Define Continuous Load

[joseywales]

Define Continuous Load, do I need additional service?

Planning on adding a subpanel for my garage. Currently, it's all 15AMP outlets. I would leave those in place, going to the main panel, doing whatever menial task they have, such as LED lighting. But I want to add at least 4, 20AMP breakers for: drill press, battery minder (motorcycle), freezer, possibly a fridge, you get the idea.

- Main panel is 200AMP, 280 L - 205 R, so 485 in total breakers are in that main panel, and it is full.

- I need to clear several spots, for various reasons, but need to clear two spots, for breakers to support the subpanel.

- For the to pole breaker, to support the subpanel, what size is that? A double 40? I plan on 4, perhaps 6 20AMP breakers for the subpanel.

- If I have 485 in total breakers in the main, how would an inspector calculate continuous load? 160 of those AMPs, support the A/C and a mini-split.

I guess the question I'm asking is, will I need more service to support a subpanel?

 

[wyliesdiesels]

you need to do a load calc. adding up breakers in a panel is not the proper way to do so.

A continuous load is one that is expected to run for 3hrs or more.

 

[joseywales]

you need to do a load calc. adding up breakers in a panel is not the proper way to do so.

A continuous load is one that is expected to run for 3hrs or more.

Thanks. Understood and I’d read about the 3 hours. To clarify, I’m asking, which items do you all, and inspectors, believe run for 3 hours or more? I can’t think of anything running for 3 hours, other than digital clocks, cable boxes, lighting, wall wart and other chargers, and whatever our PCs and their monitors draw. Our water heater is tankless.

In the summer, I do run the pool pump 24/7, so I’ll add that.

What other common items are expected to run for 3 hours, when they are on? I struggle to think of many honestly.
What standby load is typical for a fridge or freezer?
Or is the only way to get a load calc to physically check the draw at the panel?

I'm just trying to get a guestimate, until I have the entire panel reviewed.

 

[klassenl]

Re: Define Continuous Load, do I need additional service?

I guess the question I'm asking is, will I need more service to support a subpanel?

No....unless you have 150a of electric heat. A few extra circuits in the garage for random tool loads shouldn't cause too much problem.

I'm a general sense, as I have come to understand it is that everything (at least connected loads) is considered a continuous load unless it can be proven otherwise. But don't dig too deep there, at home in the house it doesn't affect things.

 

[MerlinsBeard]

Thanks. Understood and I’d read about the 3 hours. To clarify, I’m asking, which items do you all, and inspectors, believe run for 3 hours or more? I can’t think of anything running for 3 hours, other than digital clocks, cable boxes, lighting, wall wart and other chargers, and whatever our PCs and their monitors draw. Our water heater is tankless.

In the summer, I do run the pool pump 24/7, so I’ll add that.

What other common items are expected to run for 3 hours, when they are on? I struggle to think of many honestly.
What standby load is typical for a fridge or freezer?
Or is the only way to get a load calc to physically check the draw at the panel?

I'm just trying to get a guestimate, until I have the entire panel reviewed.

In general, you won't be guaranteed to get an accurate load calculation from measuring equipment current at the panel unless you measure it over a long period of time, because the power usage over time can vary. For example, the current draw will be different when the freezer is actively cooling vs when its not, or whether the door is open and the light is on (yes the load is small but still relevant). There may be large current transients that you could miss as well from starting a motor or compressor.

If you're trying to determine the size of a subpanel or determine wire gauge of a branch circuit, you really are after max instantaneous current draw on the panel or branch circuit. A refrigerator could have a typical operating current of around 3A but could have transient current draw much higher. Usually if there is a large transient, it'll be documented as max or startup amps on the nameplate. If the equipment does not have a large current transient, you can get an estimate by taking the volt-ampere rating and dividing it by the line voltage (usually 120 or 208/240 for heavier duty equipment). Other times there are rule of thumb equations like 180VA per duplex outlet or 3VA/square ft.

You may want to have lighting or certain equipment on dedicated circuits as well. You may not appreciate inadvertently losing your lighting or a freezer getting its power cut because you happened to use a drill on another duplex outlet on the same circuit and you happened to time the max amps of the drill motor and freezer cooling system at the same time and end up tripping the overcurrent protection device (circuit breaker).

If you're interested in more, you can register at NFPA website for free and read the NEC in your web browser. Read Article 220 as a starting point.

 

[AntonLargiader]

A load calc is a formulaic method of estimating your feeder needs. There's a certain baseline demand per square foot for lighting and convenience outlets, then you add specific appliances, apply certain deratings, etc. There are spreadsheets online that walk you through it which I haven't found to be incredibly intuitive, or you can just read about the methods and do your best. The fact that your 200A panel doesn't trip with over 200A worth of breakers is what load calcs explain. You might run a load calc on your house and find that you need 132A. No matter how you spread the load out among breakers, it will still be 132A.

Continuous load usually comes up in the context of derating certain circuits. If you have a load that draws for more than three hours, you need to supply a circuit with 25% more ampacity. As you can see, it only kicks in as circuits get heavily loaded. If your EV pulls 13A for over 3 hours, it needs to be on a 20A circuit because 13A*1.25 is more than 15. For most purposes, continuous load is not about your fridge and toothbrush charger running continuously.