HP Ratings for Plugs and Receptaclse for Mixed Loads

[GoldDigger]

For this equipment, the motors are wired as 3 phase and the heaters and transformer are single phase.....how does this effect my calculation?

The transformer will be, in effect, an unbalanced motor load, so I would multiply it by 1.732 when adding in its HP equivalent.
Then instead of looking at the VA available for your single phase resistive loads, look at the maximum current on each pole from the 30A limit.

But if you multiply out the transformer, you end up with more than 10HP. <sigh>
The inrush current of the transformer will probably not be as great as that way of estimating gives you, so you may still be just fine.
How much power will you actually be drawing from the transformer secondary?
Can you make sure that the resistive loads are on different phases from the transformer?

 

[fifty60]

The 7.5KVA transformer is stepping down the 460V incoming voltage to 230V. The equipment is entirely powered by the secondary of the 7.5KVA transformer.

I just spoke with someone from Hubbell and he said that he did not think the transformer would have to worry about HP ratings. He did have the disclaimer that he was just an applications guy and did not really know about electrical engineering. I asked to be taken higher up the ladder to someone who knew and they said that it was not possible.

7.5KVA transformer, single phase, 460V. I am still confused on how to rate the plug and receptacle for this. If you could try to clarify some more, I would appreciate it.

Also, the HP ratings are important for being able to break the circuit under load, right?

 

[GoldDigger]

The 7.5KVA transformer is stepping down the 460V incoming voltage to 230V. The equipment is entirely powered by the secondary of the 7.5KVA transformer.

I just spoke with someone from Hubbell and he said that he did not think the transformer would have to worry about HP ratings. He did have the disclaimer that he was just an applications guy and did not really know about electrical engineering. I asked to be taken higher up the ladder to someone who knew and they said that it was not possible.

7.5KVA transformer, single phase, 460V. I am still confused on how to rate the plug and receptacle for this. If you could try to clarify some more, I would appreciate it.

Also, the HP ratings are important for being able to break the circuit under load, right?

Since the transformer will just be providing power to everything else, then as long as you take steps to insure that you do not plug the unit in with the controls in a configuration that will immediately apply voltage to the motor, then you can ignore the transformer completely, IMHO. The increased VA from the transformer power factor will not be an issue either.

Just figure the total of motors then calculate the available remaining VA and compare that to the resistive loads. You should be fine with the 30A setup.

 

[fifty60]

When the transformer is plugged in, the operator must wait 10-15 seconds for the HMI to initialize. When the HMI is initialized, then the operator can turn on the events that will start the motors and heaters. So the loads on the secondary are not energized instantly.

GoldDigger: "Just figure the total of motors then calculate the available remaining VA and compare that to the resistive loads. You should be fine with the 30A setup."
I am not 100% what you are meaning here...


I have already calculated the load to size the transformer. I have the 7500VA transformer, at 460V 1 phase. This gives me 16A total on the primary. I fused the primary with a 25A fuse and also have 40A fuses on my secondary. Since the equipments motor and heater loads will not have energy immediately applied to them, I can go ahead forgo the HP ratings and size it off of the current and voltage requirements for the primary?
The secondary has an FLA of 28A.

The way the Meltric switches are designed, a pawl on the receptacle has to be pressed before the plug can be removed. Pressing the pawl breaks the circuit and the plug has to be rotated 30 degrees before it can be removed. They have the patent on this design, and to do it with a different manufacture a larger mechanical interlock must be used. Does this make the plug safe to unplug under load since the circuit is broken before the plug can be removed?

 

[fifty60]

I need to add that the transformer primary is fused through the receptacle. The plug and chord connect directly to the primary of the transformer. The secondary is fused at the equipment. If I fused the primary (by attaching fuses to the transformer) would the transformer then become a "Branch Circuit", and allow for me to avoid the motor circuit requirements all together?

 

[jim dungar]

The HP rating of a device, has to do with its ability to 'make' inrush current and 'break' locked rotor current. For the most part, there is about a 6X multiplier used on the continuous current rating of the motor nominal FLA, from the NEC.

So as long as the continuous rating of the device is sufficient for the total nominal load, your only other concern is that the motor/inrush loads do not exceed the HP rating.

The inrush characteristics of transformers are very different than that of motors. I can't ever remember seeing any specific application data about contactors and plugs in regards to general purpose transformer inrush. So for the most part I use KVA=HP, and then go on to another task.

 

[fifty60]

The HP rating of a device, has to do with its ability to 'make' inrush current and 'break' locked rotor current. For the most part, there is about a 6X multiplier used on the continuous current rating of the motor nominal FLA, from the NEC.

So as long as the continuous rating of the device is sufficient for the total nominal load, your only other concern is that the motor/inrush loads do not exceed the HP rating.

The inrush characteristics of transformers are very different than that of motors. I can't ever remember seeing any specific application data about contactors and plugs in regards to general purpose transformer inrush. So for the most part I use KVA=HP, and then go on to another task.

Does this mean that if the plug/receptacle is rated for 2HP, the the FLA for a 2HP motor from table 430.248-250 is multiplied by 6 and that is used as the current the receptacle can safely break?

Can you please explain a little more here....I am almost understanding...My equipment has an FLA of 23A, my plug is rated for 30A, as long as the motor loads are within the HP rating, and the entire equipment load (resistive and inductive) are within the continuous rating of the plug, I am fine? Are you saying I do not have to include the resistive loads in the HP calculation?

