Drawing more than 15 amps from a 15 amp circuit

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Old 03-09-18, 02:54 AM
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Drawing more than 15 amps from a 15 amp circuit

I know, I know.. it can't be done.. but in my situation, I think it's possible, but it's a very unique situation.

Here's the problem: I own my own catering business and I have a few food warmers that I want to operate on the same 15 amp circuit. Since I do these events at random places with random layouts, I can't always rely on having 2 or 3 individual circuits within easy access. In addition, most catering situations don't allow me to stretch an extension chord 100 feet to another outlet. It looks tacky and it will be complained about.

To keep it simple, lets just work with 2 warmers. Each one is 110v/1300w. The advantage I have to making this work is that food warmers are all or nothing. They aren't variable. Water temp drops below a certain level, and the warmers kick on. Full 1300w draw until the water reaches the peak temp, then the draw is cut off completely (outside of a few watts to maintain the circuitry monitoring temp and controlling the internal relay).

My initial thought was to use a current sensing setup. Unit 1 would be plugged into outlet A, and unit 2 would be plugged into outlet B. When Unit 1 turns on, Outlet A detects the current being drawn and cuts power to Outlet B. When Outlet A no longer detects the higher current being drawn from it, it restores power to Outlet B. In a perfect world, this works. My problem is; What if Outlet B currently has power being drawn from it when Outlet A suddenly gets a power surge from the Unit 1 turning on? Unfortunately, by the time the current sensing device detects power being drawn from Outlet A in order to trip the relay to turn off Outlet B, I've already tried drawing 2600w from the 15 amp circuit, and I trip a circuit breaker (This is all theory, I haven't tried it, but realistically, this makes sense. I'd need a break before make circuit, and this is a make before break).

The only way I can see making this work is having to have some sort of continuity between Outlet A and Outlet B, so if voltage is sensed on either one, they automatically turn off the previous one until power is no longer being drawn at high levels. The only other way to do it would be to have some sort of timer that goes back and forth, turning one outlet on for 5-10 minutes, then switching to the other. The electric steam trays I'm using are pretty efficient/insulated, so they don't drop in temperature very quickly. Even with food trays almost completely empty, since they are preventing steam from leaving the steam tray.

Anyone have thoughts on how to make this work without spending an arm and a leg on some $200+ electrical switcher? I do have a Niles APC-2 lying around that I can use within my custom circuit which may help a lot. It however turns a built in outlet on when current is sensed, with variable level of detection, as well as 12v triggers, relays and so on. Would love some ideas
 
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Old 03-09-18, 03:03 AM
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Perhaps a wig/wag circuit with a 5 minute timer tripping a relay back and forth between Outlet A and Outlet B? Most of the wigwags I've seen out there however, are usually no more than 1 second each. I don't think my units would be happy having power cut every second and then being power surged again.
 

Last edited by Maverick3n1; 03-09-18 at 03:28 AM.
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Old 03-09-18, 04:34 AM
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Your situation is why they make fuel (Sterno, butane...) powered warming and serving trays. It frees you from relying on an unknown power supply at all the various locations you will set up.
 
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Old 03-09-18, 05:25 AM
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You will need to restrict your client base to those who can provide the needed electrical power.

Specify the number of circuits needed assuming 15 amp circuits.
 
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Old 03-09-18, 07:52 AM
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Best I can think of would be a rotary switch with one steamer pan wired to each selector position. The downside is that you would need to manually select which pan gets power, but you could perhaps work it into your routine -- check the buffet every 5 min and move the switch to the next position.

There are motorized rotary switches out there you could rig to a timer, but it's all intended for the industrial controls market. I'm sure you'll be in several hundred dollars that route.

If you wanted to go really deep down the hobby project rabbit hole, you could get some 5VDC x 120VAC relays, a small computer like a raspberry pi or an arduino, write a simple switching/cycle program, and wire the computer board outputs to the relay coils. Each pan would be wired through the relay contacts and your program could control the on/off cycle rate for each pan and ensure that only one is energized at a time. There are a ton of prefab expansion boards out there too, I'm sure you could find a relay board that does what you want.

Oh, and make sure the whole thing is on GFCI protection.
 
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Old 03-09-18, 08:48 AM
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I would be very cautious about a DIY solution since you are operating commercially and in customer's facilities. Your safety and liability exposure would be pretty high if you DIY something for handling power in a non-standard and non-approved manner. At most I would, if you can find it, purchase something that already exists and has a UL or CE or other approvals appropriate for where you are located.
 
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Old 03-09-18, 01:52 PM
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A Honda generator is another good option. They are very quiet. Or get a pile of batteries and a 1500 watt inverter.
 
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Old 03-09-18, 04:06 PM
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I think you're trying to solve the wrong problem.

1) You need a well insulated "hot box" in your catering van.
2) You might want a custom heat-duct feeding the hot-box in your catering van.

3) You need to invest $18 each for soft-sided insulated carriers.
https://www.webstaurantstore.com/cho...4FCARRVNL.html

4) you need to accept spending $10 per event on disposable sterno-fired-chafing dishes
https://www.webstaurantstore.com/ful...22CHAFKIT.html
 
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Old 03-09-18, 06:19 PM
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Those blue bags are horrible. Good way to throw away money. I use full on Cambros. They'll hold food temp and the food will only drop like 3-4 degrees in a 4 hour period. Food in the blue bags would be cold by then.

