Two loads not intended to run simultaneously

[Core_d]

Hello to all, new poster here and not an electrician by trade,

If you had two loads of equal size that are never intended to run at the same time, is there any way to make this compliant and allowed to share a single circuit?

I could describe the specific application better if necessary, thanks in advance.


Sent from my iPhone using Tapatalk

 

[GoldDigger]

This comes close to a DIY question by a related trade, but I will allow it for now as a theoretical question.
The NEC does not, in this situation consider intent. If on separate circuits you might not need to take both at full value for upstream amperage calculations.
To allow them to share a circuit which can only power one at a time you would need some form of switching or interlock that only allows one at a time to run.
The details of how this can be accomplished should be referred to a qualified electrician or control technician depending on what the loads are.

Sent from my XT1585 using Tapatalk

 

[Core_d]

Thank you very much and that is what i was hoping to hear. Can you reference NEC article that would allow such an application?


Sent from my iPhone using Tapatalk

 

[Craigv]

Thank you very much and that is what i was hoping to hear. Can you reference NEC article that would allow such an application?


Sent from my iPhone using Tapatalk

There are several articles that state the circuit load shall not exceed various percentages of the OCP for the circuit. This depends on the type of load and circuit, so the specifics of your scenario would help narrow it down.

An an obvious solution that usually isn't very feasible but is worth mentioning is to size the circuit to handle both loads. Just mentioned it in case...

 

[Adamjamma]

wouldnt sizing for both loads be the best, just in case they somehow both operated at same time? Unless the were on a rotary switch where only one could ever operate?

 

[Core_d]

Thanks again, this is in a way theoretical.

The application is a commercial grow room where the ac,s main heat gain is lighting. The idea is when the lights go off electric heaters come on to provide a sensible heat load to allow dehumidification.

In my state your not allowed to use electric heat for reheat purposes so even if it could be NEC compliant it still wouldn’t fly.

This is actually a spin off of another thread in an HVAC forum and mainly just out of curiosity.

The scenario could be useful other application like a redundant or stand by unit where down time is critical.


Sent from my iPhone using Tapatalk

 

[Craigv]

wouldnt sizing for both loads be the best, just in case they somehow both operated at same time? Unless the were on a rotary switch where only one could ever operate?

That obviously works fine, but "best" is very dependent upon the exact scenario. It is quite inefficient use of materials...wire and OCPD, any conduit, fittings, brackets, etc. are larger than needed and cost far more than sizing correctly and using a simple switching mechanism. A DPDT switch could be all that's required depending upon the application.

 

[sameguy]

A 3way switch, hot on common. Load 1, load2 . 3way is no/nc

 

[MAC702]

A 3way switch, hot on common. Load 1, load2 . 3way is no/nc

Unless, of course, you ever wanted both off, without using the circuit breaker.

 

[sameguy]

Get an on off on than. I was going with the day load night load as per the op, but you do have a good point.

 

[winnie]

I don't recall seeing an explicit code article stating 'use this switching technique and thus you have non-coincident loads'.

Instead you have that circuits are supposed to be sized for the load served, and you have examples in load calculations of only using the larger of non-coincident loads. (Eg. you only need to consider the larger of air conditioning and heating)

Presumably it is left up to common sense or inspector interpretation to determine if two loads are 'non-coincident'.

IMHO if you can easily turn both loads on at the same time, and there isn't some explicit control that would make this uncommon (eg. thermostats for heating and cooling) then I would consider them potentially coincident. On the other hand if (in your example) the heat strips were turned on by a contactor only when the lights were off, then I'd consider those non-coincident. But I am not an inspector.

-Jon

 

[winnie]

Oh, for re-heat it really makes sense to have some way to use the heat of the condensing refrigerant. Otherwise you are generating heat indoors and then pumping it right out again.

-Jon

 

[kwired]

Oh, for re-heat it really makes sense to have some way to use the heat of the condensing refrigerant. Otherwise you are generating heat indoors and then pumping it right out again.

