On Wed, 20 Nov 2019 16:28:02 -0800 (PST), trader_4
wrote:

On Wednesday, November 20, 2019 at 4:47:03 PM UTC-5, Jim Joyce wrote:
On Wed, 20 Nov 2019 12:53:55 -0800 (PST), trader_4
wrote:

On Wednesday, November 20, 2019 at 2:35:33 AM UTC-5, Jim Joyce wrote:
On Tue, 19 Nov 2019 20:10:58 -0800 (PST), trader_4
wrote:

On Tuesday, November 19, 2019 at 10:33:25 PM UTC-5, Jim Joyce wrote:

6. The house has a 240VAC breaker box on the exterior wall, with no room
for an additional breaker, so I'm planning to buy a small 2-circuit
sub-panel that I'd attach to the house, right next to the existing 240VAC
panel. I would ask the electrician to connect the sub-panel to the existing
240VAC panel.

Q: I assume the electrician can create a new circuit from the 240VAC panel.
Are there any issues with that plan?

Should be OK, depending of course on what other loads are on the existing
panel.

That panel is dedicated to the kitchen range, the clothes dryer, and the
HVAC.



Doh! I hope there is enough capacity. Did you add up all the loads compared to the capacity?

No, I did not. Ugh!

IDK, with the sub-panel full and those major loads already on it,
it doesn't sound too good. I hope it's bigger than 100A.

Dryer 30
HVAC 30
Stove 40 or
Ovens + Cooktop 60....



Load calcs don't usually work that way. If he has resistance heat you
can rack up some numbers pretty fast but the FLA on AC compressors are
nowhere near the breaker you need to support them. Ranges get
calculated at 8 KVA (33a). Dryers typically have a name plate rating
around 5500-5600 VA (22-23a). You also can use a 75% demand factor if
there are 4 or more in the calc.
The 2020 is going to revisit the load calcs in Article 220 but it will
mostly be in commercial. I haven't really seen all the changes tho.



Does the new sub-panel need a plywood backer board? It'll be mounted to the
exterior of the (brick) house.

IDK of any code reqt that say so. I think it depends on what the subpanel
is being used for. If it's in a basement and and romex is going to run
into it, then they use a backer board which gives you a place to staple
the cables to as the enter the panel. But I've seen other sub-panels
outside for pools and such that are just mounted directly to the house
siding and where the wires enter and exit via conduit. That' what you'd
have, with the UF cable going through conduit until it gets underground
or if you use conduit and individual wires. With brick exterior, I'd
probably use a backer board, just to keep it off the brick which can be
damp.


What the code does say is if this is against a masonry wall, the
enclosure needs to be spaced out 1/4" if it is not on a board. (The
AHJ may consider any exterior masonry wall a damp location)

As you say, it makes supporting the cables or conduits a lot easier
with some kind of board.
On outdoor installations the (NEMA) 3R enclosures usually have raised
points where the mounting is to get you that air space so plywood is
not needed. I am not sure plywood will hold up that well outside
anyway. Mounted vertically it would certainly have to have the edges
sealed (paint or something) or it will be de laminating in a year or
two.
I have never seen a rule on color but I have seen some AHJs say they
wanted flame retardant paint. I am not sure where they get that rule.