capacity is needed they usually either go with multiple units or even a multi-compressor unit, but each compressor is usually supplied by an individual circuit, but there may be some larger units out there.

So unless you have a situation where you have increased conductor size for voltage drop - you usually can get away with using 6-2 with a 10 AWG ground for single phase AC units or if you get into a situation where you are permitted to use 6 AWG conductor but need overcurrent protection over 60 amps (mostly will be either for a motor or an AC unit), you will need larger then a 10 AWG EGC and the 6-2 with 10 AWG ground will not meet code.
 
kwired said:
Where there is more confusion is when you increase conductor size for reasons like voltage drop, then the EGC needs increased in a similar ratio from what it would otherwise have been required to be. This isn't so much a problem for most to figure out with 30 amp and less circuits, but take a conductor with a required minimum ampacity of 34 amps and max overcurrent protection of 60 (could happen with AC units or motor circuits) and increase conductor size because of voltage drop - you may need to increase the EGC size to something larger then 10 AWG even though overcurrent protection is still only 60 amps.
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Not sure I'm following this 100%.

If your OCPD can be a max of 60 amps why would you need larger than a #10 egc?
 
Dexie123 said:
Not sure I'm following this 100%.

If your OCPD can be a max of 60 amps why would you need larger than a #10 egc?
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Because of what is written in 250.122(B). - but only if you run larger ungrounded conductors then typically needed, maybe for reasons like to correct 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.
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So for our thread topic here with an AC unit - if the MOCP for the unit is 60 amps - chances are the MCA is going to be less then 60 so we are looking at probably 6AWG being the largest minimum sized conductor needed, but in many cases I bet it can be 8 AWG. 10 AWG EGC is all that T250.122 calls for on the EGC.

NOW, lets say for voltage drop reasons we increase that 6 AWG to 4AWG. This is a condition that makes 250.122(B) kick in. We now need to increase the EGC proportionally in size the same amount we increased the ungrounded conductors. The reason here is to ensure we didn't increase circuit impedance which ultimately will lessen the amount of fault current that does flow and will make it take more time for the overcurrent device to respond to that fault.

It is a silly rule to some extent though. I mean if we don't increase the 6 AWG to 4 AWG we likely have a higher circuit impedance then if we did increase to 4 AWG (which is why we did it to reduce voltage drop in the first place). It makes sense in some situations and doesn't make sense in others, they basically made a one rule fits all situation out of it.

So the 6 AWG to 4 AWG is an increase of 26,240 to 41,740 circular mils of conductor a 1 to 1.59 increase in size. The EGC needs to increase by the same ratio so multiply the circular mils of 10 AWG (10,380) by 1.59 and you get 16,504 circuilar mils needed. 8 AWG is 16510 - barely made it with 6 circular mils to spare or we would have had to go with 6 AWG.
 
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