Derating Quandary

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Jon456

Senior Member
Location
Colorado
I am installing new wiring in EMT conduit for a garage workshop. The workshop needs power for the following circuits:

1. Welder/plasma cutter (230V / 30A max)
2. Air compressor (230V / 15A)
3. Mill/Lathe (230V / 15A max)
4. GFCI outlets (115V / 20A max)
5. Isolated ground circuit for PC/DRO (115V / 15A max)

I was designing the branch circuits as follows (to match the loads above):

1. 30A breaker; 2 #10 hots + 1 #10 ECG
2. 20A breaker; 2 #12 hots (EMT for ECG)
3. 20A breaker; 2 #12 hots (EMT for ECG)
4. 15A breaker; 1 #12 hot + 1 #12 neutral (EMT for ECG)
5. 15A breaker; 1 #14 hot + 1 #14 neutral + 1 #14 ECG

My fill calculations specify a 3/4" EMT raceway @40% fill (downsized to 1/2" EMT as the branches split off).

But my problem comes to derating as it would appear that I have 10 current-carrying conductors. At 9 conductors (70% derating) I would be fine. But at 10 conductors (50% derating), everything is too small. Of course, I'm trying to avoid up-sizing everything.

If it helps, there would never be a time when all circuits are in use simultaneously. For example, the welder circuit would never be in use at the same time as the mill/lathe circuit. Also, most of the circuits will draw less current than I've specified. I've sized the branch circuit ampacities to allow for in-rush current and possible future increases (which is why I specified "max" on all the circuits except the air compressor which actually is rated at 15A).

Any recommendations or suggestions to save me?
 

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
I missed the IG part, unless speced, why would an IG cir require a dedicated grounded conductor?
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Also, most of the circuits will draw less current than I've specified.
In terms of derating for the number of current-carrying conductors, this does not change anything.
If it helps, there would never be a time when all circuits are in use simultaneously. For example, the welder circuit would never be in use at the same time as the mill/lathe circuit.
That, however, changes everything. But if I were the inspector, or even the design engineer, I would make you prove it. If, for example, you installed some type of interlock that absolutely precluded simultaneous operation of these two items, then you would not need to count both sets of conductors in the equation. But if all you can say is that these types of machines are not normally used at the same time, or that a single person could not operate both, you can't use this to help you. I would require proof that they could not both be drawing current, any amount of current, at the same time.


 

Jon456

Senior Member
Location
Colorado
Use one grounded conductor for items 4 & 5. Why 2 EGC's?

Is it code-compliant to share a neutral like that for two separate branch circuits? And if so, how will that affect the isolated ground circuit? Obviously I don't want spikes, surges, and electrical noise from power tools damaging the electronics on the isolated ground circuit.

There are actually 3 ECGs: the #10 for the welder/plasma cutter; the #14 for the isolated ground; and the EMT conduit for the rest of the branch circuits. The isolated ground circuit, by definition, requires a separate ECG for ground path isolation. The welder/plasma cutter ECG is somewhat redundant to the conduit, but considering that the ground is an important part of the operation of that machine and a poor ground can create lots of high-frequency noise, I felt it would be worth the expense of having that extra dedicated ECG.

Besides, the ECGs do not contribute to derating calculations.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Understood, asking why an IG cir can't be part of a MWBC.
Good point. I misunderstood what you were trying to say. If you are suggesting that circuit #2 share a neutral with circuit #3, and that circuit #4 share a neutral with circuit #5, that would reduce the total CCCs to 8. I am not sure about a #12 conductor and a #14 conductor sharing a neutral, however. I also don't know why circuit #4 has a 15 amp breaker protecting #12 conductors.
 

ike5547

Senior Member
Location
Chico, CA
Occupation
Electrician
Good point. I misunderstood what you were trying to say. If you are suggesting that circuit #2 share a neutral with circuit #3, and that circuit #4 share a neutral with circuit #5, that would reduce the total CCCs to 8. I am not sure about a #12 conductor and a #14 conductor sharing a neutral, however. I also don't know why circuit #4 has a 15 amp breaker protecting #12 conductors.

Aren't ckts. 2 & 3 240v each? Ckts. 4 & 5 should be okay with a #12 neutral.

And the EMT can replace the #10 ECG.

I've never heard there is a problem with IGs used with multiwire ckts.
 

Jon456

Senior Member
Location
Colorado
If you are suggesting that circuit #2 share a neutral with circuit #3, and that circuit #4 share a neutral with circuit #5, that would reduce the total CCCs to 8.
Circuit #2 and #3 are 230VAC so no neutrals to share.

I also don't know why circuit #4 has a 15 amp breaker protecting #12 conductors.
Sorry, this was an error in my original post. Circuit #4 should have been listed as a 20A breaker. Only circuit #5 is supposed to be a 15A breaker.
 

Jon456

Senior Member
Location
Colorado
I missed that circuit #4 is on a 15 amp breaker, so I don't know why there is a #12 in the mix.
Sorry, this was an error in my original post. Circuit #4 should have been listed as a 20A breaker. Only circuit #5 is supposed to be a 15A breaker.
 

Jon456

Senior Member
Location
Colorado
Cir 4 20A 12 wire, cir 5 15A 14 wire, shared neutral 12 wire. Golden.
Questions:
1. What effect would the shared neutral have on the "clean power" needs of the isolated ground circuit? Realize that circuit #5 (isolated ground) is for a computer system and digital read-outs (DROs) for the mill & lathe. Whereas circuit #4 is for general utility outlets and might have power tools or other motors plugged into them.

2. In order to share a neutral, wouldn't these need to be a multiwire branch circuit? That wont work for a number of reasons, including the fact that I wouldn't want a breaker trip on circuit #4 to also de-energize circuit #5.
 

Smart $

Esteemed Member
Location
Ohio
Yeah, if I had to. I was hoping I wouldn't have to. :(
What's the problem? :roll:

derating.gif


Once you get to 6 or less ccc's in the conduit with the 30A circuit, you can drop down to #10.
 
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