Control Circuit Wiring Methods and OCPD Sizing

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Headdog

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(THE BACKGROUND SUMMARY POST )

I have a project that involves controlling some 460VAC-3P motors, some 230VAC-3P motors, some 115VAC electric valves, a 115VAC light circuit, and a low voltage DC lighting circuit all from a common control enclosure. This is for a military training device on a base that will not be inspected for meeting the NEC, but I would like to meet the NEC requirements as much as possible (and hopefully completely).

I am having trouble understanding when, where, and how it is permissible to use circuits with smaller gage conductors (smaller than solid 14 AWG) to "run" and/or "control" devices that need much smaller currents to operate. I also want to use stranded conductors in the control box so the door can be opened and closed for maintenance, troubleshooting, etc. Finally, I need to understand what is permissible (or not permissable) with regards to the main OCPD (in the panelboard for example) and individual circuits "tapping" into that power source (such as a single motorized valve for example) that may only need a fuse.

I can see that there is flexibility in the NEC to use smaller conductors with smaller currents. I just want to be sure I understand how and when to use them properly (as much as possible). However when all is said and done, it has to be a workable, maintainable, system that makes sense (even if it wouldn't pass inspection for a home for example).

I would like to keep this all on a single "thread" (and sub-threads) where I can ask specific questions (and number them) to keep any answers/comments straight. Please try to PRECEDE your comments/answers with this identifying topic number [i.e. Topic 1 -- (answer)]. I will add to this thread as my questions get better defined, and we will see how it goes!

Thank you to everyone in advance for your help!
 
I have a project that involves controlling some 460VAC-3P motors
Click to expand...

Branch Cir Cond - use FLC or Table 430.250. 125% of FLC (430.22A)

Branch Cir OCPD 430.52(C)(1)

Feeder = 125% of the largest and 100% of the rest. 430.24

Feeder OCPD 430.62(A)

you would do the same for the 230VAC motors and their source. i hope my fingers got this right.
 
Headdog said:

I am having trouble understanding when, where, and how it is permissible to use circuits with smaller gage conductors (smaller than solid 14 AWG) to "run" and/or "control" devices that need much smaller currents to operate. I also want to use stranded conductors in the control box so the door can be opened and closed for maintenance, troubleshooting, etc. Finally, I need to understand what is permissible (or not permissable) with regards to the main OCPD (in the panelboard for example) and individual circuits "tapping" into that power source (such as a single motorized valve for example) that may only need a fuse.
Click to expand...

If you check out Table 430.72(B), you'll see the ampere ratings of the conductor sizes for control circuits that are seperately powered and also control circuits that are powered from the motor branch circuit. the motor branch circuit chart has two columns, (B) is for conductors that do not leave the enclosure and column (C) is for control circuits that do leave the enclosure.

Rick
 
Many of the minimum wire sizes in industry were developed to prevent mechanical damage in industrial installations. These requirements are above and beyond the scope of the NEC. These are not legally binding unless your company has officially adopted the NFPA79 standard.

See NFPA79:2007 12.6 Conductor Sizing
http://www.nfpa.org/aboutthecodes/li...ookie_test=1

Generally speaking for industrial use (See standard above for specifics)
Minimum 14 awg for power circuits;
Minimum 16 awg for lighting and control circuits;
Minimum 18 awg for control circuits confined within an enclosure;
Minimum 24 awg for electronic I/O and static control within raceways;
Minimum 26 awg for electronic I/O and static control confined within an enclosure;
Minimum 30 awg for electronic I/O and static control in a jacketed multiconductor cable or cord.
 
Topic 1 -- 75 hp motor circuit

Topic 1 -- 75 hp motor circuit

(These questions may be answered by anyone . . .)

Ok, since you jumped in there I will ask some questions regarding the largest motor (75 hp). This is one of the two underground conduit circuits you discussed with me on a separate thread. The current existing power circuit is for an old 75hp motor. I want to connect to that "feeder/branch" circuit (the power source) inside the existing controller enclosure, and then run it through a new conduit run (about 350') to the new motor installation area.

