sizing SO cable

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SceneryDriver

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I have a machine that requires 56A 3-phase, according to the nameplate. It will be cord-and-plug connected with 100A pin&sleeve connectors, fed from a 100A breaker. This machine will be considered a continuous load, so I multiplied the 56A nameplate by 1.2 to get 67A (thus, the 100A connectors and breaker). Do I size the SO cable for 56A, or 67A? I'd select 2/5 SO and just be done with it, but I have concern about the #2 conductors fitting in the supplied lugs. #4 wire should fit, no problem.

Thanks,

SceneryDriver
 
SceneryDriver said:
I have a machine that requires 56A 3-phase, according to the nameplate. It will be cord-and-plug connected with 100A pin&sleeve connectors, fed from a 100A breaker. This machine will be considered a continuous load, so I multiplied the 56A nameplate by 1.2 to get 67A (thus, the 100A connectors and breaker). Do I size the SO cable for 56A, or 67A? I'd select 2/5 SO and just be done with it, but I have concern about the #2 conductors fitting in the supplied lugs. #4 wire should fit, no problem.

Thanks,

SceneryDriver
Click to expand...

This is approaching a DIY type project given the lack of understanding that your question itself shows.

I don't know where you came up with the idea to multiply the FLA by 1.2. This is not something supported by any electrical code that i have seen. Normally if it is a continuous load the multiplier to get the ampacity of the cord would be 1.25. 56 X 1.25 = 70 Amps.

I think that by 2/5 cord you mean five #2 conductors. I don't know why you would need 5 conductors. Normally the 3 phase conductors and a ground would be adequate, although sometimes you might need a neutral which could account for the 5th wire.

The use of cords is covered in article 400.

T400.5(A)(1) will tell you what the allowable ampacity of SO cord is. For #2 conductors it is 80 Amps. For #3 it is 70 Amps. At least in the 2017 code.

You can make the cord larger than the minimum required.

Regardless of the wire size, you have to protect the wire at its ampacity. Article 240 will tell you what the maximum size circuit breaker or fuse is allowed for the ampacity of the conductors. 240.4 allows you to select the next standard higher rated CB above the conductor ampacity under 800 Amps. T240.6(A) tells you what the standard breaker and fuse ratings are. Since 70 and 80 are standard ratings, you would need to protect #2 SO cord with an 80 A rated device, and #3 SO cord with a 70 A rated device.

Incidentally, the instructions that come with the connector will tell you what the maximum size of the conductors can be. It is not whether they will fit or not. It is whatever the instructions say.

Personally, I might be looking at using a cord other than SO that has higher ampacity ratings at smaller conductor sizes, keeping in mind that many devices are rated for a maximum of 75 deg C terminations so even if the conductor insulation can go higher than that, as a practical matter you may well be limited to the 75 deg C column. The connector instructions should tell you what the temperature limits are. if they don't say, it is 75 deg C.

Please be careful to read the notes at the bottom of the tables. The ampacity ratings are different depending on how many of the conductors in the cord are considered current carrying.
 
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This is approaching a DIY type project given the lack of understanding that your question itself shows.

Do you really believe this, based on the other the other questions and answers I've posted to this site, or is this just a snarky response? I'm not often required to size SO cable for machines this large. I'm also dealing with a control cabinet I didn't design, so I'm working with the size limitations of the existing terminal blocks. Thus, my question.

I don't know where you came up with the idea to multiply the FLA by 1.2. This is not something supported by any electrical code that i have seen. Normally if it is a continuous load the multiplier to get the ampacity of the cord would be 1.25. 56 X 1.25 = 70 Amps.

Forgive me. I left off the "5" when I typed the question, and did the math in my head. 1.25 is of course the multiplier for a continuous load.

I think that by 2/5 cord you mean five #2 conductors. I don't know why you would need 5 conductors. Normally the 3 phase conductors and a ground would be adequate, although sometimes you might need a neutral which could account for the 5th wire.

