Basement Subpanel

[Todd.Brock]

I was wondering if you all could take a quick look at my load calcs and make sure that I am on the right page for a basement subpanel. This is about 600 square feet for residential use. My thought was to use #8 THHN and # 10 ground to the sub panel in pvc conduit from box to box. That would allow for 55 amps . According to fill chart, ¾” conduit will suffice for 6 conductors, but 1” will allow for 10. My thought would be a little more room is a good thing?

This is for a typical suburban two story home built in 1999 with 2 car attached garage in Cincinnati Ohio (my next door neighbor). The panel is on the outside wall of the garage. My intent was to use PVC conduit 18” deep against the side /back of the house and then through the band board into the basement. This would be approx. a 60-70 ft run with all the bends, turns, etc .

1. I do not know how to figure for not all the load being used at the same time. How do I assume for partial load, not all receptacles.
2. I would use a 50amp double pole breaker at the main panel and then use a main lug only panel for the sub. What is the rule about the grounding bars with sub panels? They have to be removed?
3. Not entirely sure how I would divide up the load yet into individual circuits. Suggestions?


Load (spec'ed from an internet site)
Minifridge (specs fullsize in case need grows) 1200w/hour
TV/Bigscreen/Entertainment center 650watts
Computer 120watts
10 can lights 6500 watts,
Microwave 1100 watts
Total of 9570watts/240v = 39.8 amps

Figure 6-8 recepticles around the room, etc.

 

[Highbeam]

My thought was to use #8 THHN and # 10 ground to the sub panel in pvc conduit from box to box. That would allow for 55 amps .

This is a trap that you fell into. I can't blame you for getting tricked but the 55 amp rating is for a particular temperature at which your other components such as breakers do not operate.

Breaker the 8 gauge THHN at 50 amps.

I just pulled a 70 foot run of #8s in 1" conduit and it was fine. Not super easy since I had 4 bends but fine. I would not go smaller than 1".

 

[Highbeam]

Load (spec'ed from an internet site)
Minifridge (specs fullsize in case need grows) 1200w/hour
TV/Bigscreen/Entertainment center 650watts
Computer 120watts
10 can lights 6500 watts,
Microwave 1100 watts
Total of 9570watts/240v = 39.8 amps

Figure 6-8 recepticles around the room, etc.

What are you going to put in each of the ten can lights that use 650 watts each. I think you mean 65 watts each. Your fridge will have a startup surge at 1200 for a second but will usually run at 150 watts with an occasional defrost cycle to 500 or so.

 

[Todd.Brock]

Thank you for the feedback!! Yes I meant 650 watts. 10 cans @65 watts each! So that reduces the wattage by 5850. So...9570-5850 =3720 watts. Divided by 240v gives me 15.5 amps. So , could I use a 30 amp panel or stick with 50? I want to right size it with a little room to "grow" before drywall goes up. Meaning, oh I want this or didn't think about that b however, 50 amps seems a bit much??

 

[theoldwizard1]

...I would use a 50amp double pole breaker at the main panel and then use a main lug only panel for the sub. ...

That type of installation (no disconnect in the sub-panel) is only allowed if the main panel is in the same room as the sub panel (preferable within immediate proximity).

So , could I use a 30 amp panel or stick with 50? I want to right size it with a little room to "grow" before drywall goes up.

Sub-panel size (slots) and the "main" breaker in the sub don't really matter. You could put a 100A sub-panel with a 100A input breaker if you wanted (the real purpose of the input breaker is to provide a disconnect so that you can work on that panel "cold"). What matters is the size of the breaker in you main panel and the size of the wire.

According to the Southwire Voltage Drop Calculator, if you only need a maximum of 30A in the sub, you could use #10 THWN Cu in conduit for a 75' run coming off of a 30A breaker.

 

[pattenp]

Close, but no prize..... A main lug can be used with no main disconnect if it is in the same structure/building as the main service panel. There is no NEC provision that the sub-panel without a main disconnect needs to be in the same room or within immediate proximity of the main service panel. Only one main disconnecting means is required for the ungrounded conductors that supply the structure/building.

That type of installation (no disconnect in the sub-panel) is only allowed if the main panel is in the same room as the sub panel (preferable within immediate proximity).
.

 

[Highbeam]

PattenP, how do you like that 55 amp on the #8 THHN? I bet he read the big chart at HD as we discussed a few weeks back.

 

[pattenp]

Yep.. 55A on #8 THHN is wrong. It's 50A @75 deg C or 40A @ 60 deg C. The 55A is @ 90 deg C which is used for derating purposes only.

