resizing motor ground

[mjmike]

We have a design where the motor started out as 15HP. The phase conductors were sized for this at 1.25% and the ground per 250.122(A). The supplier is proposing a more efficient motor to do the same job at 10HP. Based on the revised motor size, the phase conductors are now larger than necessary, which is never a problem, but this now effects the ground size. Since the phase conductors are now technically "upsized" the ground would need to be proportionally upsized as well, correct? Originally, there were 3-#4 and 1-#6 gnd. By code for the new motor it would be 3-#6 and 1-#8 gnd. Upsizing #6 to #4 would cause the ground to become a #4. (41740/26240)x(16510) = 26262.4 = #4.

Secondly, if the phase conductors were sized for the OCPD instead of the 1.25% FLC, this would require ground upsizing correct?

 

[Jraef]

Since the phase conductors are now technically "upsized" the ground would need to be proportionally upsized as well, correct?

Read that again and stop over thinking this...

1. The ground was the correct size for the larger conductors in the first place.
2. The load is SMALLER, so what would trigger the need for a LARGER ground?
3. If you want make the argument that because the conductors are larger, someone could possibly put in a larger motor in the future without changing the conductor sizing, go back to #1.

 

[petersonra]

Read that again and stop over thinking this...

1. The ground was the correct size for the larger conductors in the first place.
2. The load is SMALLER, so what would trigger the need for a LARGER ground?
3. If you want make the argument that because the conductors are larger, someone could possibly put in a larger motor in the future without changing the conductor sizing, go back to #1.

take a look at 250.122 (B).

 

[wwhitney]

Upsizing #6 to #4 would cause the ground to become a #4. (41740/26240)x(16510) = 26262.4 = #4.

Shouldn't that be #6? I thought AWG was a geometric series, so if the EGC is 2 AWG sizes smaller, then a proportionately upsized EGC would always be 2 AWG sizes smaller.

Cheers, Wayne

 

[don_resqcapt19]

Read that again and stop over thinking this...

1. The ground was the correct size for the larger conductors in the first place.
2. The load is SMALLER, so what would trigger the need for a LARGER ground?

A smaller OCPD would trigger the need for a larger EGC. 250.122(B)

3. If you want make the argument that because the conductors are larger, someone could possibly put in a larger motor in the future without changing the conductor sizing, go back to #1.

 

[Smart $]

Shouldn't that be #6? I thought AWG was a geometric series, so if the EGC is 2 AWG sizes smaller, then a proportionately upsized EGC would always be 2 AWG sizes smaller.

I proposed a Code change to allow determination by this method where only AWG sizes are involved but it got rejected. The problem with the current proportional method is Code uses commercial cmil sizes which are rounded to 4 significant figures. No one tells you this so when you do a calculation, you don't even round and you end up a with certain combination that results in just a few cmils over the commercial size, and presto, you're next size up... or two in some cases.

 

[don_resqcapt19]

I still think it would work better if Table 250.122 was based on the ungrounded conductor size and not based on the OCPD size.

 

[Ingenieur]

What voltage?
what is the ocpd for the 15 hp?
is it being lowered for the 10 hp?

based on ocpd egc
#6 is 200 amp ocpd
#8 is 100 amp ocpd

assuming 208/3
15 hp 46.2 amp 125 amp ocpd at 250%
10 hp 30.8 amp 75 (100?)

does 250.122.B apply if 250.122.D is used?

?????

 

[mjmike]

What voltage?
what is the ocpd for the 15 hp?
is it being lowered for the 10 hp?

based on ocpd egc
#6 is 200 amp ocpd
#8 is 100 amp ocpd

assuming 208/3
15 hp 46.2 amp 125 amp ocpd at 250%
10 hp 30.8 amp 75 (100?)

does 250.122.B apply if 250.122.D is used?

?????

208V 3-phase
125A
80A
very good point made in your last statement.

 

[mjmike]

Read that again and stop over thinking this...

1. The ground was the correct size for the larger conductors in the first place.
2. The load is SMALLER, so what would trigger the need for a LARGER ground?
3. If you want make the argument that because the conductors are larger, someone could possibly put in a larger motor in the future without changing the conductor sizing, go back to #1.

You are correct, but if you do the math strictly looking at the circuit as a 10HP motor, the phase conductors are larger than needed. Would this require a ground size i8ncrease per 250.122(B)?

 

[mjmike]

All good comments above, but let me try to simplify. If motor conductors are sized per 430.22 which state the conductors "shall have an ampacity of not less than 125%..." and they are changed from the minimum size to a larger size for no reason other than you had extra large wire (or just to match the OCPD at 250%), does the ground need to be increased as well per 250.122(B)?

Since 430.22 indicates "not less than", it is saying you can really use any size you want as long as it is greater than 125%. So is 250.122(B) applicable?

 

[Smart $]

...
Since 430.22 indicates "not less than", it is saying you can really use any size you want as long as it is greater than 125%. So is 250.122(B) applicable?

IMO. yes...

...because all ampacity requirements are stipulated as "not less than...", while 250.122(B)'s "from the minimum size that has sufficient ampacity for the intended installation" counters that stipulation in all cases.

 

[mjmike]

IMO. yes...

...because all ampacity requirements are stipulated as "not less than...", while 250.122(B)'s "from the minimum size that has sufficient ampacity for the intended installation" counters that stipulation in all cases.

Good point, I agree.

 

[wwhitney]

The problem with the current proportional method is Code uses commercial cmil sizes which are rounded to 4 significant figures. No one tells you this so when you do a calculation, you don't even round and you end up a with certain combination that results in just a few cmils over the commercial size.

Sounds like a job for . . . Significant Figures!

Cheers, Wayne

 

[Smart $]

Sounds like a job for . . . Significant Figures!

You can do that out of the gate provided you know commercial cmil sizes are rounded to 4 significant figures, and just know that the result is going to be the same as the relative change in AWG size [numbers], you just have to convince the AHJ it is permitted. :happyyes:

 

[JFletcher]

We have a design where the motor started out as 15HP. The phase conductors were sized for this at 1.25% and the ground per 250.122(A). The supplier is proposing a more efficient motor to do the same job at 10HP. Based on the revised motor size, the phase conductors are now larger than necessary, which is never a problem, but this now effects the ground size. Since the phase conductors are now technically "upsized" the ground would need to be proportionally upsized as well, correct? Originally, there were 3-#4 and 1-#6 gnd. By code for the new motor it would be 3-#6 and 1-#8 gnd. Upsizing #6 to #4 would cause the ground to become a #4. (41740/26240)x(16510) = 26262.4 = #4.

Secondly, if the phase conductors were sized for the OCPD instead of the 1.25% FLC, this would require ground upsizing correct?

The phase conductors are not technically upsized. The motor is downsized. The phase conductors are being left alone.

 

[iwire]

The phase conductors are not technically upsized. The motor is downsized. The phase conductors are being left alone.

I don't see that as making a difference at all.

 

[Ingenieur]

what is the rationale?
since conductor is larger, R is lower, fault current is higher?
so egc must be upsized to handle it

goofy rule

 

[iwire]

goofy rule

Yes.

 

[jtinge]

I don't see that as making a difference at all.

I must be missing something in this logic. If the EGC is appropriately sized for the motor load conductors, then the motor is downsized, the EGC is already adequately sized, even though the phase conductors are now larger than are required for the smaller motor.

If the EGC was sized originally based on the correct size phase conductors for the smaller motor, then the phase conductors were upsized to account for voltage drop, I would agree now that the EGC would have to be upsized also.