Continuous Duty - Residential Dwelling

[nizak]

How would one go about figuring what the Continuous Duty might be on a residential electrical service without doing any power monitoring?

Would it be accurate to say the following items would constitute a 3 hr running time and could all be operational at once?

- HVAC equipment
- Vehicle charger
- Lighting
- Pool equipment

Thanks

 

[nizak]

I should have said Continuous Load. My mistske

 

[infinity]

IMO there are no loads in a dwelling that will be considered continuous. The definition of continuous load is not merely that it runs for 3 hours but it runs at maximum current for 180 continuous minutes.

 

[wwhitney]

IMO there are no loads in a dwelling that will be considered continuous.

Storage-type water heaters are considered continuous loads for the purposes of sizing the branch circuit (422.13). Depending on heater element size relative to tank volume and cold water temperature, from a cold start they may operate at maximum current for over 3 hours.

EVSEs are considered continuous loads (625.42). Depending on EVSE rating compared to the size of the EV battery, they can easily draw their full rating for over 3 hours.

Cheers, Wayne

 

[frofro19]

IMO there are no loads in a dwelling that will be considered continuous. The definition of continuous load is not merely that it runs for 3 hours but it runs at maximum current for 180 continuous minutes.

Which makes me wonder why the NEC considers a residential water heater a continuous load as most water heaters cycle on and off as needed. Also could heating systems be considered continuous in a rare case when it's extremely cold and a heater would run at full capacity for 3 hours or longer trying to get the temp up to desired range?

 

[frofro19]

Storage-type water heaters are considered continuous loads for the purposes of sizing the branch circuit (422.13). Depending on heater element size relative to tank volume and cold water temperature, from a cold start they may operate at maximum current for over 3 hours.

EVSEs are considered continuous loads (625.42). Depending on EVSE rating compared to the size of the EV battery, they can easily draw their full rating for over 3 hours.

Cheers, Wayne

Would residential water heaters ever take 3 hrs to heat up from a cold start? Most heaters I've seen would heat up from a cold start and take 30-45 minutes

 

[infinity]

Storage-type water heaters are considered continuous loads for the purposes of sizing the branch circuit (422.13). Depending on heater element size relative to tank volume and cold water temperature, from a cold start they may operate at maximum current for over 3 hours.

EVSEs are considered continuous loads (625.42). Depending on EVSE rating compared to the size of the EV battery, they can easily draw their full rating for over 3 hours.

Good point but the NEC requires that these loads be considered continuous even if they really aren't. I'm not convinced that they are by the Article 100 definition.

 

[frofro19]

Good point but the NEC requires that these loads be considered continuous even if they really aren't. I'm not convinced that they are by the Article 100 definition.

I know it's code, but often wonder why the NEC would consider those loads to be continuous.

 

[nizak]

I thought that an AC condenser could run for 3 hrs at maximum amp draw.

Seems when we had mid /upper 90 deg temps day after day mine never shut off.

 

[wwhitney]

Would residential water heaters ever take 3 hrs to heat up from a cold start?

OK, I was just speculating as to the reasoning behind 422.13, but now we'll have to do the math.

Say for a worst case we have a 100 gallon water heater filled with 40F water and the set point is 140F. So we need to heat 833 lbs of water by 100F, which requires 83,300 BTUs, ignoring the heat loss through the enclosure during heating. If that is going to take 3 hours, that means our heater needs to be smaller than 83,300 / 3 = 27,770 BTUs/hr. 1 kW = 3412 BTUs/hr per google. So if our heater rating is less then 8138W, then yes, it would take over 3 hours.

For other cases, if you want a 100F temperature rise on startup, and you ignore heat losses, when the ratio of tank capacity in gallons to heating element kW rating is greater than 100 / 8.138 = 12.3 then you can get continuous load behavior.

Definitely a real possibility.

Cheers, Wayne

 

[petersonra]

Does it make any real difference?

 

[wwhitney]

Good point but the NEC requires that these loads be considered continuous even if they really aren't. I'm not convinced that they are by the Article 100 definition.

EVSEs for sure generally meet the Article 100 definition. Plenty of 60 kWh plus EVs out there (probably the majority of those sold these days), and I expect the most common EVSE rating is 30A @ 240V = 7.2 kW.

Cheers, Wayne

 

[tortuga]

For a dwelling service calc is no requirement to size for continuous loads.
For dewlling branch circuit calcs there is.

 

[infinity]

For a dwelling service calc is no requirement to size for continuous loads.
For dewlling branch circuit calcs there is.

Which is exactly what the OP was asking about.

 

[wwhitney]

For a dwelling service calc is no requirement to size for continuous loads.

Service Entrance Conductors are required to be sized with a 125% continuous use factor, see 230.42. This does not apply to Service Conductors, Overhead or Service Conductors, Underground.

A service disconnect OCPD must be sized with a 125% continuous use factor (unless it is 100% rated), as it is the feeder OCPD for the supplied feeder (even if that feeder is just a busbar in an attached panelboard). 215.3.

Cheers, Wayne

 

[nizak]

Which is exactly what the OP was asking about.

My question is related to a question that was asked by an inspector on a job I did awhile back.

He asked me what the total continuous load of a larger home was where I had a service rated transfer switch acting as the main disconnect.

The items that peaked his curiosity were a 60 amp EVSE along with 2 inefficient AC condensers and a small detached shop with a 12KW pottery kiln.

The switch was installed for a possible future generator as I had to change out a meter base that had failed over time due to loose connections and was a perfect opportunity to do it.

The switch was labeled something to the effect not to exceed 80% of circuit breaker rating.

The new meter base tag stated 200 amp continuous duty.

 

[jaggedben]

Pottery kiln would be continuous.

 

[wwhitney]

Pottery kiln would be continuous.

How often does the initial ramp of the temperature cycle actually require full current for more than 3 hours? I.e. the target rate of temperature rise, plus the heat loss rate, exceeds what the heater can puts out, so it stays at full current?

Cheers, Wayne

 

[Little Bill]

Fixed electric heat would be continuous.

 

[Eddie702]

Any HVAC load would be considered a continuous load in my opinion. Except a bath fan or range hood