Feeder Cable Installation

[joshnelms]

I'd like to install permanent feeder cable extensions in our building, but I'm not sure the best way to do it. We already have cam loc tails from our disconnect panel, but would like to make up some cam loc extensions to have my disconnects available in another room.

I have some 3" conduit running between the two rooms, but wasn't sure if I'd be OK to run the feeder cable through that conduit. It would be about a 70ft. conduit length.

Does anyone have any useful information on this? I wasn't sure if heat would be an issue with this or not. Also, do I need to use SC cable for this or is there more of an install grade cable that would be better to use?

 

[epimetheus]

I'm gonna go out on a limb here and say this work is easily in the realm of a licensed electrician. Plus, there's most likely issues with using a temporary connection on permanently installed cables.

 

[mstaylor]

There are a ton of variables that we could never answer without seeing it, plus it may violate the TOS for the site. You need to get a local electrician that can inspect your local conditions and recomend the proper size and type to run.
Yes, it is possible to install extensions but it isn't as simple as that.

 

[FMEng]

Portable type cables are not intended for use in a conduit. They will overheat because they are designed for use in free air. I would also recommend consulting with a licensed electrician.

 

[mstaylor]

Portable type cables are not intended for use in a conduit. They will overheat because they are designed for use in free air. I would also recommend consulting with a licensed electrician.

You guys are exactly right and that's what I meant by getting an electrician to spec size and type. I don't think he has the actual wire yet so he needs to he local contractor to give him specs.

 

[mrb]

you absolutely cannot do that. cords may not be used as a replacement for building wiring. Type SC is a cord, not building wire. The ampacity is wrong for use in conduit. and a whole slew of other reasons I cant think of at the moment.

 

[ship]

On the other hand, you can perhaps help run/pull the licenced electrician with the cable run in doing the work to save money and learn lots.

One summer, I designed and plotted out the wiring of the second and third floors in new layout for an old catholic grade school attached to a 1926 German opera house. This design and after the install of the studio theaters on both floors I spent helping the electrician move the old service from the stage to the school part and installing the new service to the stage.

Stuff like running the conduit and helping to pull like a 400' run of 400MCM thru conduit from the stage, over the fly space cored thru the procenium, over the house into the second floor of what was once a church's grade school now rental offices and theater spaces. Great learning experience, bought my pair of ratcheting cable cutters during it I still have and constantly use.

Also learned that the Polish electricians the general contractor hired for the job could neither read a blue print, nore knew much about electrical wiring. Their way of shutting off a circuit breaker was to cause a short, that and beyond every outlet for one side of the school was run off the same circuit breakers. Two circuit breakers because there was no neutral used... So much for my calculated balanced loading and distribution, in the end it was all about just getting enough circuits available for photo copiers etc. that the renters would be needing and the panel was a mess by the time it was done. Polish electricians... jokes asside in me being Polish, they were not just a joke but dangerous.

Side note, the General Contractor for the building side of the renovation was different than the person bringing in a new service to the stage and moving the old power to the school's main panel. What the above Polish electricians did with the power after the main panel was the only problem in the install other than at times late at night after the carpenters were done having to move walls also at times. Blue print verses I feel like putting this wall here...

 

[joshnelms]

Thanks for all the advice. I'll have an electrician come and take a look at it. I appreciate the quick response.

 

[STEVETERRY]

Thanks for all the advice. I'll have an electrician come and take a look at it. I appreciate the quick response.

Make sure he sizes the wire considering the neutral as a current-carrying conductor, which is required for non-linear loads like phase-control dimmers.

ST

 

[derekleffew]

Make sure he sizes the wire considering the neutral as a current-carrying conductor, which is required for non-linear loads like phase-control dimmers.

Try as you might, there still seems to be some confusion on this. Given a permanently installed system, in conduit, should the grounded conductor be sized:
a) the same as
b) 130% greater than
c) 150% greater than
d) 200% greater than
any one of the hot legs (per the NEC)?

 

[mrb]

there are actually two points to consider. First, is the neutral is a CCC since the load is unbalanced (3ph wye). Now you have 4 CCC (considering 3ph wye) in the conduit and need to derate since you have more than 3 CCC. Sometimes the wire size doesnt change since you use the 90deg column for your ampacity for derating, and 75deg for the actual current youre putting through the wire.

Second is the issue of neutral harmonics and overheating due to non linear load. This is up to the engineer to specify: ive seen no oversizing, to 200%. Does the transformer supplying the dimmers have a high K factor? Is the intermediate switchgear 200% neutral rated?

Where power, space, and building design permits, its nice to see 480V supply, 480D-208Y120 high K rated transformer next to the dimmer racks, and 200% neutral to the dimmer racks.

Where the building supply is 120/208Y and you have a long way to go from the service to the dimmer racks, it is good to place a 208D-208Y120 transformer at the dimmer racks, this way you only need to supply it with 3 hot conductors, and the neutral harmonics are circulated in this transformer and kept out of the rest of the electrical system.

As always, theory only -consult a licensed professional.

 

[STEVETERRY]

there are actually two points to consider. First, is the neutral is a CCC since the load is unbalanced (3ph wye). Now you have 4 CCC (considering 3ph wye) in the conduit and need to derate since you have more than 3 CCC. Sometimes the wire size doesnt change since you use the 90deg column for your ampacity for derating, and 75deg for the actual current youre putting through the wire.

