Continuous Duty - Residential Dwelling

[wwhitney]

The graph below may be informative, it shows my household power draw over time for Saturday.

The huge plateau in the morning is from charging the EV. It was under 3 hours, but the EV was only down maybe 40% (and it's a small EV, 40 kWh). If it had been down 60% or more, as happens, it would have lasted over 3 hours.

This illustrates both that the EVSE is a continuous load, and that its power consumption profile is unlike any other load in my residence.

Cheers, Wayne

 

[ramsy]

Update turns out a Level 2 EVSE will not see its maximum current for 3 hours or more when charging lead acid or lithium.
The charge algorithm is constant current for a while then constant voltage, meaning the current will vary.

Excellent chart, which suggests reduced load (current) as battery approaches full charge.

While branch circuits are selected for full-load current, Article 220 treats each appliance separately. Proving a 125% load calc in Article 220.18 requires motor-only inductive current rise for 3 hours at full rating, since other inductive-load appliances may be subject to nameplate current, or specific demand factors like welders.

Non-motor inductive loads like cell towers, radio transmitters, radar, sound system, or inverter equipment with no nameplate, or demand factors, can certainly be checked for continuous-inductive current rise at full rating, before it overloads undersized service equipment.

Proving a 125% load calc is harder when continuous-inductive current rise at full rating only occurs when battery charge is <~25%, or requires max setting of vehicle charge current, not using multiple-charger network limits, and only when ambient is below critical ~100.F.

In my area, afternoon ambient in residential garages can be >100.F for several months per year, and getting a full charge from 9pm-5am never needs vehicle charge current set >20 Amps @ 240vac.

The 120vac 20A mobile chargers take longer to charge, have no network limits, or charge settings, and are more likely continuous-inductive loads at full rating.

 

[wwhitney]

Excellent chart, which suggests reduced load (current) as battery approaches full charge.

Again, the graph posted is misleading, because in practice the constant charge phase is say 90% of the energy delivered, and the constant voltage phase is only 10%. The posted graph has been truncated on the left; if you look at the full charge cycle starting at the beginning, for many combinations of starting battery state of charge, battery size, and EVSE size, the constant current phase will be over 3 hours long.

Cheers, Wayne

 

[ramsy]

Again, the graph posted is misleading, because in practice the constant charge phase is say 90% of the energy delivered, and the constant voltage phase is only 10%. The posted graph has been truncated on the left; if you look at the full charge cycle starting at the beginning, for many combinations of starting battery state of charge, battery size, and EVSE size, the constant current phase will be over 3 hours long.

Cheers, Wayne

Nice graph you posted. Most of my clients are like you, commuting less than 100 miles, draining less than half the battery.

Regarding charge current graphs, you remind me that listed equipment only works as designed if installers are subject to listed instructions, as required by AHJ's during the permit & inspection process. The question becomes how are AHJ's calculating that load during permits. Forgot to ask how that is being calculated.

Knucklehead endeavors usually skip the permit process, with no regard for load calcs, much less inductive-current rise @ 125%.

 

[brycenesbitt]

Well if I wanted to cheat, I'd just program my load sharing device to take a two minute breather after 2 hours and 59 minutes.

You can't depend on the car having a particular behavior, because each vehicle model and chemistry is different.


(Image is of a peak load shedding test, involving two EVSEs, on the RV50A and EVTest circuit

 

[tortuga]

Yet there is no wording in the NEC to support any such distinction between commercial and residential. If your residential service has continuous loads, when sizing the SECs under 230.42, you need to consider what fraction of the total load is continuous.

Where 310.12 applies (the entire load of dwelling) it modifies 230.42 and allows the service conductors to be sized off the service rating which is just the calculated load (230.72) .

 

[wwhitney]

Where 310.12 applies (the entire load of dwelling) it modifies 230.42 and allows the service conductors to be sized off the service rating which is just the calculated load (230.72) .

230.72 is "Grouping of Disconnects" and doesn't say anything about service rating or load.

I believe the common interpretation is that 310.12 is modifying the effect of 230.42, but 230.42 never uses the term "service rating." So I think under close inspection the wording chosen in 310.12 fails to do that properly.

If 310.12 is successfully modifying the effect of 230.42, it would be to have the effect that the word "ampacity" there would be replace by "service rating." Which means the service rating would still need to be at least 100% of the non-continuous load plus 125% of the continuous load, unless the service OCPD is 100% rated.

