Sharing one circuit for two applications

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  #1  
Old 10-13-15, 02:37 PM
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Post Sharing one circuit for two applications

I currently have a circuit on my board devoted to a central vacuum cleaner. It is rated for 20A at 240v.

My breaker board is quite full and we are trying to come up with solutions so that we can also add a 240V line for use in charging our new electric vehicle (the chargers vary--some require 20a and others require 40a).

I had the thought that I could use the same circuit/breaker for both the vacuum and charger. What I would like to do is direct the circuit to the car charger unless the vacuum cleaner is turned on. When the vacuum is on (which is rarely at the same time as vehicle charging anyway), I would want to cut off the supply to the car.

I wondered if I could do this with an AC Contactor or Relay. Since the current requirements are quite high for both the vacuum and the car, would I need a contactor or relay that can support such high currents for both the coil as well as the load? Does anybody have pointers for the type of contactor I could use?

Thanks for any suggestions or ideas!
 
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Old 10-13-15, 03:04 PM
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Why not add a subpanel? Move some of your light use circuits to that panel and keep you main panel for the heavy loads. It would be a low cost easy DIY project in most cases.
 
  #3  
Old 10-14-15, 03:10 PM
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Thanks for the reply, Ray.

Adding a subpanel is probably the straightforward option (and gives more functionality--in case we want to vacuum and charge at the same time, for instance).

You mention moving the lighter use circuits. I'm wondering if there is any reason to choose that strategy. I suppose that it allows me to move the maximal number of circuits off the main panel while keeping the subpanel load reasonably low. Would the new subpanel need to support 240V (esp assuming that I don't move and 240V circuits to it)?

I'm still curious about how one would design the switchover option just for my own edification.

Thanks for this great resource!
 
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Old 10-14-15, 03:31 PM
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I suppose that it allows me to move the maximal number of circuits off the main panel while keeping the subpanel load reasonably low.
Yes, that is the reasoning.
Would the new subpanel need to support 240V (esp assuming that I don't move and 240V circuits to it)?
Subpanels are normally 120/240. This distributes the load between the two 240 legs. (Actually any common panel you buy is 120/240. 120 panels just aren't commonly sold or used.)
 
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Old 10-14-15, 03:34 PM
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It is customary for a subpanel to handle 240 volts as well as 120 volts.

For the main panel to handle a number of new 240 volt circuits it is best for some 120 volt circuits on each side of the 120/240 volt service to be moved to the subpanel, thus the recommendation for the latter to handle 240 volts

Usually a subpanel under the main panel will have a 60 to 100 amp rating.

With a 60 amp rating the subpanel can easily handle eight to ten smaller 120 volt (15 and 20 amp) branch circuits, freeing up that many slots in the main panel for larger breakers (including the breaker pair for the subpanel's feed). Whereas leaving the main panel largely intact, a 60 amp subpanel might only handle two or three new big 240 volt circuits even though there are many more slots.

You should do a load analysis (paperwork) before adding a big new load with or without a subpanel. This is to reveal a possible need to upgrade the service to the house.

The coil (controlling part) of a contactor draws a miniscule, you could say negligible, amount of power even though the contacts may switch a large current (load).
 

Last edited by AllanJ; 10-14-15 at 03:52 PM.
  #6  
Old 10-14-15, 05:53 PM
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You MAY be able to swap out the circuit breaker for the vacuum cleaner to a quad model that will supply two 240 volt circuits. Some panels will accept a quad CB and others won't. We would need to know the manufacturer and model of the panel as well as the circuit breaker itself. Picture of the panel and the schedule on the door (readable picture) would help.
 
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Old 10-15-15, 02:06 PM
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These are the images that you requested, Furd.

Also, Allan, thanks for the clear explanation about how to decide what to move to the subpanel.

Re the contactor: I understand that the coil itself will draw very little current. My concern (and maybe it is a concern that comes from my limited knowledge of electricity) is that the entire current of the vacuum cleaner would also be going through the coil of the contactor (and heating things up). Perhaps I just need an education on how I would wire in a contactor such that the vacuum would trigger the coil without also running the 240V and 15A of current through the coil.
 
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Old 10-15-15, 02:16 PM
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A subpanel is your only option. The panel cannot use tandems.
 
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Old 10-15-15, 03:08 PM
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...central vacuum cleaner. It is rated for 20A...
What is the car charger amperage requirement?
 
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Old 10-15-15, 03:16 PM
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Probably 20 amps at 120,or 40 amps at 240. Depending on whether it is a fast charger.
 
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Old 10-15-15, 03:19 PM
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Originally Posted by ray2047
What is the car charger amperage requirement?
The ideal 240V charger would draws 30-32 Amps (most information I see recommends a 40 Amp circuit for it). There are other chargers that are 240V but only require 16 Amps (20 Amp circuit).
 
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Old 10-15-15, 04:53 PM
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So at 30 or 40 amps you can't use the same breaker for both vac and charger so more reason to install a subpanel.
 
  #13  
Old 10-16-15, 12:24 PM
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Originally Posted by ray2047
So at 30 or 40 amps you can't use the same breaker for both vac and charger so more reason to install a subpanel.
Correct. I can't use the same breaker for both--which is why I was exploring the idea that I'd use one breaker that was either/or with the contactor/relay idea.

I'm sure the right answer is the subpanel, so I'll start looking into that option.

Thanks for all your help!
 
  #14  
Old 10-16-15, 02:05 PM
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I'll also throw out the observation that you have 10 circuits labelled "lighting" in that panel. They might be mislabelled and actually have other things on them, but if it's really that many lighting circuits you can probably combine some that are under-utilized. A typical house usually needs only 2 or 3 lighting circuits. It doesn't seem unreasonable you could free up 2 slots that way.
 
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