Relays to shed branch circuits

[enigma]

Hi All,

Pardon me if this is a question that is too simple, but I'm not an electrician but rather a product designer that has something in mind.

For residential application, I would like to install some load shedding relays downstream of the branch circuit breakers, just outside of the main load center. I have some designs for very small, latching relays that are single pole. The idea is that they switch on and off in priority sequence to limit the amount of draw on a standby generator.

The question is: is there any scenario where I can use a single pole relay for 240V, 2-wire circuits? I.e. if the neutral line is not used like in a how water heater. Switching just one of the hots would interrupt current to the load. I know the the circuit breakers need to be 2-pole because we are worried about safety in the sense that both poles need to trip at the same time so that one side is not left hot relative to ground. But in the case of a downstream switching device does the NEC require 2 poles to be switched? We still have the protective breaker installed and the circuit can be shut off and would still trip both poles. But for the purposes of load shedding, only one pole would drop the load from the generator.

Thanks!

 

[Jim W in Tampa]

You should be fine as far as NEC because your not making a disconnect. What your doing is control circuits. Should be a good market for it.

 

[paul]

As Jim said, for controls you only need to switch of the legs to stop the unit. In this case it would be one.

I would, however, put a sign/label on the HW tank to let others know this is being controlled by disconnecting only one leg of the power.

 

[Jim W in Tampa]

depending on cost i would be very interested in owning one.

 

[cadpoint]

Call me a box of rocks but I don?t agree with Paul or Party Tampa.

For residential application, I would like to install some load shedding relays downstream of the branch circuit breakers, just outside of the main load center. I have some designs for very small, latching relays that are single pole. The idea is that they switch on and off in priority sequence to limit the amount of draw on a standby generator.
The question is: is there any scenario where I can use a single pole relay for 240V, 2-wire circuits?

No. All ungrounded circuits must be disconnected at the same time.

I.e. if the neutral line is not used like in a how water heater. Switching just one of the hots would interrupt current to the load.

The neutral is just re-labeled and is now a current carrying conductor, and not a neutral any more.

I know the circuit breakers need to be 2-pole because we are worried about safety in the sense that both poles need to trip at the same time so that one side is not left hot relative to ground.

Correct

But in the case of a downstream switching device does the NEC require 2 poles to be switched? We still have the protective breaker installed and the circuit can be shut off and would still trip both poles.

Yes correct, if you have two poles, put it this way the water heater might depending on the internal wiring, still might be demanding an element to fire.

But for the purposes of load shedding, only one pole would drop the load from the generator.

Again, No.

First off there are very few 2 pole loads in a house, the range, the water heater, maybe the HVAC system and maybe a base board heater. The water heater is a 2 pole load in most average size houses.

by Paul
I would, however, put a sign/label on the HW tank to let others know this is being controlled by disconnecting only one leg of the power.

Any equipment that has two means of disconnects should be labeled, or is that multiply sources of power.

I can?t find my Code book or I?d quote the code articles. So I?ll take the heat and lumps for not having the reference material or for expressing my far-sided thoughts.

I've never heard of a relay used in this manner but I'll just say that why where here is to learn!

 

[iwire]

Call me a box of rocks but I don?t agree with Paul or Party Tampa.

I used to work with a guy we called 'box of rocks'. :grin:

No. All ungrounded circuits must be disconnected at the same time.

Going to have to ask for code reference.

A disconnecting means must disconnect all ungrounded conductors at once but controllers do not (in general) have to.

 

[sameguy]

It's controls so sp relay should be ok; didn't some one have a bunch of plc controlers they were removing that were a couple months old? Power relays?








?

 

[sii]

It's controls so sp relay should be ok; didn't some one have a bunch of plc controlers they were removing that were a couple months old? Power relays?








?

ptonsparky

 

[Jim W in Tampa]

Is 1 valid concern on any 120/240 loads as it might not remove total load but are ways around that.
He is only doing control work. Most typical water heaters only break 1 conductor
Unless I am missing something I see no violation. They are not disconnects.
Some of my best friends are rocks LOL

 

[LarryFine]

All ungrounded circuits must be disconnected at the same time.

Never seen one of these?

 

[tallgirl]

depending on cost i would be very interested in owning one.

What parameters do you want to use to control the device?

Which is questionese for "I have a product which does some of that already and I sell it."

 

[yankj]

I agree with Larry. What's wrong with using a two pole relay? Safety should dictate that you break both hot legs.

 

[Jim W in Tampa]

I agree with Larry. What's wrong with using a two pole relay? Safety should dictate that you break both hot legs.

Larry did not show you a 2 pole, look again

 

[sameguy]

this is a control circuit not a means of disconnection so 1 pole is fine.

 

[busman]

Larry did not show you a 2 pole, look again

Larry showed a single pole contactor that is most commonly used on residential A/C units. Those units usually have sign that alerts the maintainer to the presence of voltage with the unit off.

Mark

 

[Jim W in Tampa]

Larry showed a single pole contactor that is most commonly used on residential A/C units. Those units usually have sign that alerts the maintainer to the presence of voltage with the unit off.

Mark

Does look like typical compressor relay and I would hope without a label that the man working on it knows enough but we can't protect idiots

 

[Jim W in Tampa]

What parameters do you want to use to control the device?

Which is questionese for "I have a product which does some of that already and I sell it."

My problem is sheding loads off of my 200 amp service when having parties. Would like to shed water heater first, then hot tub followed by select select central airs. Likely far more money than practical. Situation is I have tripped the 200 main breaker and even melted triplex drop in two. Been converting what we can to propane. Goul would be to back loads off enough to stay under about 160 amps. Proplem is only for 6 hours each week when we need plenty of AC to cool about 100 people and about 3600 feet of rooms. Items such as range or cooktop is not in the picture as they are off.

 

[dbuckley]

I agree with Larry. What's wrong with using a two pole relay? Safety should dictate that you break both hot legs.

+1

If the NEC doesn't require it, then the NEC is lacking, in my opinion.

Why? Because "off" should mean off.

 

[ptonsparky]

Did some simple load shedding with my first plc project. We had way more HP than service.

The NEC is a minimum standard, of course it is lacking.

 

[busman]

+1

If the NEC doesn't require it, then the NEC is lacking, in my opinion.

Why? Because "off" should mean off.

Why would you trust a relay/contactor as your disconnecting means? I sure wouldn't. Thing could close at any time. I prefer the single pole. It encourages use of the required disconnect.

Mark