I was planning on using the existing "feeder/branch" OCPD to protect the new conduit conductors running to the new installation area from an overcurrent condition. After exiting the conduit I plan on connecting them to a simple "disconnecting means" (knife blade type switch) enclosure to be able to disconnect the incoming power (for installation and/or service of the new motor and it's controller). If needed I could also install another OCPD, but I'm thinking it is redundant with the current one (so it is unnecessary). (Q1)What do all of you think?

(Q2) Where is the "feeder" circuit ending and the "branch" circuit beginning (for both the old and the new circuits)? I have been calling the trench circuit a motor control "feeder" but it may just be a long long "branch" circuit. I have always thought of a "branch" circuit as something that is the final circuit (after) the last OCPD. For this case I'm thinking the existing OCPD protects the "feeder" and the circuit from the new motor controller (with integrated OCPD) to the motor is the "branch" circuit.

(Q3) If the service is sources from an "ungrounded Delta secondary" as I suspect, is it OK to NOT run the EGC through the conduit? I can't see what purpose it would have since there will be another EGC circuit in the new motor area. I would think I need to run it (through the conduit) if the source is a "wye" (since it is 4-wire by definition).

(Q4) If the "existing" OCPD for the old pump is not acceptable to protect the new conductors to be installed in the conduit, can I install some "slow-blow" fuses in series with each phase conductor (with the new terminal block) to protect them? I'm afraid the existing OCPD would "compete" with any new circuit breaker installed in the old existing pump enclosure and I don't want nuisance trips that require someone to walk back to this old controller box (if it can be avoided). My first preference is relying on the existing OCPD.

(Q5) Am I required to install an OCPD with the "knife-blade" disconnecting means to protect the conductors after the DM (at the new motor area)? I am thinking I can just run the conductors up to the new motor controller enclosure (containing another intergrated DM, motor overload protection relay, and integrated OCPD). Again the motivation is to avoid multiple nuisance trips. That way only the motor controller's OCPD and/or the switchgear's OCPD would trip.

I don't want to violate the NEC but this is all not very clear to me becuase of the uniqueness of the situation.
 
Q3. Why do you feel that bonding is not required after the 'new splice point'.

Q3. Where did that second ground rod come from ?
 
Topic 1 -- Reply to benaround

Topic 1 -- Reply to benaround

Q3. Why do you feel that bonding is not required after the 'new splice point'.

OK, I'm going to give up on this one and put in the EGC (4th wire). I will bond it to my new grounding system.

I guess I was worried about multiple grounding locations on this electrical system (one possibly at the existing switchgear and my new one). I am used to thinking of avoiding ground loops wherever possible.

I see now in the NEC 2008 "handbook" that Exhibit 250.17 shows multiple buildings with grounded service conductors and EGC all bonded together. Also 250.4(B)(4) mentions the case of two ground faults on different phases as the reason for the EGC as a fault clearing path (instead of the Earth). If one fault occurred on one side of my conduit run and the second one occurred on the other side the EGC would need to be present to clear the fault without relying on the Earth.

I am considering Q3 to be "answered". Thank you.


-------------------------------------------

Q3. Where did that second ground rod come from ?

We will have a new ground rod for our new system, plus two more grounding rods for lightning protection. All will be bonded together at one point and to the structure. In other words it will have a fantastic earth grounding system.
 
Reply to pfalcon

Reply to pfalcon

Yes it looks like many of my concerns are covered by NFPA 79. I did not know it existed. I have been "stressing" on this subject a lot wondering how I use stranded wire.

At quick glance, it looks like once the power enters my enclosure it leaves the jurisdiction of the NEC unless or until it leaves again to power lighting fixtures and/or power sources to other equipment. Meantime, just like in an appliance for example, other wiring methods may be used (as long as they meet the NFPA 79 rules).

I will explore this later on in future posts. Thank you.
 
