Exceeding equipment ratings

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Pitt123

Senior Member
I need to supply temporary 480V power to a load which is rated at 120A. The load will be connected to a disconnect.

All I have avaliable at the moment is an 125A breaker in MCC to protect cable, and an 100A disconnect. #2 AWG cable is going to be used, and most of run is in free air. Even if part of the run is in an MCC and room can we use the free air rating for this cable?

Even if temporary would you use 100A disconnect for a known load of 120A thinking that it may not draw that much?

Is 125A breaker large enough for this load? This load is continuous, so shouldnt we size breker and cable for 125% of load rating? Even if temporary?
 
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Pitt123

Senior Member
Cold Fusion said:
I got lost right here.

And here:

The equipment has a known, continuous draw of 120A. Now, explain why one might think that the equipment would not draw that much - cause I'm not getting that.

cf
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Basically we are connecting (4) temporary blowers that are rated at 30A each for a total of 120A. In looking at this I would take this 120A x 1.25% and come up with 150A which equipment and cable would need to be rated at for continuous load.

Although these equipment ratings have been given to us, when we actually run the equipment each unit may not draw the full 30A, and therefore the combined equipment may not see the combined full 120A load. Since this is a temporary installation can we make assumptions that equipment may not draw full rated current, or must we size everything with rated full load x 1.25%?

We have a 125A breaker avaliable but to be code compliant I would think that we need to use a 150A breaker to account for 1.25%.

Also I know #2AWG cable is rated at 115A and would be too small for the calculated 150A however since this is a tray cable that is going to be installed temporarily in free air can I use the free air rating of 170A for this cable?
 
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benaround

Senior Member
Location
Arizona
Pitt123,

No where in the NEC does it state that temporary wiring can be less than code compliant.

Read Article 590, it is only two pages. You are really stressing that this is temporary

wiring, but, unless modified by Article 590, it has to be installed per regular NEC requirements.

If someone was to get injured for any reason and you installed this feeder as outlined, you

and the company would not like the outcome.
 
C

Cold Fusion

Senior Member
Location
way north
Pitt123 said:
...Although these equipment ratings have been given to us, when we actually run the equipment each unit may not draw the full 30A, and therefore the combined equipment may not see the combined full 120A load. ...
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First I would ask:
How is it you know the equipment will draw less than 30A? Unless you have test data, prior experience, or mfg information, then the only thing you have to go on is the nameplate data.

As power tech said, the proposed install is marginal, so the first question I would be asking is, "How important is it that these blowers stay running?" Cause if you are even thinking about putting it in marginal, then you shouldn't be surprised if it trips.

Maybe you can get the customer to rotate through the blowers alyways keeping one shut off.

Or if it is important to keep all the blowers running all the time - then get started pricing out a new feeder.

cf
 
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Cold Fusion

Senior Member
Location
way north
Pitt123 said:
...Also I know #2AWG cable is rated at 115A and would be too small for the calculated 150A however since this is a tray cable that is going to be installed temporarily in free air can I use the free air rating of 170A for this cable?
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Hummm - sent this post a bit ago, but I don't see it.

So again:

Cable tray is covered by NEC (2008) 392. So the #2 has to be multiconductor. Ampacities are covered by NEC 392.11. That pretty much says you use NEC 310.16. You are likely stuck with the 75C column.

cf
 
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Pitt123

Senior Member
Cold Fusion said:
Hummm - sent this post a bit ago, but I don't see it.

So again:

Cable tray is covered by NEC (2008) 392. So the #2 has to be multiconductor. Ampacities are covered by NEC 392.11. That pretty much says you use NEC 310.16. You are likely stuck with the 75C column.

cf
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Ok Thanks.

What about derating the values in 310.16?

For instance if part of the run is underground do I need to still derate even though the other half is above ground?

What if part of the cable is in a raceway and the other part is in free air? Can I use the free air ratings then?

Does the whole length of the cable have to meed a certain location in order for that particular rating to be used or is just part of the lengh exposed to a certain condition applicable?
 
mcclary's electrical

mcclary's electrical

Senior Member
Location
VA
Pitt123 said:
Ok Thanks.

What about derating the values in 310.16?

For instance if part of the run is underground do I need to still derate even though the other half is above ground?

What if part of the cable is in a raceway and the other part is in free air? Can I use the free air ratings then?

Does the whole length of the cable have to meed a certain location in order for that particular rating to be used or is just part of the lengh exposed to a certain condition applicable?
Click to expand...



By your logic, you could run a conduit 100 feet,,,,,the last 2 feet inside the panel are in free air,,,,,can i use that column? NO
 
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Cold Fusion

Senior Member
Location
way north
Pitt -
I don't mind helping and I promise I am not trying to be demeaning, but you are going to have to read some of the sections in the NEC

What about derating the values in 310.16?

I'm not sure what you are asking here. Maybe see 310.15.B.2 and exceptions. But these all limit you to less current than T310.16

For instance if part of the run is underground do I need to still derate even though the other half is above ground?

What table/section are you looking at to specify the underground ampacity? What table/section are you looking at to specify the aboveground ampacity? Generally 310.16 covers both - but if not, then maybe 310.15.A.2. Read that and see if that is what you are asking.

What if part of the cable is in a raceway and the other part is in free air? Can I use the free air ratings then?

If by "free air" you are applying T310.17 or T310.20, then note the heading at the top of tables. They apply to single conductors only, not three conductor cables. And since you are using cable tray for #2 wire, then you are not using single conductors. Read 392.

Does the whole length of the cable have to meed a certain location in order for that particular rating to be used or is just part of the lengh exposed to a certain condition applicable?

That would be covered by 310.15.A.2 and exception


cf
 
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iwire

iwire

Moderator
Staff member
Location
Massachusetts
Call me a wuss but with my hard earned license in the balance someone would be buying the correct sized conductor if they wanted me to do it.

Say you can find enough loopholes to use this smaller conductor and there is to much voltage drop for the loads to operate correctly? Or worse something heats up and lets go?

Hard to look like a pro when stuff does not work.
 
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Cold Fusion

Senior Member
Location
way north
iwire said:
...Say you can find enough loopholes to use this smaller conductor ....
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"loopholes" ?? I never thought that was a good word in this business.

Are you saying that meeting the NEC minimums is making use of "loopholes"?

Personally, NEC minimums is not a place I like to be. For the business I'm in there is generally no money in it. But for a short term, highly monitored, temp power issue, as long as I was inside of the damage curves, appropriate for the circuit criticality, and the ahj didn't get their shorts in a knot - I'd be okay.

cf
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Cold Fusion said:
"loopholes" ?? I never thought that was a good word in this business.

Are you saying that meeting the NEC minimums is making use of "loopholes"?
Click to expand...

:grin:

At times yes, I am saying that the NEC is a large document and you can make it work for you if your cleaver. But cutting down on conductor sizes is not a place I like to stretch and manipulate the NEC.

The fact that it is temp would only strengthen my resolve to use (for lack of a better term) 'full size' conductors as temps often get added to and the time frame extended.
 
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lakee911

Senior Member
Location
Columbus, OH
Like I have said before, the NEC is not a design guide. It is a set of minimum standards. You need to design for the application and not try to find out just what you can get away with.


Jason
 
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