Second is the issue of neutral harmonics and overheating due to non linear load. This is up to the engineer to specify: ive seen no oversizing, to 200%. Does the transformer supplying the dimmers have a high K factor? Is the intermediate switchgear 200% neutral rated?

Err....a neutral that is simply carrying imbalance current does not have to be considered a current carrying conductor. Thus, for wire-sizing, a 208Y/120V four-wire feeder has only three current-carrying conductors if it is feeding a linear load. For a non-linear load like a phase-control dimming system, it has four current carrying conductors and ampacity of all conductors must be derated to 80% per table 310.15(B)(2)(a).

As to the optional neutral oversizing you describe, it is important to realize that the four wires in the conduit heat as a system, and neutral overcurrent on a phase-control dimming system (typically a maximum 130% of phase current) only occurs when the other three conductors are at significantly less than full load. This is different than the harmonic neutral overcurrent behavior of other devices like VFD's or switch-mode power supplies. With these types of devices, neutral overcurrent can occur with the other conductors at full load.

BTW, another good transformer solution that provides better power quality and more no-load efficiency than a K-rated transformer is the Harmonic Mitigating Transformer or HMT. See Transformer, Harmonic Mitigating in the Glossary for some details.

ST

 

[tjrobb]

310.15(b)(2)(a) doesn't apply to the branch circuits of the dimmers, but DOES apply to the feeders. The demand factor involved for branch circuits can invoke table B.310.11 (back in the annexes). Without this table, electricians can run into a LOT of problems with the sheer number of wires involved.

Thanks to STEVETERRY for help on the clarification.

 

[mrb]

Err....a neutral that is simply carrying imbalance current does not have to be considered a current carrying conductor. Thus, for wire-sizing, a 208Y/120V four-wire feeder has only three current-carrying conductors if it is feeding a linear load. For a non-linear load like a phase-control dimming system, it has four current carrying conductors and ampacity of all conductors must be derated to 80% per table 310.15(B)(2)(a).

As to the optional neutral oversizing you describe, it is important to realize that the four wires in the conduit heat as a system, and neutral overcurrent on a phase-control dimming system (typically a maximum 130% of phase current) only occurs when the other three conductors are at significantly less than full load. This is different than the harmonic neutral overcurrent behavior of other devices like VFD's or switch-mode power supplies. With these types of devices, neutral overcurrent can occur with the other conductors at full load.

BTW, another good transformer solution that provides better power quality and more no-load efficiency than a K-rated transformer is the Harmonic Mitigating Transformer or HMT. See Transformer, Harmonic Mitigating in the Glossary for some details.

ST

You're correct. I mispoke and failed to mention the neutral is a CCC when 50% of the 3p4w load is non linear. The other 1/2 is the neutral in a 3w circuit supplied from a 3p4w service is a CCC but that doesnt apply here.

The bulk of the specifying I have done (200% neutrals and K rated transformers) was to supply loads which consisted entirely of switching power supplies. Admittedly I am a bit rusty when it comes to powering dimmer racks.

Thank you for clarifying this.

 

[STEVETERRY]

310.15(b)(2)(a) doesn't apply to the circuits of the dimmers, however, only the feeders. The demand factor involved invokes table B.310.11 (back in the annexes). Without this table, electricians run into a LOT of problems with the sheer number of wires involved.

Actually, Table 310.15(B)(2)(a) applies to all conductors in all circumstances, both feeders and branch circuits.

However, Annex B provides theatre installations with an option: Table B.310.11 describes an alternate derating for a given number of conductors where 50% diversity is applied. This means all the circuits are only loaded to half of the overcurrent device rating. It also allows up to 85 conductors in a conduit or wireway.

While 50% diversity may be the norm on most theatre branch circuits (see Load Diversity in the Glossary for proof), such diversity is unlikely to exist on feeders, so B310.11 is rarely, if ever, used in feeder applications.

ST

 

[TimMiller]

And people just think you can just stick some pieces of wire through a conduit.........

 

[mrb]

And people just think you can just stick some pieces of wire through a conduit.........

and people wonder why it costs $25,000 to install a 400a 3ph panel....come on! electrical work is easy...brother in law said he would do it for beer if i buy the parts at home depot. its not like this is something hard like plumbing...

 

[STEVETERRY]

and people wonder why it costs $25,000 to install a 400a 3ph panel....come on! electrical work is easy...brother in law said he would do it for beer if i buy the parts at home depot. its not like this is something hard like plumbing...

You would be amazed at how many engineers and contractors need help with NEC issues on entertainment lighting system installations! That is one of the reasons that we keep working on the NEC to clarify these issues.

ST

 

[mrb]

You would be amazed at how many engineers and contractors need help with NEC issues on entertainment light system installations! That is one of the reasons that we keep working on the NEC to clarify these issues.

ST

oh..im not amazed at all. the best are the ones that argue with you about requiring neutral per circuit on the output side of a dimmer rack.

it goes both ways though....had an install (i wasnt part of the job until after this happened and was done) where the audio guys said they need '400amps of power' -what they meant was they needed 400amps worth of 120v, or 20, 20a circuits. The engineers and electricans heard '400amps' and installed a 400 amp main breaker with shunt trip panel (i remember the panel itself being around $12K). I metered the feed to it with the sound system at full volume and got something around 40 amps per leg...

 

[TimMiller]

I have never seen a sound system that pulls 400 amps yet. Lighting systems thats easy.