Okay... but looking at the meltric plugs/recepts they are switch rated. AFAICT, they don't have any 1? that are switch rated. Do the plugs/recpts need to be switch rated?
Derating 3-Phase Receptacles
- Thread starter fifty60
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Works for me... I don't know of any required method. But that certainly is not factoring by sqrt of 3, right?
Also, if you look at the meltric receptacle HP ratings for various voltages and phasing, they pretty much hold true to ampere per pole, but it appears the ampere per pole value diminishes a bit as voltage increases.
http://www.meltric.com/
Standard duty vs. switch rated choices on page.
Switch rated means they can be disconnected under load.
Probably because of the switch rating.
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My reasoning for root 3:
For a resistive load the power in one winding of a three phase load is 1/3 of the total power.
But if we are driving only one phase we can increase the load current by root 3 and keep the same maximum current in the line wires.
Extend that by analogy to a motor load.
Tapatalk...
For a resistive load the power in one winding of a three phase load is 1/3 of the total power.
But if we are driving only one phase we can increase the load current by root 3 and keep the same maximum current in the line wires.
Extend that by analogy to a motor load.
Tapatalk...
I am looking at their DSN60 Decontactor line, and the 2Pole and Ground 480V plug/recept are listed there. Everything in that line is switch rated as far as I can tell. Still not certain what to do, wish I had some code to back up my decision.
The 2P+G 460V 60A Plug/Recept are listed in their online brochure as switch rated, but they DO NOT have any HP ratings....I do not want to give the customer a receptacle and tell them to leave a phase disconnected. But I also do not want to give the customer a plug that is not rated for the transformer load. I spoke with their people several times and they seem indecisive about whether or not I can use the 2P+G for this transformer.
The 2P+G 460V 60A Plug/Recept are listed in their online brochure as switch rated, but they DO NOT have any HP ratings....I do not want to give the customer a receptacle and tell them to leave a phase disconnected. But I also do not want to give the customer a plug that is not rated for the transformer load. I spoke with their people several times and they seem indecisive about whether or not I can use the 2P+G for this transformer.
I don't believe we can increase the load current [rating]... more like we can keep the load current the same or less. Let's say we have a 230V 3? 10HP rated plug/receptacle combo. Motor FLA for that size is 28A. Max 230V 1? motor is 5HP with an FLA of 28A.
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I am saying that the line current for the three phase motor is less than the individual phase current(s), and the HP rating of the disconnect is based on the line current, as you say.
I also would not go by the FLA, since if you actually run the numbers the line current at rated HP for the connector is a lot lower than the FLA, presumably to allow for the inrush current on make as well as the starting current.
I have spent too long thinking about this already, but I still may be wrong.
Tapatalk...
I also would not go by the FLA, since if you actually run the numbers the line current at rated HP for the connector is a lot lower than the FLA, presumably to allow for the inrush current on make as well as the starting current.
I have spent too long thinking about this already, but I still may be wrong.
Tapatalk...
Line current is greater than phase current by ~sqrt 3... assuming a delta connected 3? motor.
Surge current will be roughly the same for 1? and 3? motors of the same voltage and having the same FLA.
Okay... I didn't look at the sub-models.
A 7.5kVA xfmr has a 480V primary rated current of <16A. While it depends on the xfmr protection schema ocpd rating, a 60A recepltacle/plug appears to be extreme overkill...
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So you agree with me, then.
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Not if you consider that the connector mating surfaces have to be rated to actually make contact carrying the full inrush current of the transformer.
For a resistive load the peak current at make and break is no higher than the average current.
In this respect only, a transformer primary and a motor are similar. But the motor surge current will last many times as many cycles as the transformer inrush current.
Ummm... not exactlySo you agree with me, then.
That's correct. I understand the principles involved. Under NEC compliance, ratings aren't base directly on surge current. Let's take for example xfmr size of 7.5kVA. That's be way too small is we based its rating on surge current. We cover surge current with up-factored ocpd... and conductor ratings are based on the ocpd rating. Let's say we opt to use primary and secondary protection schema. Max ocpd protection is 250% of 15.625, or 39A. Upsizing is not permitted for this schema, so max ocpd standard rating is 35A. Now the only reason I can see a 60A receptacle be used is the next lower size is 30A... but what if we could get by with 30A primary ocpd???
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I also understand the process of rating conductor size and the implications of time versus current for breaker sizing.Ummm... not exactly
That's correct. I understand the principles involved. Under NEC compliance, ratings aren't base directly on surge current. Let's take for example xfmr size of 7.5kVA. That's be way too small is we based its rating on surge current. We cover surge current with up-factored ocpd... and conductor ratings are based on the ocpd rating. Let's say we opt to use primary and secondary protection schema. Max ocpd protection is 250% of 15.625, or 39A. Upsizing is not permitted for this schema, so max ocpd standard rating is 35A. Now the only reason I can see a 60A receptacle be used is the next lower size is 30A... but what if we could get by with 30A primary ocpd???
What I would like to point out is that the sizing of contacts --- whether snap switch, CB, contactor or just a load breaking (and therefore also making) connector --- involves additional considerations.
If the connector were not being rated as load breaking, there would be no reason for the steady state current limit to vary at all with the nature of the load.
Look at the load-specific ratings of the humble snap switch for some examples.
Based on experience (not my own, fortunately) a 15A snap switch will not last through many on-off cycles on the primary side of a transformer with an operating current of 15A.
The contacts in general purpose breakers are particularly beefy for just that reason (among other design considerations)
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I'm following you guys for the most part, but will probably have to read over everything another time or two to understand everything (but that goes for almost every thread on this forum)
I am really hoping to be able to use the correctly configured plug and receptacle. I'm not sure if I should concern myself with the HP rating of the plugs/recepts. Does not having the HP rating mean that the plug/recept should only break resistive loads?
The transformer certainly has different inrush and breaking characteristics than a motor, but the characteristics are momentarily similar. But transformers are certainly not pure resistive loads, unless saturated. Would oversizing the plugs to 60A and going with the single phase plugs be a better option than derating the 3 phase plugs?
I am really hoping to be able to use the correctly configured plug and receptacle. I'm not sure if I should concern myself with the HP rating of the plugs/recepts. Does not having the HP rating mean that the plug/recept should only break resistive loads?
The transformer certainly has different inrush and breaking characteristics than a motor, but the characteristics are momentarily similar. But transformers are certainly not pure resistive loads, unless saturated. Would oversizing the plugs to 60A and going with the single phase plugs be a better option than derating the 3 phase plugs?
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Not directly, but it means that the manufacturer has not tested other uses and so will not make any commitment as to suitability.
Not sure what if anything it means in terms of NEC compliance for that use.
That was my reasoning for going with the 3 phase plug. They have published HP ratings for the plug. Now my concern I am properly derating to use as a single phase load only. And if using the 3 phase plug is any different than the single phase plug ratings wise. Should I be reasonably comfortable using a single phase switch rated plug that is oversized for amps for a transformer load? As I mentioned before, the motors and heaters will not be on the instant the transformer is plugged in, there is a delay at startup....
should I even be worried about HP ratings for a transformer in the first place...should I look at the no HP ratings as don't use it on motor loads, and just look at voltage and current ratings for sizing it for the transformer.
should I even be worried about HP ratings for a transformer in the first place...should I look at the no HP ratings as don't use it on motor loads, and just look at voltage and current ratings for sizing it for the transformer.
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All very good questions, and I am running short on answers.
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