I am trying to size switch rated plugs and receptacles for a piece of equipment I am working on. My concern is with the HP rating. For motors, compressors, I just add up the respective HP's of each motor/compressor. What about transformers and resistive heater loads? Do I divide the total watts by 746 to convert to horsepower?
HP Ratings for Plugs and Receptaclse for Mixed Loads
- Thread starter fifty60
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That would give you a very conservative figure. The HP rating of a connector will be lower than the wattage (current) rating of the connector divided by 746 because a motor load will have both starting surge current and inductive kick on disconnect.
I do not know of a formal way of allowing for adding resistive loads to the motor load. For a transformer load, the inrush and inductive kick considerations are such that dividing the rating by 746 and adding it to the real motor HP may not be a bad approximation.
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When you have multiple motors, you can just add the HP values to be conservative or you can ask whether the motors will be A. starting and B. running at the same time or not.
[Answer moved from original Off Topic thread where fifty60 first posted it. ]
Sorry I moved the thread, I thought I was creating a new thread. Long day and I am a little under the weather.
The equipment will have a main fuse of 30A. The incoming voltage is 460V 3PH 60Hz. I have 2 compressors that will have 2HP each for a total of 4HP. There is also a fan motor that will add another 1/4HP. I have a 3KVA general purpose transformer and a 3340W heating load. I am looking at these IEC rated disconnect rated plugs.
http://www.meltric.com/digital-catalog-2013/index.html#/27/
I am looking at the 480V 3P+G part number 63-34043 and 63-38043. These plugs are rated at 10HP. If I add up the above I get 4.25 (compressors) 4hp (transformer kva/746) and 4.5HP (heater load). This comes up to a grand total of 12.75 which would be over my 10HP rating.
Is this the right approach to sizing these switch rated plugs and receptacles?
If I take the 460V * the 30A ratings of the plugs, I get 18.49HP rating for the plug....
The equipment will have a main fuse of 30A. The incoming voltage is 460V 3PH 60Hz. I have 2 compressors that will have 2HP each for a total of 4HP. There is also a fan motor that will add another 1/4HP. I have a 3KVA general purpose transformer and a 3340W heating load. I am looking at these IEC rated disconnect rated plugs.
http://www.meltric.com/digital-catalog-2013/index.html#/27/
I am looking at the 480V 3P+G part number 63-34043 and 63-38043. These plugs are rated at 10HP. If I add up the above I get 4.25 (compressors) 4hp (transformer kva/746) and 4.5HP (heater load). This comes up to a grand total of 12.75 which would be over my 10HP rating.
Is this the right approach to sizing these switch rated plugs and receptacles?
If I take the 460V * the 30A ratings of the plugs, I get 18.49HP rating for the plug....
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- Placerville, CA, USA
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- Retired PV System Designer
You did start a new thread, you are not that far out of it.
But you deleted the question from your post in the original thread while I was answering it.- Location
- Placerville, CA, USA
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- Retired PV System Designer
Since you are not changing the voltage requirements of your equipment, I would go for the higher current connectors.
The higher HP rating for the higher voltage connectors is based on your running the motor at that higher voltage.
I just spoke with Eaton regarding contactors. They told me that I do not have to consider HP ratings for their Contactors if I am dealing with a resistive load. Why do I have to include the heater in my HP ratings of plugs and receptacles? Shouldn't the HP rating only be sufficient for the motor loads and maybe the transformer, but only the Voltage and Current rating for the heater?
I just spoke with another plug/receptacle manufacturer and they told me that I do not have to consider the HP ratings of the Plugs and Receptacle because the compressor's, transformers, heaters, etc all have their own branch protection.
Does this make sense to anyone else out there? Does having branch protection change the HP requirement for the plug and receptacle?
Does this make sense to anyone else out there? Does having branch protection change the HP requirement for the plug and receptacle?
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- Placerville, CA, USA
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- Retired PV System Designer
No.
Tapatalk...
Tapatalk...
If I increase my plug and receptacle to the next Amp size, do I also have to increase the AWG of my cable? For example, the equipment I am working on now will require a 30A main fuse (24A FLA) so I would normally use 8/4 SOOW cable. The 30A rate plug and receptacle is only rated for 10HP. Counting my heaters and transformer I have 12.6 HP from my equipment.
Does using the larger receptacle necessitate using a larger AWG cable?
Does using the larger receptacle necessitate using a larger AWG cable?
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- Placerville, CA, USA
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- Retired PV System Designer
In general, no.
There are some specific cases (probably not affecting you) where you cannot just install a larger receptacle. 120V 20A for example.
The OCPD must match the wire, not the receptacle.
Tapatalk...
There are some specific cases (probably not affecting you) where you cannot just install a larger receptacle. 120V 20A for example.
The OCPD must match the wire, not the receptacle.
Tapatalk...
Would you be comfortable using a plug rated for 10HP on a 7.5KVA transformer with 25A primary fuses?
