Multiconductor Cable Ampacity

[charlie b]

Existing facility has plans to replace a 40 HP pump with a 100 HP pump. I am told the conduit from power source to motor controller is 1-1/4”. To me it looks like RGS. There is one multi-conductor cable in the conduit, and I am told it is #4. If I replace the cable with a larger one, how do I figure ampacity? Am I right in believing that the NEC tables don’t apply to other than single conductors? How do I interpret the top of Table 310.15(B)(16): "Not more than three current-carrying conductors in . . . cable. . ."? Do I look to manufacturer data for ampacity? I am looking at one web site that tells me their #2 cable (3 conductor plus ground) has an ampacity of 130, and I need 124 for a 100 HP motor. Their table also tells me that this cable has a cross-sectional area that is below 53% of the area of a 1-1/4” RGS. So that barely satisfies the project requirements.

Voltage drop may complicate matters a bit. But is this a realistic approach to this upgrade project? I strongly suspect that replacement of the 1-1/4” conduit will be physically impossible.

 

[Dennis Alwon]

The not more than 3 current carrying conductor means the ampacities are based on that however if you have 4-6 then you have to de-rate 80%. Table 310.15(B)(3)

You will have to use note 9 from Table 9 for fill

A multiconductor cable, optical fiber cable, or flexible
cord of two or more conductors shall be treated as a
single conductor for calculating percentage conduit or
tubing fill area. For cables that have elliptical cross
sections, the cross-sectional area calculation shall be
based on using the major diameter of the ellipse as a
circle diameter. Assemblies of single insulated conductors
without an overall covering shall not be considered
a cable when determining conduit or tubing fill area.
The conduit or tubing fill for the assemblies shall be
calculated based upon the individual conductors

For fill the cable is considered one conductor but for ampacity you will need to use the Table mentioned for the number of current carrying conductor's in the cable.

 

[charlie b]

The cable will serve one motor. So there will be exactly 3 CCCs. For conduit fill purposes, there is only one cable, so I can use up to 53% of the internal area. If I have to use Table 310.15(B)(16) for ampacity, I will need a #1 (not yet addressing voltage drop). Per the only one manufacturer's data I have looked at so far, their 3/c + G #1 will take up more than 53% of the interior area of a 1-1/4" RGS. So I am looking for alternatives. Is there a reason I cannot use the manufacturer's published ampacity values?

Signed,
Desperately hoping.

 

[Dennis Alwon]

You are asking if the manufacturer's ampacity supercedes the NEC? Idk

Have you looked at Annex B and see how those ampacities compare. Since you are an engineer you may be okay using those values

 

[petersonra]

The cable will serve one motor. So there will be exactly 3 CCCs. For conduit fill purposes, there is only one cable, so I can use up to 53% of the internal area. If I have to use Table 310.15(B)(16) for ampacity, I will need a #1 (not yet addressing voltage drop). Per the only one manufacturer's data I have looked at so far, their 3/c + G #1 will take up more than 53% of the interior area of a 1-1/4" RGS. So I am looking for alternatives. Is there a reason I cannot use the manufacturer's published ampacity values?

Signed,
Desperately hoping.

IMO you can't use them because the code requires you to use the tables.

Is there some reason you cannot use (4) THW/THWN conductors instead of a cable?

 

[charlie b]

Annex B might be tricky. There is no dirt nearby. I don't think the tables and formulas related to underground ducts would apply. The "facility" is a dam on the Columbia River. The conduit is likely to be surrounded by concrete (thus my guess that replacement would be impossible). The tables would allow up to 4 #1 AWG THHN conductors in the 1-1/4" RGS. I might be able to pencil in a set of 1/0 with a smaller ground wire.

I haven't completed a VD calculation yet, as I don't know the distances involved. The spillway is about 1000 feet long, and there is a unit substation "somewhat close" to each end. But how far the farthest of these is from the location of the motor controller is something I have asked the locals to find out for me.

 

[MRKN]

You are required to use the NEC tables for determining ampacities, partly so that future projects to replace a failed motor can do so without having to validate whatever particular motor is available at that time.

 

[Julius Right]

I am sorry to disappoint you, but after NEC the 3 * #1 cable with a diameter of 1.09”[0.933 square inch] can not pass through the pipe of 1-1/4”, which for 53% only allows 0.809 square inch. In any case, NEC art.430.22 requires 1.25 times the current as per Table 430-250, that means 155 A -3*2/0 copper [1.26” diameter 1.257 square inch] and you need 2” RMC

 

[Sahib]

I am sorry to disappoint you, but after NEC the 3 * #1 cable with a diameter of 1.09”[0.933 square inch] can not pass through the pipe of 1-1/4”, which for 53% only allows 0.809 square inch. In any case, NEC art.430.22 requires 1.25 times the current as per Table 430-250, that means 155 A -3*2/0 copper [1.26” diameter 1.257 square inch] and you need 2” RMC

What about cable with compact conductors? Any possibility?

