Disp & Dishwasher on single 20A ckt?

[peter d]

Yes Pete it does happen in some areas, I know you have read those types of posts on these forums just like I have.

Just like some areas require everything to be listed, different areas different expectations.

Ok, you're right, I defer to the "Master of Electricity". :roll:

I should have specified "in this area." Inspections are an absolute joke here.

 

[jaylectricity]

One of the inspectors in Newton tried to fail me for this, but I was able to read the wording of the code to him. He didn't like passing it, likely because he couldn't wrap his brain around the code not saying what he thought it's always said.

Sometimes you just get a "rule" stuck in your head that applied to a certain situation and transfer it to other situations that aren't exactly the same.

As for manufacturer's instructions for individual circuits (which I've never seen, but I'm sure there's a dishwasher out there that can clean the seasoning off a cast iron skillet), if you're just installing a receptacle and switched receptacle the appliances don't even have to be installed.

The inspector would have to be a real ninny to demand the installation instructions for the dishwasher on the off-chance it includes that instruction.

Anybody have a list of dishwasher brands that require individual circuits?

 

[Electric-Light]

A worst case scenario 1/2 HP 115 volt motor may draw 58.8 amps according to the table.

The surge when energizing could still be more then that - but only for a few cycles, locked rotor is what it draws when the shaft is not turning and the magnetic fields are established.

The worst inrush occurs when energized at zero crossing and it depends on residual magnetism too. Something that can come into play when a large transformer is connected across the line.

I could see a disposer motor drawing 60 or more amps for a few cycles when first energized, but I bet most won't draw that much if the rotor is locked after the fields are established.

The vast majority of household disposals are meant to be brought up to speed unloaded but its fairly common for residues to become crusty or the grinder gets rusted up and exceed the starting torque and the probability is fair. It can stay energized with locked rotor for a few seconds before the thermal trip in the unit or the panel opens the circuit.

We don't need protective devices if nothing ever went wrong. Design practice needs to provide protection against disruption to other devices from fairly likely deviation. I think its pretty much a guarantee that a disposal will trip on overload on locked rotor multiple times over its useful life.

The amount of voltage drop at the receptacle depends on multiple factors. The PoCo transformer size, size and length of run from transformer to panel, and branch circuit length. The transformer size is probably so big that it might as well be a non-factor for a large apartment complex, but the single biggest determining factor for a rural cottage.

What if the dishwasher with the digital control has a 1/2 hp motor?

The responsibility falls on the engineer to consider real world field conditions.
I'd say any rotating components in a dish washer motor is lighter than a disposal and the duration of sag is shorter.

Test laboratories don't always have a utility fed 480/277 at the bench. The available fault current on 480/277 from a bench power supply is almost nothing. The very first thing in many electronic power supplies (such as GSFL and LED ballasts) is a rectifier and a big capacitor. It draws a tremendous amount of current on power-up. A test bench power source will sag significantly and limit current. The input stage sees substantially more inrush current when fed a native 277v from the utility and devices that didn't consider this had a drastically high failure rate in the field.

 

[Netpog]

View attachment 15625
Goldstar's interlock switch gets the design award of the day.

1) Disconnects Dish.Washer when disposal runs, isolating the circuit to one appliance at a time.[...]

Thanks for this! I've been almost getting around to installing an instant hot under my sink, and this will save me running another circuit. (I'd already been planning to replace my batch-style disposer with a switched one, so this solves two problems.)

Now, for the wall-switched disposers, you could accomplish the same objective (turning off the instant hot when the disposer is actuated) by replacing the disposer's switch with a SPDT wall switch. All I need is a maintained-contact double-throw switch that does not have a center-off position.

EDIT TO ADD A NOTE TO SELF: Dummy! You've dozens of them on-hand. They're call three-way switches. Now go to bed and wake up smarter.

 

[JohnE]

Am I the only one wondering whether the OP may just want to run the question by the inspector for this jurisdiction and get his interpretation on this potential installation?

 

[ramsy]

Now, for the wall-switched disposers, you could accomplish the same objective (turning off the instant hot when the disposer is actuated) by replacing the disposer's switch with a (3-way switch).

I admit your brilliance, with a 3-way switch. Never saw that coming, but will capitalize on your idea, and charge appropriately.

 

[jaylectricity]

I admit your brilliance, with a 3-way switch. Never saw that coming, but will capitalize on your idea, and charge appropriately.

Make sure you have a separate disconnect. If they're both cord and plug you're fine. If not, you need an additional disconnect switch for the circuit.

Sometimes I do attic fans on three-ways where one traveler energizes the fan and the other energizes the thermostat. But the common is fed from a single pole right next to it in the box.

 

[Netpog]

Make sure you have a separate disconnect. If they're both cord and plug you're fine. If not, you need an additional disconnect switch for the circuit.

Good point, that disconnects are necessary, but if the instant hot is cord/plug, isn't that enough? Toggling the three way down kills the disposer. Then you unplug the HW, and everything is dead, under the sink.