Cheers, Wayne

 

[tortuga]

230.72 is "Grouping of Disconnects" and doesn't say anything about service rating or load.

Sorry Type'o that was supposed to be 230.79 'rating of Service'
There was a recent discussion where Ben summed up the recent history of 310.12
I think the intent of the CMP's is that 310.12 can be used with the calculated load in article 220 (AKA rating of the service), and there is no need to apply 230.42 to a SFD service.

 

[wwhitney]

Sorry Type'o that was supposed to be 230.79 'rating of Service'

It's "Rating of Service Disconnecting Means" which is not quite the same as "Service Rating," although getting close.

Even if you could skip 230.42 (not sure why you think so), the service OCPD is also the 215.3 OCPD for the feeder supplying all of the dwelling unit (even if that feeder consists only of the panelboard busbars). So I don't see how you get around the "100% non-continuous plus 125% continuous" language there.

Cheers, Wayne

 

[tortuga]

It's "Rating of Service Disconnecting Means" which is not quite the same as "Service Rating," although getting close.

What would be an example where they are different?

Even if you could skip 230.42 (not sure why you think so),

Because 310.12 says

Single-Phase Dwelling Services and Feeders. For one-
family dwellings and the individual dwelling units of two-family
and multifamily dwellings, service and feeder conductors sup-
plied by a single-phase, 120/240-volt system shall be permitted
to be sized in accordance with" ...

the service OCPD is also the 215.3 OCPD for the feeder supplying all of the dwelling unit (even if that feeder consists only of the panelboard busbars).

Panelboard busbars are part of a listed assembly they are not feeders, and they don't need to be sized for continuous loads, see 408.30.
Other wise if I pull a permit for a service change I'd also need a permit for a 'feeder'
Overcurrent protection of a panelboard is per 408.36

 

[jaggedben]

Nah, panelboard busbars are feeders (or sometimes service conductors). It's just that other rules about panelboards and service equipment take precedence and any ampacity tables referring to wire type conductors clearly don't apply.

And I believe that panelboard busbars do end up needing to be sized for continuous load. For example if I have two 48A EVSE on a subpanel, then if I follow 215.2 and 215.3 then both my wires and breaker need to be sized for not less than 120A, i.e. 125A for the breaker. If I then use a 100A panelboard (greater than the actual load of 96A) I've violated 408.36.

 

[tortuga]

When I pull a permit to replace a typical residential service panel in a single family dwelling, and all the branch circuits originate out of that one panel, I pull a permit for a service, not a service and a feeder. Inside that panel are busbars and sure they are a type of feeder but the rules of 215 dont apply to busbars in a listed enclosure.

 

[jaggedben]

When I pull a permit to replace a typical residential service panel in a single family dwelling, and all the branch circuits originate out of that one panel, I pull a permit for a service, not a service and a feeder. Inside that panel are busbars and sure they are a type of feeder but the rules of 215 dont apply to busbars in a listed enclosure.

I don't see how the manner in which an AHJ decides to manages permits changes how the code defines things. (I've never seen a AHJ base permitting on a feeder, anyway. Around here it's usually a fee for either a service panel or a subpanel. If I replaced a service panel and also ran a new feeder to an existing panel it would still be a service panel fee. But in any case I don't see the relevance.)

 

[tortuga]

I don't see how the manner in which an AHJ decides to manages permits changes how the code defines things. (I've never seen a AHJ base permitting on a feeder, anyway. Around here it's usually a fee for either a service panel or a subpanel. If I replaced a service panel and also ran a new feeder to an existing panel it would still be a service panel fee. But in any case I don't see the relevance.)

The AHJ is the person we answer to so if my AHJ does not think a service panelboard is a 'article 215 feeder', he does not require a permit for it as such, and I don't think its a 215 feeder, and we both agree the ampacity of a service panelboard is just the calculated load per 230.79 or 408.30. 480.30 points to 220 not 215, there is no one to argue with.

But perhaps I am the odd man out not doing a second continuous load calc for a dwelling, how do you all MH forum members implement continuous loads in SF dwelling load calc?
Do you include:

• Lighting not covered by 210.70?
• Pool and spa pumps?
• Yard lighting?
• Water heaters?

 

[wwhitney]

[*]Water heaters?

Water heaters are not continuous loads for the purpose of feeder or service calculations. 422.13 says that storage type water heaters of 120 gallons or less are "considered a continuous load for the purposes of sizing branch circuits." By my favorite principle, that means they are not considered continuous loads for the purposes of sizing feeders or services.

Cheers, Wayne