We use Chafing dishes/canned heat all of the time. The problem with Chafing dishes is they do not accept any pans deeper than 2". Our electric Chafing dishes take 6" deep pans. We use them at street fairs primarily, but occasionally we like to use them at catered jobs especially if there are things like sauces that need to be served hot. Ladling sauce out of a 2" deep full size pan can be very frustrating for the end user on a self serve buffet. In addition, if I have 3+ different types of sauces to serve, I'm not going to have 3 full size chafing dishes/canned heat with just sauce. Instead, I have a 6" deep electric chafer with 4 - 6" deep 3rd pans, each containing almost the same capacity as a full size 2" pan, while making it much easier for customers to ladle their own sauces.

As for DIY solutions, when I do these things, I don't do them amateur hour style. I have 15 years experience in home and commercial automation. As someone brought up, I could use a computer to automate the process with relays. I honestly didn't think about it, but there are some mini Crestron Controllers that are old/outdated that I could probably buy off of ebay and program to trigger a relay every 5 minutes. I have one lying around here, but it's a full size controller and I'd rather not drag a full size rack mount controller around with me. The mini controller however, I could mount to the back of a double gang box a 120v relay mounted on one side and a dual outlet on the other side. The only thing that sucks is I was hoping to contain all of the system in a single exterior electrical rated box. This solution would require a double gang outdoor rated box plus the automation module, and a power chord/transformer for the module... it has my thinking the right direction though

Any more suggestions would be great!
 
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Old 03-09-18, 07:44 PM
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The advantage I have to making this work is that food warmers are all or nothing. They aren't variable.
That's a pity. That's your problem. Once the product is heated it probably doesn't need the 1300w to keep warm/hot.

If it was me..... I'd try a BIG diode in series with the hot leg. When the diode is inline.... the heating current is half. You could actually install a hi/lo switch that bypassed the diode. You'd want at least a 20A 200v diode.

You could try it on one warmer and see if in low it was hot enough.
 
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Old 03-10-18, 09:04 AM
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I don't think I've ever seen a 15 amp receptacle circuit in a commercial building.
 
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Old 03-11-18, 11:32 AM
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@CasualJoe, who said anything about a commercial building? I have clients with 15,000 sq ft homes. Most outlets other than kitchen are 15 amp. Often times the buffet is setup in their great room which can be as large as 2,000-3,000 sq ft. If I was doing a ballroom etc, they all typically have 20amp. Churches are hit or miss. When I'm doing a booth at an event, my options are hit or miss. Some have commercial power lines with 100-200amp circuits in a line of spider boxes. Some are sharing a generator with multiple booths. Especially when sharing a generator, I have to be careful about my current draw.

Some street fair style events supply power but charge you based on your power draw requirements. If I require 30 amps, my fee will be much higher than 15.
 
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Old 03-11-18, 11:36 AM
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@PJMax, 20amp diode would still let more than 15 amps of power through the line, popping the breaker. In addition, while I know that cutting the power for 5 minutes and re-applying the power for 5 minutes won't hurt the unit, impeding the power flow so it can't draw what it needs may cause damage to the heating element.
 
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Old 03-11-18, 11:43 AM
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I don't think I've ever seen a 15 amp receptacle circuit in a commercial building.
.
I would say about 75% of receptacles installed in commercial buildings I see are 15 amp. However, I rarely see 15 amp circuits in them.
 
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Old 03-11-18, 03:41 PM
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Maverick, PJ has the right idea IF your heating units are AC/DC compatible. That is, if they have simply resistance heating elements AND mechanical thermostats. Adding a simple diode in series with one lead would effectively create a 1/2 wave rectifier circuit and give about 66% power to the element. The element itself (if purely resistive, and almost all are) won't care if it gets AC or pulsing DC. The one caveat is if you have an electronic thermostat that requires AC for operation. Even then you could insert the diode in the leads to the heater rather than the input leads.

OR, you could use a time-proportioning PID controller driving a solid state rectifier and then it would pulse the power to the element proportional to the difference from the set temperature.

I would, however, use a much larger rectifier diode than PJ stated, at least a 40 ampere unit rated at 400 PIV. A nice stud-mount on a heat sink shouldn't set you back much. You could then use either a heavy-duty mechanical switch to bypass the diode or a solid state relay.
 
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Old 03-15-18, 08:14 AM
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I would say about 75% of receptacles installed in commercial buildings I see are 15 amp. However, I rarely see 15 amp circuits in them.

Agree. I can't count the number of times I have sent service trucks out to replace 15 amp receptacles with 20 amp receptacles, on 20 amp circuits, for office copy machines.

As far as residential catering, I would put the responsibility to furnish adequate power for the warmers on the homeowner. Otherwise, there would be an additional charge for a quiet generator.
 
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Old 03-26-18, 09:15 PM
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Look into if you can program one of these to do what you want.

See https://dlidirect.com/products/web-power-switch-7. This is designed to control networking equipment -- reboot if connectivity is lost, but it allows direct programmed control of multiple outlets. I think outputs are rated for up to 15 amps -- enough for your spec. And since it is your intent to only activate one at a time you'd be OK. And it is a nice neat UL rated box. See if the scripting language it supports meets your need.
 
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Old 03-26-18, 11:36 PM
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20 Amp solid state relays SSRs would be good starting point.

Put 5 of these inbox with oulets for warmers to plug into

Next you need low voltage 3 to 32 volt DC sequencer to trigger 4 or 5 of these in series

https://www.ebay.com/sch/i.html?_fro...0A+SSR&_sop=15
 
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