Energy Management and Over-current situations

[Rossn]

The questions:
1) Given an instantaneous 50A load (makeup air heater), what is a reasonable tripping threshold to target for an energy management system overseeing a 200A OCPD?

If it was set to 150A, then the EMS would not benefit me because it is not providing extra capacity. At 160%, it is really only freeing up 10A. At 200A, it is most useful, but it means that the OCPD could see 250A for a short period before it disconnects the device, and this could be a regular occurrence.

2) Is one to infer from the time-current curves of an OCPD and NEC that residential electrical systems seeing a load 25% beyond OCPD for a few seconds (until EMS kicks in), on a regular basis is acceptable and within the capabilities of a NEC compliant electrical system (mainly thinking about feeders)? It would be great to understand a professionals' point of view on this.

The background:
In my remodel, I have a situation by which my load calculations will (eventually) exceed my capacity (namely one 50A heating load for the makeup air system), thus there will be a need for an energy management system. There is a possibility that I may be able to meet NEC needs by monitoring usage, but need to have a clear solution in the event that does not pan out.

In reviewing available energy management systems (many of which are oriented towards EVSEs), many use CTs to monitor load on the feeders for a given panel or service, and then cut power to a device if the current draw for the panel or service exceeds that threshold. Different EMS's use different thresholds. Some 80%, some configurable at 80% or 100%, some can be configured or any threshold.

I believe my main breaker trip curves are below (awaiting confirmation from Schneider). From this it appears that, for example, on this 200A OCPD:

250A (125%) load trips between about 200s and 10,000s (or greater). And a 300A (150%) load trips between 125s and 1250s. Having not looked at time-current main breaker curves previously, I was surprised. I thought they would be more sensitive. Perhaps this implies that it is reasonable for a feeder of a 200A breaker to also see the higher (say 250A) current for shorter periods of time (seconds).

 

[wwhitney]

The questions:
1) Given an instantaneous 50A load (makeup air heater), what is a reasonable tripping threshold to target for an energy management system overseeing a 200A OCPD?

For 200A OCPD and 200A conductors, the EMS should be able to see all the load (including the make up air heater), and have the ability to drop a load whenever the observed load is above 200A.

From your previous thread, you have 180A conductors protected at 200A, for which the EMS should be set to do that at 180A.

If your load calc is 250A (or 230A), I bet that EMS will kick in at most once a year (more likely never).

If your EMS is designed to monitor everything except the controlled, and that 50A nominal load has a consistent real load (50A, or maybe 40A), then you'd just subtract that consistent load from the EMS setting above. That would give you the same effect.

If your 50A nominal load is variable, you are obviously better off monitoring the total load and dropping the 50A load only when the real total load exceeds the ampacity of your conductors.

2) Is one to infer from the time-current curves of an OCPD and NEC that residential electrical systems seeing a load 25% beyond OCPD for a few seconds (until EMS kicks in), on a regular basis is acceptable and within the capabilities of a NEC compliant electrical system (mainly thinking about feeders)? It would be great to understand a professionals' point of view on this.

As you observed, from the trip curve this particular OCPD will allow a 125% overload for at least 200 seconds. So a few seconds is not a problem. A motor starting against a heavy load (where it takes a few seconds to hit the design rpm) might do that on normal startup.

Cheers, Wayne

 

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[Rossn]

For 200A OCPD and 200A conductors, the EMS should be able to see all the load (including the make up air heater), and have the ability to drop a load whenever the observed load is above 200A.

From your previous thread, you have 180A conductors protected at 200A, for which the EMS should be set to do that at 180A.

Yes, agreed, that the EMS would be monitoring the main feeders, and for purposes of this discussion I am assuming those have been upgraded to support 200A.

If your load calc is 250A (or 230A), I bet that EMS will kick in at most once a year (more likely never).

I believe you on that. My intent right now is to make sure there is an EMS solution (provided load-monitoring-on-existing-dwelling doesn't cut it) that would work for me, and to put in place any necessary infrastructure between the garage and mechanical room that would be needed to support it in the future. What I am hearing from mfg's is that generally speaking shielded 2-conductor 18-gauge wire should be used for the CT extensions and/or dry contact connections.

If your EMS is designed to monitor everything except the controlled, and that 50A nominal load has a consistent real load (50A, or maybe 40A), then you'd just subtract that consistent load from the EMS setting above. That would give you the same effect.

The stelpro duct heater technical support says that their heater is modulated, but time-modulated, via SCR, and that I should count on seeing the full 50A load anytime it is running. If you happen to be an engineering type and believe there is a lower RMS value that will be felt at times when not operating at full load, I'm all ears.

If I reduce my EMS set point to, for instance 150 or 160A, that doesn't really buy me much. Instead, I'd prefer to have that set point at something like 190 Amps

If your 50A nominal load is variable, you are obviously better off monitoring the total load and dropping the 50A load only when the real total load exceeds the ampacity of your conductors.

