Plug HP Rating

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fifty60

Senior Member
Location
USA
When sizing a plug for a motor load, if the plug is not the disconnecting device for the equipment, do you have to consider HP when sizing the plug?

For example, if the plug is interlocking and a seperate UL 98 equipment disconnect is provided with the machine, can you size the plug just off of Voltage and Current, and not consider the HP?
 

fifty60

Senior Member
Location
USA
If a plug is not current interupting, and used on a motor, should it be sized to the inrush current of the motor? Looking at Meltric, for example, they have their switch rated plugs and their non switch rated plugs. The non switch rated plugs do not have any HP ratings. If I use a non switch rated plug for a motor load, do I size it directly to the motor FLA or does the inrush and LRA come into play at all?
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
As I understand it, if you have a proper HP rated disconnect switch, you do not need the plug / receptacle to be HP rated, in which case inrush has no bearing. But then you CANNOT use that plug to disconnect under load, you must use the disconnect switch first. That's why many companies offer the disconnect switches that have a mechanical interlock for the receptacle that prevents someone from pulling the plug out unless the disconnect is open.
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What Meltric (and I hear there are others now) offer is that their plug/receptacle combos are actually UL listed AS the disconnect device. They have a set of contacts behind them that break before you fully separate the pins and sleeves. In their HP ratings of the devices, inrush current is part of the criteria, so again, you don't need to worry about it.
 

fifty60

Senior Member
Location
USA
The non-HP rated plug would still be affected by the inrush though, right? Is the thinking that it is a short enough duration that the insulation will not be heated enough by the inrush current to degrade the insulation?

I am trying to get a feel for how much the inrush will degrade the insulation, if at all...
 

GoldDigger

Moderator
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Location
Placerville, CA, USA
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Retired PV System Designer
The non-HP rated plug would still be affected by the inrush though, right? Is the thinking that it is a short enough duration that the insulation will not be heated enough by the inrush current to degrade the insulation?

I am trying to get a feel for how much the inrush will degrade the insulation, if at all...

I would be much more concerned about damage to the contact surfaces by the inrush arc striking and the rubbing contact carrying that high current than about thermally induced degradation of insulation.
 

Smart $

Esteemed Member
Location
Ohio
... Is the thinking that it is a short enough duration that the insulation will not be heated enough by the inrush current to degrade the insulation?
...
No need to think. It is a fact... at least it is when using nominally rated products. :D
 

fifty60

Senior Member
Location
USA
Do you think when they are designing the nominally rated plugs, they take inrush currents into consideration when it comes to the insulation?l Or is it simply only a concern for the arc carrying contact that is making and breaking the circuit....also, does the inrush make this arc greater in magnitude, or is the arc only a function of the voltage?
 

Smart $

Esteemed Member
Location
Ohio
Do you think when they are designing the nominally rated plugs, they take inrush currents into consideration when it comes to the insulation?l Or is it simply only a concern for the arc carrying contact that is making and breaking the circuit....also, does the inrush make this arc greater in magnitude, or is the arc only a function of the voltage?
I believe they take inrush into account for plugs that are listed disconnects.

AFAIK arc intensity is directly related to peak inrush.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Inrush takes place on starting a motor. The only way a plug will see motor inrush is if the feeder/receptacle is energized and the plug going directly to the motor is inserted. This is also covered in the rating of a HP rated device. if you have a plug that does not have a HP rating, you cannot use it this way, so no, it will not be designed to handle this. However if your plug is rated for the HP of the load, it's fine. Not a good idea, but technically capable. If already plugged in when something else turns it on, the concerns over inrush /starting current are no different than for a conductor. Do you size a conductor for inrush?

But inrush isn't the biggest problem, drawing an arc by pulling a plug under load is the thing you need to be concerned about, and doing so on an inductive load is especially hard on the device because the inductance of the motor sustains the arc longer, causing more heat and surface damage. Having that HP rating takes that into account and the pins and sleeves will have more material and different treatments to deal with it, or again, the Meltric type system uses a spring loaded set of contacts inside, just like a contactor.
 

Smart $

Esteemed Member
Location
Ohio
... The only way a plug will see motor inrush is if the feeder/receptacle is energized and the plug going directly to the motor is inserted. ...
That is incorrect. The plug experiences the inrush any time the circuit is energized, whether that is a result of energizing by plugging in or already being plugged in and closing the circuit elsewhere. The difference is whether there is an arc between plug and socket contacts to satisfy the inrush.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
That is incorrect. The plug experiences the inrush any time the circuit is energized, whether that is a result of energizing by plugging in or already being plugged in and closing the circuit elsewhere. The difference is whether there is an arc between plug and socket contacts to satisfy the inrush.
But the rest of that point was that the plug is no different than any conductor in the circuit as to how it "experiences" inrush current. So do you worry about the cable being able to handle inrush current?
 

Smart $

Esteemed Member
Location
Ohio
But the rest of that point was that the plug is no different than any conductor in the circuit as to how it "experiences" inrush current. So do you worry about the cable being able to handle inrush current?
No. That is my point about the plug being inserted prior to energization... but aimed toward fifty60... not you.
 
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