Maximum # of NM-B per drilled hole?

[Guest]

I came across this:

Question 8: How many Romex can be placed in a bored
hole?
Answer: Bored (drilled) holes may contain no more than
three (3) Romex cables that do not fill more than
50% of the hole.

My NEC books, handbooks, and cheat sheets have not arrived yet (should be here Thursday). Can somebody direct me to the code section that covers the above statement? Thank you. I am very grateful for your help and I am very appreciative of all your constructive answers. I am not an apprentice. I'm just old, rusty, and somewhat dusty

[ This post was spell checked with: ieSpell prior to posting ]

 

[iwire]

Re: Maximum # of NM-B per drilled hole?

Awwt, I know of no NEC requirement that is close to this, in general terms you can bundle as many NN Cables as you like up to a maximum of 24" in length.

Beyond 24" you would have to start derating the cables just as you would for a pipe job.

That said, do not forget this is a national forum and many places amend the NEC with local rules and codes.

It is very difficult to make a blanket statement and not have someone say that in their area it is different.

Some areas use no NM cable at all for instance, wood framed house would be EMT.

Bob

[ September 02, 2003, 05:08 AM: Message edited by: iwire ]

 

[pierre]

Re: Maximum # of NM-B per drilled hole?

Try to remember that a bored hole through a wood framing member is not considered a raceway. There are some building inspectors who will limit the size of the hole, and rightly so, but there are no specifications in the NEC.
Common sense dictates here!!

Pierre

 

[tom baker]

Re: Maximum # of NM-B per drilled hole?

In washington state we only run three NB cables per bored hole in a stud. Anything more is considered bundling.

 

[george t. everett]

Re: Maximum # of NM-B per drilled hole?

This question was ask in the IAEI mag.(when they had questions & answers) The answer was, "The number of cables and the size of the hole are not restricted by the requirements of section 300-4. Physical size of the hole alludes to only installing that number of cables which will pull through the hole without damaging the cable sheath during installation."
Bob Sallaz, CMP-6

 

[Guest]

Re: Maximum # of NM-B per drilled hole?

I called the plan checker in my town today at Building Inspection and he said:

1" hole: 3.
3/4" hole:2.
5/8" hole: 1.

I did not quarrel with him. I'll wait until I have a hole with more than three. Then I'll throw the book at him. The one without the code in it

I guess he's thinking that in a 3.5" stud or wall plate that a 1" hole is the maximum without a nail plate or notch plate (i.e.- 3.5"-1"?2=1.25" minimum thickness).

 

[iwire]

Re: Maximum # of NM-B per drilled hole?

What code article is he citing?

1" hole: 3.
3/4" hole:2.
5/8" hole: 1.

14/2s 6/3s?

I would still say this is not a NEC rule, what if you make a slot, keeping back the required 1.25" in the top plate to drop the NMs to the panel?

Look to 310.15(B)(2)for the info on bundling.

Tom is that a local code as it seems the NEC is clear that you could bundle NMs up to 24", so if you are just going through one stud (like at the top or bottom of the wall) would you still be limited to 3 NMs

Bob

[ September 03, 2003, 10:36 PM: Message edited by: iwire ]

 

[curt swartz]

Re: Maximum # of NM-B per drilled hole?

awwt,

I believe your in California correct? If so and the inspector says that this is a local code ask them to show you proof that the state has approved this amendment. The state of California must approve any changes to the CEC. Unfortunately they don't spend much time reviewing the requests and just file the paperwork. There is nothing in the NEC or CEC that restricts the number of NM cables per hole. If you keep them bundled tightly between holes you need to derate the cables accordingly. I have been told that San Francisco restricts the number of cables per hole but have never done and residential work there.

P.S. Do you have a first name we can use to address you? Most members here either have their name as part of their profile or sign their messages.

Curt

 

[earlydean]

Re: Maximum # of NM-B per drilled hole?

I am a master electrician, an electrical inspector, and a building official, certified with BOCA (now ICC) as a Master Code Official. I have responded to this question this way: Cables in bored holes must be installed such that the size of the hole is sufficient to allow installation of the cables without damage to the cables. If the cables are installed in bored holes further than 24", then this is "bundling" and the derating rules of 310.15(B)(2)(a) come into force.
Two cables contain 4 current carrying conductors, Three cables contain six current carrying conductors, and four cables contain 8 current carrying conductors. What about three conductor cables? If the cable is used for power, the neutral is not current carrying. If the cable is used for three way switches, only two wires are used at any one time.
Article 334.112 tells us that "Romex" must be 90 degree wire, and 334.90 allows us to use the 90 degree ampacity of Table 310-16 for derating.
The ampacity of #10 is 40 Amps, #12 copper is 30 Amps, and #14 is 25 Amps. Table 310.15(B)(2)(a) allows only 70% of the ampacity for 7-9 current carrying conductors. Seventy percent of 40 is 28 Amps, Seventy percent of 30 is 21 Amps. Seventy percent of 25 is 17.5 Amps.
Article 240.4(D) allows maximum 15, 20 and 30 ampere overcurrent protection for #14, #12 and #10 conductors, and Article 334.90 allows only the 60 degree column to be used for the maximum ampacity, which is 20, 25 and 30 Amps.
Article 240.4(B) allows us to ?bump up? the #10 Romex to 30 Amps, as #10 is not used for a multiwire branch circuit. (usually for a clothes dryer or maybe a range top)
The bottom line is: Up to 4 cables may be pulled in bored holes for distances longer than 2 feet, shorter distances have no limits on the number of cables.

