Derating Quandary

[chris kennedy]

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.

Question 1 I can't answer. Chances are the manufactures of the DRO's wouldn't be happy with a shared neutral. Question 2, yes that would be rather inconvenient.

One conduit and a sub-panel out of the question?

 

[Smart $]

Questions:
..
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.

.... Question 2, yes that would be rather inconvenient.

Same inconvenience as with a supply outage... there should be an UPS for that purpose.

 

[Jon456]

What's the problem? :roll:

This only works with a shared neutral for circuits #4 and #5, thus bringing the total number of current-carrying conductors to 9. Unfortunately, I can't share the neutrals on those two branches.

Btw, if I only had 9 current-carrying conductors, couldn't I still use #10 wire for the welder circuit?

40A (#10 @ 90?C) x 70% = 28A. Could the breaker be sized to the next largest (30A) or would it have to be sized down (20A)? The question is more academic; I do agree that #8 would be better conductors for this circuit.

I believe the ECG for circuit #1 would only have to be #10, even with #8 current-carrying conductors.

Where are you getting your cross-sectional areas for your wire? For example, the calculator I'm using lists 0.0366 as the wire area for #8 THHN. Using the number and size of conductors in your chart, I calculate a fill of 0.209 (with an allowed fill of 0.213 @40% in 3/4" EMT, thus leaving only 0.004 available).

 

[Jon456]

Question 1 I can't answer. Chances are the manufactures of the DRO's wouldn't be happy with a shared neutral. Question 2, yes that would be rather inconvenient.

Even with a UPS and surge suppression, I would hesitate to share the neutrals on these two circuits. It would really seem to defeat the whole purpose of an isolated ground, and I think it would stress the surge suppression to have a lot of motor noise on that shared neutral.

One conduit and a sub-panel out of the question?

There is already a sub-panel in the design for the machinery loads. But the isolated ground is coming off the house main panel, again for isolation purposes. I could bring the power for circuit #4 off that same main panel, but I'm still not comfortable with making those two circuits into a multiwire branch circuit.

 

[infinity]

Btw, if I only had 9 current-carrying conductors, couldn't I still use #10 wire for the welder circuit?

40A (#10 @ 90?C) x 70% = 28A. Could the breaker be sized to the next largest (30A) or would it have to be sized down (20A)?

Yes, you could use a 30 amp OCPD on this circuit.

I believe the ECG for circuit #1 would only have to be #10, even with #8 current-carrying conductors.

No, since your upsizing from #10 to #8 you would need to increase the size of the EGC as well. 250.122(B).

 

[ike5547]

Btw, if I only had 9 current-carrying conductors, couldn't I still use #10 wire for the welder circuit?

Unless your system is fed from a Y connected source you would only have 8 CCCs with that shared neutral.

40A (#10 @ 90?C) x 70% = 28A. Could the breaker be sized to the next largest (30A) or would it have to be sized down (20A)? The question is more academic; I do agree that #8 would be better conductors for this circuit.

What's the ampacity according to the name plate?

I believe the ECG for circuit #1 would only have to be #10, even with #8 current-carrying conductors.

I wonder if 250.122(B) comes in to play here?

 

[Cow]

Making a mountain out of a molehill here....

Why not run a dedicated conduit to the closest piece of equipment and then combine the rest of the circuits in the other one? Boom.......you're done.

 

[hardworkingstiff]

Why not run a 30-amp circuit for 2&3 and install a fused disconnect or breaker before the equipment or outlet?

 

[Jon456]

Why not run a dedicated conduit to the closest piece of equipment and then combine the rest of the circuits in the other one? Boom.......you're done.

What I'll probably end up doing is running a second conduit (or possibly MC) just for the isolated ground circuit.

 

[Jon456]

Why not run a 30-amp circuit for 2&3 and install a fused disconnect or breaker before the equipment or outlet?

While there would never be a time when circuit #1 (welder/plasma cutter) is run with circuit #3 (mill/lathe), circuit #2 (air compressor) can be expected to run at any time with the other circuits. Compressed air is required to operate a plasma cutter. And compressed air can also be used for cooling (often in conjuction with liquid coolants) of cutting tools in mill and lathe operations. And the compressor will automatically start any time the tank pressure drops below a pre-set level.

 

[Jon456]

[The fact that circuits #1 and #3 would never be operated simultaneously] 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.

Well, it's unfortunate that "one man/one operation" is not good enough. An interlock is far too complex/expensive for this application.

 

[ike5547]

What I'll probably end up doing is running a second conduit (or possibly MC) just for the isolated ground circuit.

You may already be on top of this, but check out 250.118(10) on the MC.

 

[hardworkingstiff]

What I'll probably end up doing is running a second conduit (or possibly MC) just for the isolated ground circuit.

