Three bathroom receptacles...

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Code defines dwelling as place for living including permanent provisions for living, sleeping, cooking, and sanitation. So Optional calculation works for the whole building. Or you could just add the loads from the two separate panels together for a quick answer. I don't think that number is going to be very large.

The 2nd floor calc would be standard using its square feet, its HVAC, one 1500VA laundry circuit, and that sounds like it. No need for 2 small appliance circuits if there is no kitchen up there.

The 1st floor would be standard using its square feet, 1500VA x 2 for the kitchen appliances, any electric ranges, electric water heaters, etc and the HVAC. Don't forget to add the load of any fastened in place item on either floor to its respective calculation or items on their own circuit (e.g. dishwashers, garbage disposals, built in microwaves, car chargers, electric water heater, electric clothes dryer, etc. The required 120V garage stall circuit is yet another circuit with no explicit calculation value. You don't even need to count the square footage of the garage in the dwelling calculation.

I don't know why a 320/400 meter was installed. Most houses that are all electric are fine with a 200A service unless they have a lot of strip heat. Use the MCA rating of the HVAC, not the breaker rating.

It sounds like the upstairs panel will be fed from a 100A breaker in the 8 slot 200A panel. In that case, it is not a feeder tap, it is just a 100A feeder. Could probably be even smaller with what is on that second floor. This puts no length limit on that feeder and no main breaker is required in that subpanel.
 
suemarkp said:
Code defines dwelling as place for living including permanent provisions for living, sleeping, cooking, and sanitation. So Optional calculation works for the whole building. Or you could just add the loads from the two separate panels together for a quick answer. I don't think that number is going to be very large.

The 2nd floor calc would be standard using its square feet, its HVAC, one 1500VA laundry circuit, and that sounds like it. No need for 2 small appliance circuits if there is no kitchen up there.

The 1st floor would be standard using its square feet, 1500VA x 2 for the kitchen appliances, any electric ranges, electric water heaters, etc and the HVAC. Don't forget to add the load of any fastened in place item on either floor to its respective calculation or items on their own circuit (e.g. dishwashers, garbage disposals, built in microwaves, car chargers, electric water heater, electric clothes dryer, etc. The required 120V garage stall circuit is yet another circuit with no explicit calculation value. You don't even need to count the square footage of the garage in the dwelling calculation.

I don't know why a 320/400 meter was installed. Most houses that are all electric are fine with a 200A service unless they have a lot of strip heat. Use the MCA rating of the HVAC, not the breaker rating.

It sounds like the upstairs panel will be fed from a 100A breaker in the 8 slot 200A panel. In that case, it is not a feeder tap, it is just a 100A feeder. Could probably be even smaller with what is on that second floor. This puts no length limit on that feeder and no main breaker is required in that subpanel.
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I like this whole general assesment of the situation. My view of demand and load calcs has changed greatly over the years and I still got to poke myself to use the real math vs the instinct to want to 2x it all.
 
I am using the 320/400 amp service for the upcoming upgrades....a pool with a heater and a car charger.....
BIG THANKS for your detailed explanation...It's not easy to find good info on residential services with more than 1 load center...
Even the codebook does not give good examples to follow...
If you have a link to a good video or book explaining residential services with multiple panels, I"m game..
But I think you did an excellent job in explanation...
Thanx Again, DHSVCS
 
dhsvcs said:
I am using the 320/400 amp service for the upcoming upgrades....a pool with a heater and a car charger.....
BIG THANKS for your detailed explanation...It's not easy to find good info on residential services with more than 1 load center...
Even the codebook does not give good examples to follow...
If you have a link to a good video or book explaining residential services with multiple panels, I"m game..
But I think you did an excellent job in explanation...
Thanx Again, DHSVCS
Click to expand...

A service calculation is one thing while calculations for each panel is another thing. The service encompasses all the panels however a separate calculation can be done on each panel also.

I can feed a sub panel with a 100 amp breaker but the load on the panel may only be 30, 40, 50 ...amps As long as the load isn't over 100 amps it is fine.
 
This is one of those things that common sense and experience sort of kick in with.

You can't apply any feeder or service calculation allowances to this individual branch circuit, it has to be able to handle whatever load might get plugged into it.

What gets plugged in that is typically the max demand item on such circuit? Hair dryers. But how long is the demand? Maybe 15 minutes max.

You possibly could go with 180VA for this receptacle. If you had two or even three receptacles at same bathroom vanity that gives you 540 VA, but those hair dryers are usually 1500 or even 2000 VA. Might even be some up to 2400. But again the fact they only run maybe 15 minutes max kind of messes with the reality of true load and what the system can temporarily tolerate.

Reality on the branch circuit itself is it can handle 2400 VA or at least 1920 for continuous duty.

If you go with the general lighting approach of 3 VA per square foot, and assume receptacles and lights are fed by same circuit or by same panel then you maybe have up to 150 VA for the entire room, many bathrooms are smaller and could only end up being maybe 60 VA for the entire room.

Art 220 really doesn't have any good way of determining actual load for situation like OP has. though the 180 VA or the 3 watts per square foot still kind of show that the average load for "general lighting" or for non dedicated receptacle loads is pretty diverse in most cases. The results of either of those is low VA per outlet yet high enough over a particular area that it will cover you in most instances. Note the laundry and small appliance circuits are typically more heavily loaded and they do need 1500 VA load calculation regardless what actually gets used on them.