Hmmm, KVA=HP...So 7.5KVA Transformer, the 7.5HP rated plug? The problem with the Meltrics is that their single phase 480V plugs do not have any HP ratings, they are still in the process of publishing the ratings....

 

[jim dungar]

Does this mean that if the plug/receptacle is rated for 2HP, the the FLA for a 2HP motor from table 430.248-250 is multiplied by 6 and that is used as the current the receptacle can safely break?

That is a reasonable explanation for the result, of course the actual UL procedure is more complicated (it includes PF and timing). The 6X multiplier is how much 'inductive current' the device can make/break, it would be possible for a device to be rated for making only 24A of inductive load yet be able to handle 30A of resistive load.

Can you please explain a little more here....I am almost understanding...My equipment has an FLA of 23A, my plug is rated for 30A, as long as the motor loads are within the HP rating, and the entire equipment load (resistive and inductive) are within the continuous rating of the plug, I am fine?

Yes.

Are you saying I do not have to include the resistive loads in the HP calculation?

Yes.

Hmmm, KVA=HP...So 7.5KVA Transformer, the 7.5HP rated plug?

Yes, but do not use it backwards.

The problem with the Meltrics is that their single phase 480V plugs do not have any HP ratings, they are still in the process of publishing the ratings....

Personally, I would trust their tech support, but I'm not a UL Inspector.

 

[fifty60]

their tech support shy'd away from saying they would recommend the 480V 30 amp rated 2P+G plug/receptacle with the 7.5KVA transformer. They actually seemed a little baffled by the question. Meltric said their 2P+G was in the process of having the UL testing done...The 3P+G however does have the HP rating but I don't think I can use the ratings because the transformer will be connected as a single phase load.

The support at Hubbell said they didn't think i had to consider the HP ratings for just the transformer, but then became a little defensive when I started to ask more specific questions...they said they could only really recommend the much more expensive mechanical interlock receptacle...which I don't want to buy if I do not have to.

 

[GoldDigger]

their tech support shy'd away from saying they would recommend the 480V 30 amp rated 2P+G plug/receptacle with the 7.5KVA transformer. They actually seemed a little baffled by the question. Meltric said their 2P+G was in the process of having the UL testing done...The 3P+G however does have the HP rating but I don't think I can use the ratings because the transformer will be connected as a single phase load.

The support at Hubbell said they didn't think i had to consider the HP ratings for just the transformer, but then became a little defensive when I started to ask more specific questions...they said they could only really recommend the much more expensive mechanical interlock receptacle...which I don't want to buy if I do not have to.

OK, I think that I am finally getting the picture. Even your motor loads are all single phase, and all of them will end up on the same phase since they all go through the transformer.

You will have to derate the HP rating of connector by 1/1.732 (or more) for a single phase feed.
The other way to proceed, as you were thinking earlier, would be to go for a single phase connector. Unfortunately if you are using 460V delta for your supply, you will need to use a connector that can handle two ungrounded phase wires and a circuit ground. If one terminal of the connector is identified as a grounded conductor you will be out of luck. Fortunately a connector for two ungrounded phase lines may be available.

 

[fifty60]

So I would need to derate by 1/1.732 if i use a 3 phase plug and receptacle? The power supply is 480Y/277. I am actually not 100% sure that it is not delta. 480Y/277 is the more common configuration in the US, right? I cannot use the 2P+G configuration if the supply is Delta?

I am not using a neutral, I am just using 2 phases and a ground (2P+G plug/receptacle). Do you see a problem somewhere with this GoldDigger? The customer is installing the receptacle, so no need to give them a 3P+G receptacle/plug right?

Or are you saying that I will need to derate the HP ratings if were actually using 3Phase power and all single phase loads? That is not the case with the 7.5KVA transformer. Single phase all the way from the primary of the the transformer to the loads on the secondary of the transformer.

My options are this: Use the Meltric 480V 2P+G 30A plug and connector that does not have any motor disconnect ratings, but only branch circuit disconnect ratings. I could use the 60A version of the same plug just to oversize for safety. The problem is the single phase 60A plug does not have HP ratings, just voltage and current ratings. Same for the 100A meltric single phase 480 plug. I could purchase the 3P+G plug, but again I only need single phase 460V.

I could go the hubbell route and buy the much more expensive mechanical disconnect receptacle. They are rated for 20HP. I do not want to do this if it is not necessary.

 

[fifty60]

OK, I have my plan here. I will use a 3 Phase 480V plug that is rated at 60A and 20HP. Since I am only using 2 phases, 460V to supply my transformer I will derate by 20HP/1.73 and be left with 11.56HP. This will be sufficient HP for my 7.5KVA transformer.

The 2Pole plug does have 2 hots and a ground (i think what you were meaning earlier GoldDigger is that maybe it only had 1 hot, 1 neutral, 1 ground) but it does not have any published HP ratings, so I do not feel comfortable using it.

Does anyone see any problems with the 3P + G 480V plug on a 460V single phase and ground system as long as I derate the HP ratings? Would I also derate the Amp ratings?

 

[fifty60]

I would like to put a meltric receptacle on the transformer secondary so that the equipment can be plugged directly into the transformer.

The secondary is 230V. Could I use a 250V 2P+G receptacle or should I use a 1P+N+G receptacle? There is no centertap on the transformer to get 230V...