This would be the best option, but for some reason I thought from the OP that it was not viable.

While there would never be a time when circuit #1 (welder/plasma cutter) is run with circuit #3 (mill/lathe), circuit #2 (air compressor) can be expected to run at any time with the other circuits. Compressed air is required to operate a plasma cutter. And compressed air can also be used for cooling (often in conjuction with liquid coolants) of cutting tools in mill and lathe operations. And the compressor will automatically start any time the tank pressure drops below a pre-set level.

A 30-amp circuit would handle both 2&3 running at the same time. If you are concerned, install a 40-amp (just 10's are easier to work than 8's).

 

[Smart $]

This only works with a shared neutral for circuits #4 and #5, thus bringing the total number of current-carrying conductors to 9. Unfortunately, I can't share the neutrals on those two branches.

I wouldn't worry about it. Most psu's of any delicate electronic equipment are going to have internal line filters. If not, you can add one. And being an IG circuit has nothin to do with "clean" or "dirty" power. The IG is simply for the reduction of RFI.

If you are under the 2008 code, yes you'll have to use a handle tie for the MWBC... so make sure it is very loose but yet will not fall off D), allowing either circuit to trip without tripping the other one.

Btw, if I only had 9 current-carrying conductors, couldn't I still use #10 wire for the welder circuit?

40A (#10 @ 90?C) x 70% = 28A. Could the breaker be sized to the next largest (30A) or would it have to be sized down (20A)? The question is more academic; I do agree that #8 would be better conductors for this circuit.

Not if your numbers are accurate. A max of 30A is more than 28A, and you're only permitted to go down to the total non-continuous load rating on the circuit.

I believe the ECG for circuit #1 would only have to be #10, even with #8 current-carrying conductors.

Nope. As stated, when you upsize due to derating you have to upsize the EGC, too.

Where are you getting your cross-sectional areas for your wire? For example, the calculator I'm using lists 0.0366 as the wire area for #8 THHN. Using the number and size of conductors in your chart, I calculate a fill of 0.209 (with an allowed fill of 0.213 @40% in 3/4" EMT, thus leaving only 0.004 available).

Your right about the CSA of #8... it's 0.0366. Ihad #10 in there and forgot to change it. Yet it still works out... 0.209 is not greater than 0.213 and that's all that matters.

 

[jim dungar]

An interlock is far too complex/expensive for this application.

How about a breaker double throw interlock kit, like a Square D QO2DTI? I think their cost is only a 'few' dollars.

 

[Smart $]

While there would never be a time when circuit #1 (welder/plasma cutter) is run with circuit #3 (mill/lathe), circuit #2 (air compressor) can be expected to run at any time with the other circuits. Compressed air is required to operate a plasma cutter. And compressed air can also be used for cooling (often in conjuction with liquid coolants) of cutting tools in mill and lathe operations. And the compressor will automatically start any time the tank pressure drops below a pre-set level.

Let welder/plasma cutter and mill/lathe be on the same circuits, only install a fused-disconnect for the mill/lathe.

 

[Smart $]

Forgot to add that if you work it such that the supply wires to the fused (20A?) disconnect for the mill/lathe are downsized to #12 and meet the tap rules, the 30A circuit becomes a feeder. Thus 220.60 can be applied (i.e. noncoincident loads).

 

[RB1]

It used to be common practice, before the 1996 Code, to use load diversity for derating puposes. See if the AHJ will allow you to use Table B.310.11 in Annex B. Worst case he says no.

 

[e57]

Not sure why you want to stuff this pipe so full..... Why not make it a 50A feeder to a small panel out there??? Put all of these circuits out of that. Up-sell it and put a CA 50A outlet fed-thru to the bottom for a spider box too....

As I do a bit of welding myself. My garage has my homes panel down there - right out of the bottom of it is a cord whip to a CA 50 cord cap. My welding cart has a panel mounted to it (right on the cart) with a 50' length of cord to feed it. THAT panel feeds my welder itself, and has 2 20a GFI circuits. The whole thing is on wheels I can take it outside - out back - any job site. (If I did it for a living...) Many times you will be wanting to take the welder to the work - rather than the other way around...

 

[Jon456]

Let welder/plasma cutter and mill/lathe be on the same circuits, only install a fused-disconnect for the mill/lathe.

One of the reasons for the separate breakers for the welder/plasma (#1) & mill/lathe (#3) was because my understanding is that 230VAC loads each require their own means of disconnect. Is that correct?

If so, then is a plug and receptacle considered a "disconnect"?

(Note: These are not large industrial machines, so I think a fused-disconnect is overkill. Unless that is a requirement for them to be on the same branch.)