Edit: I should add there are no 90 deg C rated devices so you are limited to 75 deg C for the wire amp capacity.
And the HD chart does show the #8 THHN as 55A, because the THHN is listed in the 90 deg C column. But that's misleading.

PattenP, how do you like that 55 amp on the #8 THHN? I bet he read the big chart at HD as we discussed a few weeks back.

 

[theoldwizard1]

Close, but no prize..... A main lug can be used with no main disconnect if it is in the same structure/building as the main service panel. There is no NEC provision that the sub-panel without a main disconnect needs to be in the same room or within immediate proximity of the main service panel. Only one main disconnecting means is required for the ungrounded conductors that supply the structure/building.

Thanks for clarifying that !

I'm no "code lawyer".


(Personally, I would do it my way, for the few extra dollars. Much easier to work on the sub having a disconnect right there !!)

 

[Todd.Brock]

I m not sure about the big chart at HD. I googled ampacity chart and came up with Cerro wires chart. It listed THHN in 90degree column @55 amps. I did see the 50 @75 degrees. I don't want to get caught up in 50 or 55 amps. I assume there was a discussion about a chart at HD giving misleading info, or did I miss something?
Based on the anticipated load, what amperage would you all recommend I feed the sub panel with? 30 amps? 40 amps? 50 amps? I would size the wire and breaker accordingly. Thank again!

 

[pattenp]

I'd do at least 50A to the sub-panel. If you use NM-b cable you'll need to use #6 Cu. If you use THHN in conduit, then you can use #8. Use a 50A double pole breaker in the main panel to protect either the #6 NM-b or the #8 THHN feeder.

I m not sure about the big chart at HD. I googled ampacity chart and came up with Cerro wires chart. It listed THHN in 90degree column @55 amps. I did see the 50 @75 degrees. I don't want to get caught up in 50 or 55 amps. I assume there was a discussion about a chart at HD giving misleading info, or did I miss something?
Based on the anticipated load, what amperage would you all recommend I feed the sub panel with? 30 amps? 40 amps? 50 amps? I would size the wire and breaker accordingly. Thank again!

 

[Todd.Brock]

Pattonp, is a 10 ga ground wire acceptable? What would you suggest for the number of circuits? Gut tells me do what makes sense based on layout assuming it doesn't exceed the amp for that individual circuit?

Is 18" deep enough for the trench?

 

[wyliesdiesels]

I wonder how many fires have been started because of ampacities and temperature ratings that are so easily misunderstood by newbees and DIYers?

Pattonp, is a 10 ga ground wire acceptable? What would you suggest for the number of circuits? Gut tells me do what makes sense based on layout assuming it doesn't exceed the amp for that individual circuit?

Is 18" deep enough for the trench?

#10 cu EGC(ground wire) is good upto 60a breaker.

 

[pattenp]

You can have as many circuits that you want. The total load at one time on a circuit is what you need to account for. Just keep outlet and lighting circuits separate.

18 inches is minimum for the depth to the top of the conduit.

Pattonp, is a 10 ga ground wire acceptable? What would you suggest for the number of circuits? Gut tells me do what makes sense based on layout assuming it doesn't exceed the amp for that individual circuit?

Is 18" deep enough for the trench?

 

[Todd.Brock]

Pattenp and Wylie, thanks for the info! Out of curiosity, why is the 90degree column for derating only?

 

[sberry]

Location: Modesto, CA
Posts: 1,180 Default Re: Basement Subpanel

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I wonder how many fires have been started because of ampacities and temperature ratings that are so easily misunderstood by newbees and DIYers?

My bet is very few. The 30A screw in fuse may be another matter????? Depending on minor variables I say the real load here could be served a single 20A circuit as long as you didn't micro and run a vac at same time. If this is a kitchenette you need another appliance circuit.

You wouldn't burn an 8 up with a hundred A breaker on it.

 

[wyliesdiesels]

Pattenp and Wylie, thanks for the info! Out of curiosity, why is the 90degree column for derating only?

Because there isn't any terminals, such as breaker lugs, that are rated for 90* c terminations. Most breakers are dual rated 60*/75* c. But u gotta becareful because there is older breakers which are only 60* rated.

 

[theoldwizard1]

My bet is very few. The 30A screw in fuse may be another matter?????

Oh yeah baby. I inherited a few 20A screw in fuses when I bought my house years ago. I replaced them with 15A, but kept them in a drawer until I finally realized the potential fire hazard by a person unknowingly installing one.

I still have a screw in fuse panel box, but it is all screw in push button beakers !