 

[JFletcher]

Don't forget that you can use the 90 degree tables if the terminations on both end of the wire are rated for 90 degrees i e Polaris connectors or the like. Also it seems to me that jumping from a 40 horsepower to a 100 horsepower pump maybe a bit suspect or certainly worth reevaluating, especially for a dam.... final thought on the matter is that if the conduit is buried under several feet of concrete, you may get a ampacity rating adjustment upward for a lower ambient temperature.

Eta: why run a ground wire at all? If the existing wire can be removed and new pulled in, the existing RGS is in good enough shape to serve as the EGC. No need to blow precious fill with a wire EGC.

 

[Dennis Alwon]

Why do you have to use a cable?

 

[Sahib]

Why do you have to use a cable?

To preclude the possibility of voltage unbalance, due to inadvertent, unequal separation of individual phase wires,at pump motor input.

 

[petersonra]

To preclude the possibility of voltage unbalance, due to inadvertent, unequal separation of individual phase wires,at pump motor input.

Huh??

 

[petersonra]

Why do you have to use a cable?

See post #5 where I asked the same question. You can get a supply house to twist the conductors into a bundle for you if you want.

 

[kwired]

Is this 1-1/4 RGS actually used as a raceway or is it just a sleeve? Fill requirements don't apply to a sleeve, but are worth some consideration when there is significant length to them and how difficult it may be to get the cable in there.

If 75C terminations are involved a 100 HP 460 volt motor must have 2/0 copper minimum conductor. You didn't say what the voltage is.

My motor slide calculator does show 1 AWG if it were a 575 volt motor, but you did mention you need 124 ampacity which is NEC FLA for 460 volt motor, NEC requires minimum ampacity of 125% of that though in most applications so that puts required conductor ampacity at 155. Adjustments if required can start at the 90 C rating though.

 

[charlie b]

Thanks for the comments. Here are a few responses.

• The motor will run at 480V 3 phase.
• I don't have to use a cable. But it gives me 53% of the internal area of the conduit, as opposed to 40%.
• The RGS runs up from a gutter about 12 inches. It turns left via an elbow thingy (I don't know its name), and continues out of the range of my photo.
• I think I could use the conduit as an EGC.
• The selection of 100 HP is a design choice by the mechanical engineer. I asked him yesterday what the impact might be if I said he had to use a smaller motor. He was not pleased.
• The power source (i.e., the other end of the RGS run) is a very old unit substation. I doubt it would have, or could get, 90C terminations.
• I haven't looked into compact conductors yet.
• I am told the run is closer to 650 feet than 500 feet. I would need a 2/0 just to keep VD below 3% anyway.

 

[charlie b]

Here is another approach: Table 430.22(E). The application is a sump pump, and the on/off cycles are driven by water level. I watched the existing system operate a couple weeks ago. It seemed to me that it took less than 2 minutes for the pump to bring the water level from the "on" setpoint level to the "off" setpoint level. Under worst case conditions, I am made to understand that the pump would cycle on every 11 minutes or so. We are looking at ways to extend that, so that the pump doesn't experience that frequent a start/stop cycle.

With that said, I think I can use the "intermittent duty" 15 minute rated value of 85%. So my minimum ampacity would be 85% of 124, or 106 amps. A #2 would do that job. I would still have to deal with VD issues, but this could resolve the "conduit fill" problem.

Any comments?

 

[petersonra]

Thanks for the comments. Here are a few responses.

• The motor will run at 480V 3 phase.
• I don't have to use a cable. But it gives me 53% of the internal area of the conduit, as opposed to 40%.
• The RGS runs up from a gutter about 12 inches. It turns left via an elbow thingy (I don't know its name), and continues out of the range of my photo.
• I think I could use the conduit as an EGC.
• The selection of 100 HP is a design choice by the mechanical engineer. I asked him yesterday what the impact might be if I said he had to use a smaller motor. He was not pleased.
• The power source (i.e., the other end of the RGS run) is a very old unit substation. I doubt it would have, or could get, 90C terminations.
• I haven't looked into compact conductors yet.
• I am told the run is closer to 650 feet than 500 feet. I would need a 2/0 just to keep VD below 3% anyway.

My handy dandy chart says that a 480V 100 HP motor requires conductors with an ampacity of 124*125% or 155 Amps. That would give you a 2/0. Even if you can use an allowance for lower duty cycle that would only get you to a 1/0 conductor.

A 600V motor only needs 99*125% worth of ampacity or 124 Amps. A #1 wire would do.

No reason you could not splice on some larger conductors before terminating it at the old substation.

 

[charlie b]

With that said, I think I can use the "intermittent duty" 15 minute rated value of 85%. So my minimum ampacity would be 85% of 124, or 106 amps. A #2 would do that job.

Even if you can use an allowance for lower duty cycle that would only get you to a 1/0 conductor.

How's come we are so far apart on this calculation?:?

 

[charlie b]

Aside: Is "RMC" the current and more appropriate name for what I have been calling "RGS"?