Sometimes I do attic fans on three-ways where one traveler energizes the fan and the other energizes the thermostat. But the common is fed from a single pole right next to it in the box.

I like that plan. Except that this is where I do want the center-off option, for when you just don't want the fan to run, and don't want the extra switch in that box.

 

[Electric-Light]

There's an appliance called microfridge which I don't quite understand. It's a low power microwave with an outlet that the fridge it sits on plugs into.
It disables the outlet so the fridge can not operate while the microwave is running.

The fridge can require quite a starting current for its size, so I can see why it might make sense to delay the start-up but to shut power off to the fridge while running to save 100W of demand seems silly

 

[kwired]

Good point, that disconnects are necessary, but if the instant hot is cord/plug, isn't that enough? Toggling the three way down kills the disposer. Then you unplug the HW, and everything is dead, under the sink.



I like that plan. Except that this is where I do want the center-off option, for when you just don't want the fan to run, and don't want the extra switch in that box.

Disconnecting means must be indicating (marked on/off), so if there is no cord and plug and only a three way switch - you violate this rule, can be a problem with single pole decora type switches as well sometimes.

 

[GeorgeB]

I admit your brilliance, with a 3-way switch. Never saw that coming, but will capitalize on your idea, and charge appropriately.

I think that modern dishwashers have to have continuous power to work "properly". At least with the last 3 of mine, interrupting power "destroys" a running cycle and requires resetting all defaults. One even shows an error code for power lost.

 

[Netpog]

I think that modern dishwashers have to have continuous power to work "properly".

Yes, almost all of them seem "smart" enough these days to be quite stupid about power loss.

The three-way switch concept, and the air-switch-cutout gadget, are much more useful for instant hot dispensers, which don't really care about a minute's shutoff.

But, yes, you'd need cord & plug connections or redundant toggle switches for that required shutoff, since a switch must be labeled "off". (Thanks kwired)

 

[kwired]

I think that modern dishwashers have to have continuous power to work "properly". At least with the last 3 of mine, interrupting power "destroys" a running cycle and requires resetting all defaults. One even shows an error code for power lost.

They still make units with a mechanical timer, they will resume where they were left off when power is interrupted.

Most consumers want all the bells and whistles though, so the place you see the simple timer units the most is in apartments or other rental spaces because they are usually less cost.

 

[GoldDigger]

Dishwashers which have a supplemental water heating function can pull a pretty high current for the heating element at some points in the cycle, and it can last for quite a few minutes.

I have not seen separate rating information for the heating element.

 

[Netpog]

Dishwashers which have a supplemental water heating function can pull a pretty high current for the heating element at some points in the cycle, and it can last for quite a few minutes.

I have not seen separate rating information for the heating element.

Yes, the manufacturers don't generally love giving out information. But I found a service manual online for older Bosch dishwashers. For that vintage, the heater (which is integrated with the circulation pump) draws 9.75 amps. The circulation motor 1.25 amps, for a total of 11 "Total Amps (load rated)"

(The drain pump is 0.75 amps. Although it can run simultaneously with the circulator, it won't drain while it's heating.)

 

[RICK NAPIER]

This requirement gets enforced here and I am not the only one if the manufacturer recommends a dedicated circuit then a dedicated circuit is required.

 

[goldstar]

This requirement gets enforced here and I am not the only one if the manufacturer recommends a dedicated circuit then a dedicated circuit is required.

Of course we are all under the assumption that we have this information at the time of installation. Many times on new construction you don't get that information until the last minute

 

[kwired]

This requirement gets enforced here and I am not the only one if the manufacturer recommends a dedicated circuit then a dedicated circuit is required.

Do you come and inspect a few years down the road when they replace that unit just to make sure the new one also is compliant? If that is how you/your jurisdiction chooses to do things, so be it. Many don't feel that amount of detail needs to be inspected. If the branch circuit device won't hold - it is doing it's job of protecting from overload, and any nuisance becomes an issue between the owner/user and installer/designer.

 

[JohnE]

This requirement gets enforced here and I am not the only one if the manufacturer recommends a dedicated circuit then a dedicated circuit is required.

Do you enforce the manufacturer's instructions on replacement units in existing dwellings for a situation as described in the OP, if the install is otherwise compliant with the NEC? Or is it typical that the dishwashers are already on an individual circuit, as is the case in my area. Not trying to bust chops - it is a sincere question.

 

[RICK NAPIER]

Do you enforce the manufacturer's instructions on replacement units in existing dwellings for a situation as described in the OP, if the install is otherwise compliant with the NEC? Or is it typical that the dishwashers are already on an individual circuit, as is the case in my area. Not trying to bust chops - it is a sincere question.

Not for dishwashers, ranges, clothes washer,dryers, range hood, individual fixtures, or receptacles. The contractor is required to install it per code but it does not rquire a permit or isnspection.