The International Residential Code limits the size of bored holes in lumber. If the lumber is a bearing or exterior stud the largest hole allowed is 25%. In a non-bearing interior stud, 40% is allowed. (Measurements are actual, not trade dimensions. i.e. a 2x4 is 1.5 by 3.5 inches). A ceiling joist or floor joist allows a hole of 33% of the size of the lumber, but holes cannot be bored within 2 inches of the top or bottom. On a stud, holes cannot be bored within 5/8 of an inch from the edge, but Article 300.4 of the NEC requires holes bored within 1-1/4 inch from the edge to have nailer plates

 

[karl riley]

Re: Maximum # of NM-B per drilled hole?

Excellent and complete answer. Print it!

Karl

 

[don_resqcapt19]

Re: Maximum # of NM-B per drilled hole?

earlydean,

If the cables are installed in bored holes further than 24", then this is "bundling" and the derating rules of 310.15(B)(2)(a) come into force.

Is this one continuous hole or do you consider it bundling when you are running through multiple holes that are 16" on center?
Don

 

[earlydean]

Re: Maximum # of NM-B per drilled hole?

It is bundling when we run 2 or more cables through multiple holes through the joists or studs. Joists are typically 16" on center. So if two consecutive joists are bored, then it is not bundling. When three, or more,consecutive joists are bored, we are bundling the cables.

 

[roger]

Re: Maximum # of NM-B per drilled hole?

Can you provide some documentation to back this

When three, or more,consecutive joists are bored, we are bundling the cables.

up, or is this just a personal interpretation?


After all, if for some reason someone decided to take the time to seperate the wires between these studs how could you say they were bundled, and even if they didn't, how close by any standard constitutes bundling?

Roger

[ September 05, 2003, 02:12 PM: Message edited by: roger ]

 

[Guest]

Re: Maximum # of NM-B per drilled hole?

Wow! Thanks for all that!

Here is my summary conclusion from reading all this:

1. The limit without derating is 4 per hole (8-conductors) if not bundled.

QUESTION 1: If the hole is not round does it count as two holes? Would a 1" x 2" double drilled hole (two 1" holes that overlap without separation) count as one or two holes?

QUESTION 2: Is there a hole fill limit? How much slack space does there need to be by code? I know what common sense says. If there is ample slack in the wire on both ends of the hole is it OK by code to cram the hole?

I take this all very seriously and I am a dedicated craftsman.

I am in the San Francisco Bay Area, California.

../Wayne

 

[earlydean]

Re: Maximum # of NM-B per drilled hole?

Roger,

I'll excuse the nitpicking.
Yes, if you took the time to separate the cables between studs or joists it would not be bundling.

Earl

 

[earlydean]

Re: Maximum # of NM-B per drilled hole?

awwt,

if two holes are made, the diameter is measured from one edge to the far edge of the other. if the cables are damaged, the holes are too small. I have never heard of a drill making a square hole.

 

[roger]

Re: Maximum # of NM-B per drilled hole?

Earlydean, I didn't ask you to excuse me, and you didn't provide any documentation or back up to your claim, nor did you answer my first question.

Roger

[ September 05, 2003, 02:54 PM: Message edited by: roger ]

 

[don_resqcapt19]

Re: Maximum # of NM-B per drilled hole?

Earl,
In my opinion, unless the cables are actually bundled by the use of wire ties or a similar method, derating is not required.
Don

 

[earlydean]

Re: Maximum # of NM-B per drilled hole?

Roger,

I apologise for offending you.

NEC Article 310.15(B)(2)(a)Adjustment Factors:

Where the number of current carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are stacked or bundled longer than 600mm (24 in.) without maintaining spacing and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(2)(a).

As a building inspector, it is, and has been, my interpretation (opinion with the force of law)that it is bundling to place NM cables in bored holes (without maintaining spacing) for distances longer than 24 inches. Three joists spaced 16 inches on center total 33.5 inches total distance, and therefore would be the trigger for bundling calculations.

This is just a simple rule of thumb. It has always been possible under Section 310.15(A)(1) to allow an engineer to furnish a report to the building official detailing why he would allow any number of cables to be installed in bored holes.

As far as what is proper spacing to maintain? I know of no NEC code to quote, but the rule of thumb (again opinion)I use is to space cables like the rules for cable tray. Maintain the cables one cable diameter apart. (See Section 392.11(A)(3))

 

[earlydean]

Re: Maximum # of NM-B per drilled hole?

Don,

I disagree. In my opinion, maintaining spacing is physically fastening the cables to keep them separated. The whole purpose of spacing is to allow cooling of the wires. While ty-rapping the cables is certainly bundling, not physically restraining the cables apart is not "maintaining spacing".