Topic 1 -- 75 hp motor circuit (update)

Topic 1 -- 75 hp motor circuit (update)

(by the way I recommend using "advanced mode" and "thread mode" to make all these posts fit together in a logic way; Just click on the exact message you want to answer, and hit reply)

Q1 -- (open)

Q2 -- (answered) a quick referal to the feeder/branch circuit definitions confirmed that the branch circuit is after the LAST OCPD. If you have ten OCPD the branch circuit is that last leg. The other are feeders by that definition (including my "trench run").

Q3 -- (answered) I am running the EGC through the conduit (see post to pfalcon)

Q4 -- (open)

Q5 -- (open) now I'm thinking I should just run straight to the new motor controller and use that as the DM & OCPD for the motor. I can't see any reason to provide a redundant DM and/or more OCPDs. Please take a look at the original message question (and the updated figure below).
 
some 230VAC-3P motors, some 115VAC electric valves, a 115VAC light circuit
Click to expand...

assuming a 480/240 3 ph step down xfmr - you may have to multiply any unbalance single phase load by 3 and add to the 240 vac 3 ph load to properly size the xfmr. (one that meets or exceeds).

75 hp mtr @ 480vac = 96A
breaker 200
size 4 starter and
110.4 heater amp

apply volt drop for 350', i think it's 250kcmil ( to get under 3%)* 3 + 1 #6 AWG EGC (bonded to equipment as needed) from the original equip OCPD (if the new and old mtr specs match). 40% fill = 2" ENT. And a service disconnect in site of the motor, unless you meet an allowable exception.

There may be other factors to consider for wire size, i assumed 75C terminals.

if i messed any of this up, someone please correct me.........
 
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Headdog said:
At quick glance, it looks like once the power enters my enclosure it leaves the jurisdiction of the NEC unless or until it leaves again to power lighting fixtures and/or power sources to other equipment.
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Yes, in general the NEC covers the power up to the main disconnect, however there are some NEC specifics added to help the AHJ determine if the feeder to the machine is sized properly and SCCR data.

Please read NFPA-70 Articles:
409 Industrial Control Panels
670 Industrial Machinery
 
Q1: I am not aware of any requirement for additional OCPD simply because you ran through a conduit. Disconnecting means relies on being "in sight of".
Q2:
Q3: This is industrial bonding territory and as such needs to be done correctly. The EGC and a Wye neutral are not necessarily the same conductor. In the industrial environment they are typically separate with the neutral frequently not being run at all. The bonding grid for the controller, the conduit, and the motor area all need to be bonded together.
Q4: See Q1
Q5: as billsnuff referenced: Various NEC Article 430

NFPA79 becomes a legal requirement if officially adopted at your site.
NFPA79 is good practice if not adopted at your site.
NFPA79 supplements NFPA70; it does not supercede it. It adds additional restrictions.

NFPA79 does not take over at the enclosure. It addresses additional items that are not normally found outside of industrial enclosures. NEC Article 430 is an example where the NFPA79 did not see a reason to repeat or modify the NEC.

My apologies for being pedantic here. Some of the other forum members are still struggling with the whole "adoption vs legality" issue regarding the NFPA79.
 
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Topic 2 - 480 VAC 3-ph to 240 VAC Delta 3-ph Transformer with one coil Center Tapped

Topic 2 - 480 VAC 3-ph to 240 VAC Delta 3-ph Transformer with one coil Center Tapped

I have a transformer picked out Schneider (Square D) Part EE45T151HCT that I thought would do the job well (45 kVA 3-phase). I thought it was a delta primary to delta secondary configured transformer (with one of the secondary coils center-tapped).

Now I see that the primary is actually in a "Y" configuration (three phase connections with the other side of each primary coil bonded together). The "common" (neutral?) node is NOT made available for connections. The secondary side of the transformer is in a delta configuration with one secondary coil center-tapped.

So, I am wondering if it really matters that the transformer primary is in a "wye" configuration, and the power source is actually a "delta" source (no neutral conductor). I am thinking I still would hook it up the same way (one phase conductor to each transformer phase input terminal). I would still bond/ground the C.T. connection to my grounding electrode system to get a 240/120 single phase power source (as long as it is small compared to the 240 VAC, 3-phase loads).

