It sounds like the plastic boxes you are looking at are only designed for use with cable. Boxes designed for use with conduit have rounds knockouts.

Metal boxes must be used with metallic cables or conduit.

 

 

Remember Junction boxes have to be accessible. Under the insulation is not acceptable.

Metal boxes are better when you need the strength and volume to make up a ton of wires.
The blank covers fit better than a plastic box. The holes are easily used by adding a romex connector of the right size.
I use plastic where ever there is light duty. I use metal for medium to heavy duty. My welder has a metal box

 

Remember Junction boxes have to be accessible. Under the insulation is not acceptable.

Depends on the insulation. Loose fill is OK, you don't have to damage the building structure or finish nor does the structure or finish close it in. Spray foam, on the other hand, is right out.

 

Another question is now that I’m able to run
Up to the attic. I’m thinking I want a branch for each bedroom (3) and two for the bathroom, all 12/2 with ground and maybe another for the attic and at least one 14/2 w ground for lights to all those rooms.

 

Some of those boxes are for cables only and or old work boxes.

Are you running conduit or cables?

 

I am running cable.
Am I correct in assuming that best practice a branch for each bedroom outlets and and bathroom with a dedicated line to the bathroom fan. I am also assuming that the new LED wafer lights draw so little power that really only one line needs to get up to the attic to light all three rooms, bathroom and attic. By the way the upstairs might only be 850 square feet. We are not talking about a big house here just old.

 

The blank covers fit better on a metal box than a plastic box.

 

I have actually not seen blank covers for the plastic boxes that is why I questioned their use as just junction boxes

 

The cover and the box don't have to be of the same material. But they indeed make both metal and plastic covers. They mount into the same holes (where the device yoke would be attached if their had been a device in the box).

 

If you are saying these runs are from the panel to the attic, I would run a conduit and you can then run what ever wire you need at that time.

 

Conduit would be very difficult. Just getting a 3/4 hole through the sill plate to the basement and the fire block in the attic was a challenge.

 

If it is just a bath fan or fan light it can easily share a lighting circuit. The bathroom receptacle should be a 20 amp circuit.

 

So here is the question a am looking to answer. Why would I choose a large 4x4x2.5 metal box (this is what my brain tells me to use) vs plastic box. The plastic looks like they are all designed for installing switches or outlets and not covers but they are larger and cheaper.

It's the part of your brain that knows boxes are required for good reasons, and metal is better at every single one of those reasons. Versus, the thrifty part of your brain that seeks low price.

But you're absolutely correct. Metal boxes are well optimized to have blank covers. You can "dial the cubic inches" by bumping up to 4-11/16" or 6" metal boxes, or by stacking extension boxes, or by using domed covers or mud rings which add cubic inches.

They're modular like that.

Whereas plastic boxes are highly optimized for rapid assembly-line assembly - wiring 8 houses a week instead of 4. Romex only. Things that can barely be called cable clamps. Device boxes only. Only need to last til closing papers are signed, won't be around for failures or house fires.

I fail to see the charm. Last time I compared my everyday 4x4x1-1/2" drawn steel box (99 cents normally) or Handy-Box same price... vs the blue plastic boxes (99 cents normally) and the orange "wish" boxes (99 cents) I failed to grasp the cost benefit of plastic. (don't blame plastic for "shortage pricing").

I might feel differently if I wired 8 houses a week.

They evolved in completely different directions, and that prejudices each kind of person.

"Plastic people" ignore my 99 cent mainstay box, and look for built in gangs and cable clamps and mounting flanges. Pushing them into a goofy box that is much more expensive.

Conversely, "metal people" seek out this silliness with its silly price, or for a 4x4 with with integral 1-gang mud ring, this wackiness with its wacky price.

So for metal or plastic to make sense, you have to frame-shift into the other's thinking. To enter Romex into a standard metal box, you knock out a KO and bring it in via a cable clamp, which are laughably cheap and do a really good job clamping, actually. Which changes (reduces) the need for staples.

 

IMHO, use metal boxes for exposed interior work always... that includes attics and crawlspaces.

I dislike plastic boxes unless its for outside, or roughing-in an exterior wall... where it makes the VB considerations easier.

 

I would hardly say stapling a cable at 12 inches vs 8 inches helps to reduce the number of staples.

Plastic boxes reduce the need for extra parts like ground screws and cable clamps and do not require grounding saving time. Set them the correct depth and go. Metal boxes need a proper depth plaster ring and make it harder when a DIY cuts the drywall due to the rounded edges. Plastic boxes and NM are about twice as fast to install over metal. Plastic have the built-in nail vs needing screws to secure a metal box or stapling the box to a stud.

 

I would also want metal if I'm going to be installing drywall with a Rotozip. It's too easy to cut into plastic boxes. Covers are easily available for plastic boxes IME.

 

This all make sense. I did have to laugh at the cost comparison. I think that is where the problem occurs. In the past I think cost for boxes ect. was something I never thought about. Copper cable yes when I had a farm with building far apart and a deep well but all the parts never seemed to add up to much. Today nothing is $.99. I am still not using a lot but my little remodel of a porch mud room and bathroom costs hundreds of dollars every time a go to Lowe's. Just running these 12/2 lines from the attic down to the basement and then the panel is going to send me back tonight to pick up anther couple of $150 rolls of romex.

And that ceiling fan Jim was asking about. Well I learned what a too small fan does to walls with this house plus I like a warm bathroom when I am showering. My wife and I are also getting hard of hearing so having leaf blower running up in the ceiling not going to cut it. The unit that went in the downstairs new bath was over $400 but it is quit, moves a lot of air and you can dry your hair under the heater. The dam switch for it was $60 I think.

While we are on the subject of bathroom fans let me run an idea by you guys. I have heating duct work running up to the second floor but that is it. No cold air return vents and not really enough room to run sufficient duck work to pull hot air off the ceilings back down to the basement for the central air conditioning. The system works but you can tell it needs help on a hot summer day. So here is my DIY solution. I can probably get a one run of 4x10 inch duct down the outside of the house where it won't standout to bad or maybe someplace inside but there are already four (one in each room downstairs running up to the second floor. Given that it will be under sized for the needed air flow I was thinking of force feeding the air flow with bathroom ceiling fans in each room. The attic is wide open and running 6 inch ducts to a collection point and then down to the basement is do able. The alternative is a small system in the attic with a compressor unit outside at ground level or min-splits. Both those options are going to be more expensive and probably beyond my DIY ability. Is this idea nuts? Feel free to say so. I don't mind being corrected.

 

I will assume when you say bathroom ceiling fan you actually mean an exhaust fan and not a paddle fan. 6ou do not want to pull the steam and moisture back into the house.

 

The bathroom exhaust fan in the upstairs bathroom vents outside and the room is pretty small. I don't think finding a way to pull a return air back to the air conditioning flow is probably not something I would try for. The occasional operation of the fan is probably enough to get summer heat off the ceiling in that room. Right now when ever you turn on the light in that bathroom the fan comes on. I am thinking someone was on a budget or did not want to put in a separate switch for the fan. What ever the case it needs to change but that can wait. Right now I need re arrange some wire runs so a connection box can be relocated in a craw space above the new bathroom instead of sticking out of the wall with a cover plate. Why do I always seem to do things twice.