I do mean (5) #2 wires. And no, you don't know why I need five conductors. I do in fact need the neutral, but it wasn't germane to the conversation, so I left that part out.

The use of cords is covered in article 400.

Yes. That's the table I have open on my desk right now.

T400.5(A)(1) will tell you what the allowable ampacity of SO cord is. For #2 conductors it is 80 Amps. For #3 it is 70 Amps. At least in the 2017 code.

While #2 is oversized and #3 will work, it's proving mighty hard to find #3 SO cable. Thus, my question. Even the great McMaster-Carr doesn't carry #3 SO cable.

You can make the cord larger than the minimum required.

I'm aware of that.

Regardless of the wire size, you have to protect the wire at its ampacity. Article 240 will tell you what the maximum size circuit breaker or fuse is allowed for the ampacity of the conductors. 240.4 allows you to select the next standard higher rated CB above the conductor ampacity under 800 Amps. T240.6(A) tells you what the standard breaker and fuse ratings are. Since 70 and 80 are standard ratings, you would need to protect #2 SO cord with an 80 A rated device, and #3 SO cord with a 70 A rated device.

I'm aware of that. The conductors feeding the receptacle will be #3 THHN, and protected at 100A.

However, I can ensure that the machine will never draw more than 60A, as that's the breaker in the control cabinet. Thus, my question: do I size the SO cable for 56A, or 56A x 1.25 = 70A?

Incidentally, the instructions that come with the connector will tell you what the maximum size of the conductors can be. It is not whether they will fit or not. It is whatever the instructions say.

The connector will take up to #1 conductors. It's rated for 100A. The connector isn't the concern. It's the terminal blocks in the machine. It's a European machine, and they have different rules for sizing conductors. I can swap out the TB's if I have to, but it's not my first choice to go tearing into a brand new machine.

Personally, I might be looking at using a cord other than SO that has higher ampacity ratings at smaller conductor sizes, keeping in mind that many devices are rated for a maximum of 75 deg C terminations so even if the conductor insulation can go higher than that, as a practical matter you may well be limited to the 75 deg C column. The connector instructions should tell you what the temperature limits are. if they don't say, it is 75 deg C.

I'm aware. I'm looking to use SO cable for its flexibility as well as its availability.

Please be careful to read the notes at the bottom of the tables. The ampacity ratings are different depending on how many of the conductors in the cord are considered current carrying.

I'm aware. I'm using 'Column A' ampacity values, as I will have (3) current-carrying conductors. Majority of the load is heater-based, so no need to count the neutral as a CCC; no harmonics and/or non-linear loads.



Thanks,

SceneryDriver
 
Imo it's often unclear how exactly to size conductors and ocpd for many industrial machines. They usually are not like hvac which give you mca and max ocpd. Often it's just a nameplate with some amp rating and you don't know what part of that is motors, if it includes 125% of the largest notor, if the other load are continuous, etc. Probably just going 125% of nameplate is good for conductors, but you still have to decide ocpd size.
 
petersonra said:
article 400 adds requirements for cords. the existing requirements found in chapters 1-3 still apply.
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Sure, but if the equipment is cord and plug connected, then the branch circuit ends at the receptacle. Meaning the cord is not part of the branch circuit. Is there some other requirement for a 125% factor for continuous operation that would still apply to the cord?

Cheers, Wayne
 
If the machine has a 60 amp OCPD, then it makes zero sense to upsize the cord beyond that. I would use SO or Type W cord, rated for 60 amp minimum.

Looks like #4 SO or #8 Type W using the roundup rule.
 
Cow said:
If the machine has a 60 amp OCPD, then it makes zero sense to upsize the cord beyond that. I would use SO or Type W cord, rated for 60 amp minimum.

Looks like #4 SO or #8 Type W using the roundup rule.
Click to expand...

I hadn't considered Type W cable. #8 or #6 Type W looks like the winner, and should easily fit in the lugs provided.

Thanks!