Questions:

(1) Can I connect 480VAC delta service into the 480 VAC "wye" transformer primary input? Will it still work correctly and give me a 3-phase delta 240 VAC output (as if it were a 480VAC delta primary configured transformer)? If not, please explain why it will not work, because I am thinking it would be OK (since the "Y" neutral connection is not used). I'm guessing this is an obvious "yes" to those of you in this field, but I need to know now if I am really confused! :smile:

(2) If I have a primary side circuit breaker (to protect the transformer) and then run the secondary side into a "panelboard" containing multiple branch circuits (both 3-phase and single phase 240 VAC loads, each protected by normal circuit breakers), do I need an OVERALL panel board circuit breaker (or is the primary side circuit breaker sufficient)? My guess is the overall (secondary) circuit breaker is NOT needed.
 
pfalcon said:
NFPA79 becomes a legal requirement if officially adopted at your site.
NFPA79 is good practice if not adopted at your site.
NFPA79 supplements NFPA70; it does not supercede it. It adds additional restrictions.
Click to expand...

Thanks for the info pfalcon. I don't think it is "required" in this case but I will try to follow it anyway. -- HD
 
Topic 3 - 480 VAC 3-ph Tapped Circuits and OCPDs

Topic 3 - 480 VAC 3-ph Tapped Circuits and OCPDs

Background info:
Upstream there is a 480 VAC, 3-phase (ungrounded Delta) service with a 225A breaker running to an old motor control enclosure.
In that old enclosure there is a 150A breaker (my new power source).
There will be a long underground conduit run to a "new structure" (three phase wires and the EGC).

(I think everything is good to the new structure; this question concerns the circuit from this point forward)
---------------------------------
  • The new "circuit" consists of a 460 VAC, 15 hp, 3-phase motor load AND the transformer described in "Topic 2".
  • For the purposes of this topic the transformer is a 45 kVA transformer loaded with some other smaller motor loads (i.e. a three phase heat pump 9.5 kVA is the largest load) and other miscellaneous loads (mostly small valve motors, spare outlets, fluorescent lights, LEDs, etc.)
  • I know I need an OCPD for the transformer (not the question here).
  • The 15 hp motor has it's own "turn-key" motor controller (disconnecting means, overload protection, ground fault protection, etc.). However, I am assuming that I will need another OCPD for this "feeder" run (since it is not a branch circuit yet). I just don't really know what size to make it becuase it will be "redundant" with the OCPD inside the "turn-key" controller (in my mind), since that is the only load (question #4 below).
Questions:
(1) If the 15 hp circuit requires the additional OCPD how it is its size calculated and why is it really needed (in addition to the OCPD already in the "turn-key" motor controller system)?
(2) Since the conduit run up to the new structure is protected by the 150A circuit breaker (in the old enclosure), do I need an "overall" OCPD for the two circuits I mentioned above (the 15 hp motor and the transformer) OR do I just need a "disconnecting means"? Is the disconnecting means optional (other than the one in the motor controller)?
(3) If the answer to #2 is that an "overall" OPCD is required (in addition to the 150A breaker), how do I go about figuring out its size?
(4) If the answer to #2 is that I only need (or optionally need) a disconnecting means at the point the "service" enters the "new structure area", could it be a simple switch in a small panelboard (with one circuit breaker protecting the 15 hp pump circuit and another CB protecting the transformer pirmary) or must it be the external lever (safety) type DM?
 
Headdog said:
I am used to thinking of avoiding ground loops wherever possible.
.
Click to expand...

There is normally no current on the "ground" system, so the ground loop is something for a power system that is not a concern. Sometimes we install a single point ground but this for communications systems to avoid a difference in potential.
 
Topic 2 Question 2
NFPA70:2008 450.3 Table 450.3(B)
Secondary protection is not necessarily required. See the code cited.
Secondary protection used must be overall or consist of 6 or fewer circuits. See the table cited.
 
Topic 3
At this point I feel we are going too deep for a discussion forum.
Someone that is familiar with your facility bonding, power runs, etc needs to look at this.
 
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