Experience has told me you can supply a lot of dwelling space from a 100 amp feed, particularly if it doesn't have much for heating, cooling or other fixed appliances. And the amount of space served has kind of increased with higher efficiency items entering the picture over the years, but does drop some for the simple fact we have so many things these days that we never used to have as well.
 
An approach that may be helpful (if has been stated this way already, I missed it):

- Do a load calc for the whole dwelling unit (can be optional rather than standard). That will entail a list of different loads that enter into the calculation.
- Then for each branch circuit, assign that branch circuit to one of the loads in the above list.
- Now for each panel, look at the branch circuits served, and assign the associated loads to that panel. Then you can do a standard calc for just that panel.

The only wrinkle I see in the above is how to divide the 3 VA/sq ft allowance between the two panels. If there is a strict spatial separation in how the lighting and general receptacle branch circuits are divided between the two panels, then it's straightforward to assign a square footage to each panel.

But if, say, one panel supplied all the general receptacle branch circuits for the whole house, and the other panel supplied all the lighting circuits, then as far as I'm aware there's no guidance on how to divide the 3VA/sq ft between the panels.

Cheers, Wayne
 
wwhitney said:
An approach that may be helpful (if has been stated this way already, I missed it):

- Do a load calc for the whole dwelling unit (can be optional rather than standard). That will entail a list of different loads that enter into the calculation.
- Then for each branch circuit, assign that branch circuit to one of the loads in the above list.
- Now for each panel, look at the branch circuits served, and assign the associated loads to that panel. Then you can do a standard calc for just that panel.

The only wrinkle I see in the above is how to divide the 3 VA/sq ft allowance between the two panels. If there is a strict spatial separation in how the lighting and general receptacle branch circuits are divided between the two panels, then it's straightforward to assign a square footage to each panel.

But if, say, one panel supplied all the general receptacle branch circuits for the whole house, and the other panel supplied all the lighting circuits, then as far as I'm aware there's no guidance on how to divide the 3VA/sq ft between the panels.

Cheers, Wayne
Click to expand...
The wrinkle you mention is what makes this difficult to calculate anything with any certain accuracy.

But what is accurate in the diverse loading of a typical dwelling?

Only thing you can count on to a certain extent is HVAC, or other very predictable loads like say a pool pump, most everything else is all over the place demand wise. Even water heating, well pump and similar are fixed loads yet demand can have a lot of diversity. The closer you get to actually being the final branch circuit the more you have to be able to handle the full current indefinitely with your supply wiring though.

ETA: for the bathroom receptacle - you need 20 amp conductors/OCPD because the branch has to handle that when demanded. But a feeder or service conductor that also supplies many other items kind of only sees a significantly small amount as demand for that particular receptacle. When that receptacle has a high demand it is usually for short time and the feeder/service doesn't see any significant heating from that short time loading on that one circuit.
 
The 1st floor panel will feed all of the first floor and will feed the condenser for the 2nd floor...The first floor panel will be fed at 200 amps which, as it is not that heavily loaded, will give me some play for additional load. The 2nd main breaker, which is to be 200 amps, will feed the upstairs panel
which should be under 100 amps, giving me room on the remaining 6 circuits to feed a pool panel and car charger...
Thanx for the help and your interest...dhsvcs
 
If I calculate out at 145 amps for the calculated load of my 1st floor panel, 49 amps for my 2nd floor panel (1st floor panel also feeds 2nd floor panel with a 80 amp breaker ), what size wire do I need to run to my 1st floor panel ?
There are 2 200 amp breakers in the service, the 2nd one will feed the 8 breaker "included" panel being used for a car charger and pool panel.
(320 contin-400 amp peak meter) GE/ABB model TSDR840UFCU. There are 2 main 200 A service breakers. Do I need to full size my 200 A feeder to the "main panel", or can I feed it with 2/0 copper instead of 3/0 ?
 
You can always use the standard calc. The optional calc (220.82) says "This section applies to a dwelling unit having the total connected load served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors with an ampacity of 100 or greater."

So to me, the 1st floor panel could use the optional calculation since it serves a dwelling (cooking, sleeping, living, and bathroom facilities). It just needs to serve a dwelling and be rated at 100A or more. The 2nd floor panel is probably standard calc only. Is that area a dwelling? It doesn't appear to have cooking provisions, so an inspector oculd argue no. Also, you'd have to install a 100A or greater feeder in order to use that calculation instead of the 80A you used.

The wire size question is a grey area. You are permitted to use the 83% reduction factor for dwelling service/feeder conductors when it serves "the entire load associated with a dwelling". Some will say by definition you have split the service into two 200A chunks so obviously you can't have all the associated load on one 200A breaker. But if one panel feeds a pool and outbuilding and the other feeds all of a dwelling, I would argue the dwelling feeder is supplying all of the dwelling. A similar house with no pool or detached garage fed from a 200A breaker could use that allowance, so why can't you. So like many things in the NEC, it comes down to how the inspectors interpret things. If you install 3/0 copper or 4/0 aluminum, there should be no issue. 2/0 copper off the 200A feeder to the dwelling should be OK, but you'd hate to have an inspector show up and tell you its too small and tear it out.
 
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