SceneryDriver
 
wwhitney said:
Sure, but if the equipment is cord and plug connected, then the branch circuit ends at the receptacle. Meaning the cord is not part of the branch circuit. Is there some other requirement for a 125% factor for continuous operation that would still apply to the cord?

Cheers, Wayne
Click to expand...

I'm sizing the cable at 125% of nameplate as good, conservative practice, if nothing else. I believe you're correct regarding the branch circuit ending at the receptacle. A few dollars in cable to keep the machine running cooler seems like a good idea.


SceneryDriver
 
wwhitney said:
Does the NEC require the 1.25 factor for continuous loads for cords? I don't see anything in Article 400.

Cheers, Wayne
Click to expand...

petersonra said:
article 400 adds requirements for cords. the existing requirements found in chapters 1-3 still apply.
Click to expand...

Interesting Wayne. I also do not see anything requiring the 1.25 continuous load adder were the cord is not a feeder or branch circuit.
 
SceneryDriver said:
I have a machine that requires 56A 3-phase, according to the nameplate. It will be cord-and-plug connected with 100A pin&sleeve connectors, fed from a 100A breaker. This machine will be considered a continuous load, so I multiplied the 56A nameplate by 1.2 to get 67A (thus, the 100A connectors and breaker). Do I size the SO cable for 56A, or 67A? I'd select 2/5 SO and just be done with it, but I have concern about the #2 conductors fitting in the supplied lugs. #4 wire should fit, no problem.

Thanks,

SceneryDriver
Click to expand...

GeorgeB said:
I've not seen the stranding addressed by anyone; is terminating SO or W an issue in your lugs?
Click to expand...
Are the lugs in question the lugs on the pin and sleeve connectors, or lugs somewhere else? If you are referring to the lugs on the pin and sleeve connectors, they are designed for flexible cable so will have lugs rated for that use. 100A pin and sleeve connectors are usually designed for use with #2 Type W cable, 4 or 5 conductor depending on the application (3 + G or 3 + N + G)
 
Jraef said:
Are the lugs in question the lugs on the pin and sleeve connectors, or lugs somewhere else? If you are referring to the lugs on the pin and sleeve connectors, they are designed for flexible cable so will have lugs rated for that use. 100A pin and sleeve connectors are usually designed for use with #2 Type W cable, 4 or 5 conductor depending on the application (3 + G or 3 + N + G)
Click to expand...

The "lugs" in question are actually the terminal blocks for neutral and ground in the machine, and the line-side terminals on the machine's main CB. They wouldn't have been able to accommodate the #2 wire necessary if I was going to use SO. The #6 in the Type W cable will fit nicely. I will of course ferrule the incoming conductors to deal with the relatively fine stranding of the Type W wires.


SceneryDriver
 
drcampbell said:
Why is the ampacity of Type W cable so much greater than Type SO? They seem pretty similar.
http://www.pittsburghwire.com/pdf/CatalogD13.pdf
http://www.pittsburghwire.com/pdf/CatalogD2.pdf
Click to expand...

Based on the research I've done (thanks to this thread), it appears that Type W has a different insulation; a different type of rubber/plastic. Perhaps the insulation used can cope with higher temps, or at least can dissipate heat better?

Or maybe the Type W cable manufacturers have better lobbyists :lol:



SceneryDriver
 
for Type W cable the insulation is different usually an EPDM (Ethylene Propylene Diene Monomer - 125 degrees C) based material rather THHN (Thermoplastic High Heat-resistant Nylon - 90 degrees C) is actually a Polyvinylchloride (PVC) insulation with a nylon (THHN) jacket.

But there is another reason for the higher current rating... to make W cable more flexible two things are done - first there is a slick inner layer that is not annealed to the other material - this allows the copper to slide inside the insulation. The other is that there are many strands.#6 for 100 amps as a standard has around 250 strands. this is where it gets fun! Due to skin effect and the larger surface area created by many small circles instead of 8 large ones in THHN the copper itself can carry more current.

BTW - Mining cable is even better - better material to resist petroleum-based chemicals as well. and more strands!
 
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