-Jon

Seems to me if the heat gain mostly comes from the lighting, then it would make sense to shut down refrigeration or at least portions of it when lights go down, what am I missing? Otherwise no matter what you heat with you are taking heat right back out.

Thermostatically controlled cooling should shut down anyway if source of heat gain suddenly drops.

 

[Craigv]

Seems to me if the heat gain mostly comes from the lighting, then it would make sense to shut down refrigeration or at least portions of it when lights go down, what am I missing? Otherwise no matter what you heat with you are taking heat right back out.

Thermostatically controlled cooling should shut down anyway if source of heat gain suddenly drops.

The OP is inferring the room needs constant dehumidification. When the lighting is off, the temperature drops, relative humidity increases, but the temperature shouldn't be permitted below a certain level. Running HVAC cooling to dehumidify under these conditions would cool the room too much.
The HVAC needs to be set up so that when lighting is off, the AC works like an in-room dehumidifier...the condenser heat is directed into the room to heat it, resulting in high enough temps to run the HVAC cooling to dehumidify. When lighting is on, condenser heat should be directed outside. Essentially the condenser should be fed outdoor air that is directed outdoors when cooling/dehumidifying, and fed indoor air that is directed indoors for dehumidification-only.

 

[kwired]

The OP is inferring the room needs constant dehumidification. When the lighting is off, the temperature drops, relative humidity increases, but the temperature shouldn't be permitted below a certain level.

The HVAC needs to be set up so that when lighting is off, the AC works like an in-room dehumidifier...the condenser heat is directed into the room to heat it, resulting in high enough temps to run the HVAC cooling to dehumidify. When lighting is on, condenser heat should be directed outside.

I guess my next question is what is wrong with bringing in outside air, or even some natural light for that matter? Grow houses that get mentioned here never seem to have any natural light means or much for ventilation.

 

[Craigv]

I guess my next question is what is wrong with bringing in outside air, or even some natural light for that matter? Grow houses that get mentioned here never seem to have any natural light means or much for ventilation.

I don't know either. If energy conservation is important, nothing beats Sol for lighting, and heat. I would imagine air exchange is a necessity, as plants require plenty of CO2 to replace the O2 they create, and mold/fungus issues generally require plenty of ventilation to control.

 

[gadfly56]

The OP is inferring the room needs constant dehumidification. When the lighting is off, the temperature drops, relative humidity increases, but the temperature shouldn't be permitted below a certain level. Running HVAC cooling to dehumidify under these conditions would cool the room too much.
The HVAC needs to be set up so that when lighting is off, the AC works like an in-room dehumidifier...the condenser heat is directed into the room to heat it, resulting in high enough temps to run the HVAC cooling to dehumidify. When lighting is on, condenser heat should be directed outside. Essentially the condenser should be fed outdoor air that is directed outdoors when cooling/dehumidifying, and fed indoor air that is directed indoors for dehumidification-only.

Based on your description, it sounds like a job for the HVAC guy with some duct work and a couple of automatic dampers, depending on how the condenser and evaporator coils are set up.

 

[Core_d]

I just wanted some advice for what is kosher regarding the national electrical code, i never said electric reheat was the best option, it is the least costly as far as install and material goes, in some states its not even allowed.

Can a “dedicated”circuit be “undedicated” so long as its dedicated when it runs?

Again this is theoretical.


Sent from my iPhone using Tapatalk

 

[Core_d]

Im guessing the nec does specify whether such a scenario is legit or not? It would be up to the local eyes?


Sent from my iPhone using Tapatalk

 

[Craigv]

I just wanted some advice for what is kosher regarding the national electrical code, i never said electric reheat was the best option, it is the least costly as far as install and material goes, in some states its not even allowed.

Can a “dedicated”circuit be “undedicated” so long as its dedicated when it runs?

Again this is theoretical.


Sent from my iPhone using Tapatalk

Golddigger did answer your question in post #2. Circuits must be sized to handle the load. Intent is irrelevant: if it is possible for both loads to be present simultaneously, the circuit must be sized for that total load. Hence the suggestion to use lockout switching, which can be as complex as an automatic system or as simple as a double throw switch.