Per above, I don't think I benefit unless I treat it this way... provided, of course, that few seconds at 125% is not an issue (seems like that is your take).

As you observed, from the trip curve this particular OCPD will allow a 125% overload for at least 200 seconds. So a few seconds is not a problem. A motor starting against a heavy load (where it takes a few seconds to hit the design rpm) might do that on normal startup.

Thanks for that sanity check. I didn't want to make an assumption on this one without some one who works in the industry suggesting it is reasonable.

It seems the company PSP, who has typically had solutions around generator energy management, is branching more into the utility direction, and their products seem both well thought out as well as highly configurable. For instance, they have a controller that can control multiple relays with prioritization for dropping and re-adding loads. They have a unit that was developed for use with EVSEs that is configurable for floor and ceiling current, as well as time to drop out and time to restore. CT leads actually go to a dry contact set of contacts, so there are a lot of options with their products and they have some additional ones coming out soon.

 

[wwhitney]

If I reduce my EMS set point to, for instance 150 or 160A, that doesn't really buy me much. Instead, I'd prefer to have that set point at something like 190 Amps

If the only load the EMS can shed is the 50A heater, and it is monitoring all the non-heater load but not the heater, setting it to 150A is fine. The non-heater load can still hit 200A. The EMS will just inhibit the heater unless the non-heater load is 150A or less.

Cheers, Wayne

 

[Rossn]

If the only load the EMS can shed is the 50A heater, and it is monitoring all the non-heater load but not the heater, setting it to 150A is fine. The non-heater load can still hit 200A. The EMS will just inhibit the heater unless the non-heater load is 150A or less.

Cheers, Wayne

I see what you are saying.

My gut says what will have the least overall impact, and what will have additional innovative solutions coming out will be shedding the 60+A for the EVSEs, and that would cover just about everything. Now, if I add more to that main later, potentially adding welder, hot tub (heater only, not circulating pump), and EVSE. It is nice that PSP has a controller that can manage multiple relays, and the costs of a few relays and the controller are less than a lot of other solutions. But, of course, that now, and I expect in 3 years even more options will be available.

I am thinking that for contingency I should be running 3 sets of shielded 2-conductor 18-gauge wire between garage and main panel (via mechanical room), as well as one from makeup air heater to mechanical room, and that should give me a lot of options later. On the house side, if additional capacity is needed later - I could add energy management on Steam Shower and Well pump to have 40+ amps free.... 80 if I was to add EMS to the heat pump, but IMO that is not ideal.

I'm also sensing that it makes sense to pull ethernet to each main and the two 'main' sub panels (if not all of them). It seems that the energy monitors and smart panels work best off a hardwired connection. Usually low voltage isn't allowed to run parallel close to the high voltage, but I am guessing it's acceptable in these cases.

 

[Fitter30]

If the heater elements are controlled by more than one contactor adding a time delay on the second and or third would lower the in rush currant. Only has to be second or two delay.

 

[Rossn]

If the heater elements are controlled by more than one contactor adding a time delay on the second and or third would lower the in rush currant. Only has to be second or two delay.

Thanks, and good point. When I spoke with Stelpro and asked about some options to manage the current (versus single 50 Amp draw), they did tell me that they can build the heater with multiple stages. However, I'd have to provide controls. So, I'm assuming (and should verify), they cannot install a time-delay between stages, rather it would be control dependent.

I did some poking around and could not locate a control that could handle multiple stages in the manner I needed, and offered short time cycle for the feedback loop to have closer to steady-state temps. If you, Wayne, or anyone knows of a control that can receive a 24v 'power on' feed, and then manage multiple stages to an adjustable delta-T with ceiling temp, then I would love to hear about it! It seems a lot of the HVAC controls are still pretty rudimentary, old-school controls.

It's all PLC doable, but I need something that is not a high-fuss solution. It may make sense to have it built to two stages with their standard control attached to the stages in parallel, such that I could later add a control that manages the stages independently.

 

[Fitter30]

Three stage heating wall thermostat can even get one with wifi and remote sensor. Honeywell, white rogers and others make them.

 

[Rossn]

Three stage heating wall thermostat can even get one with wifi and remote sensor. Honeywell, white rogers and others make them.

How fast do those cycle? I think the heater has to cycle off and on rapidly to adjust for the needed BTUs. I think the standard SCR control is up to 30 cycles per second. In this case, think about 750-1000 CFM of 5degreeF air inbound, and needing to modulate the heater to raise the temp by 50F, with a ceiling of 55F.

 

[Fitter30]

45°rise 1k cfm =48,600 btu's = 14 kw
Scr as fast as 8.3 millisecond most heats 2 seconds cycle rate. Depends on your process how close u want to be.

CFM calculator

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