7500/745.7 = 10.05HP
I do not have enough experience yet to say, yeah, that will be fine. I would think with a transformer you would have almost the exact HP calculation since the transformer rating is in VA...so the 10HP rated 30A plug would also be fine. I would have to go to a 60 amp rated plug to get a 20HP rating....
Do I have to consider the HP on the load side of the transformer, or would I just use the KVA/745.7?
7500/745.7 = 10.05HP
I do not have enough experience yet to say, yeah, that will be fine. I would think with a transformer you would have almost the exact HP calculation since the transformer rating is in VA...so the 10HP rated 30A plug would also be fine. I would have to go to a 60 amp rated plug to get a 20HP rating....
Do I have to consider the HP on the load side of the transformer, or would I just use the KVA/745.7?
Are the following statements correct:
1) For a resistive load current and voltage ratings only should be considered when sizing plugs and receptacles.
2) For a motor load, current, voltage and HP rating have to be considered.
3) For a mixed resistive/motor load, the current, voltage, and HP rating has to be considered. The VA from the resisitive load must be converted to HP and added to the motor HP.
I am actually having a hard time finding plugs for my equipment when I convert the resistive loads to HP. We normally do not use plugs for our equipment, and they are directly connected to a disconnect...
1) For a resistive load current and voltage ratings only should be considered when sizing plugs and receptacles.
2) For a motor load, current, voltage and HP rating have to be considered.
3) For a mixed resistive/motor load, the current, voltage, and HP rating has to be considered. The VA from the resisitive load must be converted to HP and added to the motor HP.
I am actually having a hard time finding plugs for my equipment when I convert the resistive loads to HP. We normally do not use plugs for our equipment, and they are directly connected to a disconnect...
- Location
- Placerville, CA, USA
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- Retired PV System Designer
That process will result in the combined rating be lower than a detailed analysis would indicate. Conservative and safe, but more limiting and more expensive.
I would suggest the following instead.
Since the HP rating is the maximum approved motor load on the connector and the volts x amps across all phases is the maximum resistive load, you can convert between the two as follows:
1. Divide the total VA summed over all phases by the rated motor HP.
2. Using that as a conversion factor rather than 736, figure out the HP equivalent of the resistive loads. Use 736 for the transformer loads.
3. Add them up and compare to the rated motor HP of the connector. Be careful that any single phase loads do not overload one or two of the poles.
A different way of looking at it which gives the same mathematical result is this:
1. Since the HP represents the full capability with only one motor and the VA represents the full capability with only resistive, add up the motors (and converted transformers) and divide that number by the rated HP of the connector. That will tell you what fraction of the capability is being used.
2. Subtract that number from 1 to find out what fraction is left unused.
3. Multiply that number by the rated VA to find out what how much of the VA capability is left. That is the limit on your resistive loads.
There are all sorts of other subtle points, including the fact that for opening under load having both motor and resistive loads will produce less of an inductive kick than the motor loads alone. And that in an extreme case, the inductive kick from opening the motor connection could actually cause an overvoltage on the resistive loads if they are small.
If you are careful not to ever disconnect under load even though the connectors are rated for that, these should not be issues for you.
What about a transformer load? I am working with a single phase transformer that will be connected to single phase 460V (2 conductors and a ground from a 460 3 phase).
I have just been informed that the the 460V single phase ratings for these plugs/receptacles do not exist! This company sells the 480 2P + G configuration, but they do not have any HP ratings for the plug.
Do I need the HP ratings for just a single phase transformer? There are motor circuits on the secondary of the 7.5KVA transformer, but the plug will only bring power to the primary.
I have just been informed that the the 460V single phase ratings for these plugs/receptacles do not exist! This company sells the 480 2P + G configuration, but they do not have any HP ratings for the plug.
Do I need the HP ratings for just a single phase transformer? There are motor circuits on the secondary of the 7.5KVA transformer, but the plug will only bring power to the primary.
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GoldDigger, when you say:
3. Multiply that number by the rated VA to find out what how much of the VA capability is left. That is the limit on your resistive loads.
Do you mean the rated VA of the Plug and Receptacle? So if it is a 480V/30A plug, then the VA would be 14,400VA.
The HP rating of the Plug/Receptacle is 10HP. I would subtract hp of transformer (4.02HP) + compressors (4HP) and have 8.02 total HP. Divide that total by 10HP and get .802. Subtract .802 from 1 and get .198. Multiply that by the VA rating of the plug (480 * 30) 14,400 and get 2.859KVA. As long as my heating load is below that, I can use the plug?
3. Multiply that number by the rated VA to find out what how much of the VA capability is left. That is the limit on your resistive loads.
Do you mean the rated VA of the Plug and Receptacle? So if it is a 480V/30A plug, then the VA would be 14,400VA.
The HP rating of the Plug/Receptacle is 10HP. I would subtract hp of transformer (4.02HP) + compressors (4HP) and have 8.02 total HP. Divide that total by 10HP and get .802. Subtract .802 from 1 and get .198. Multiply that by the VA rating of the plug (480 * 30) 14,400 and get 2.859KVA. As long as my heating load is below that, I can use the plug?
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- Placerville, CA, USA
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If you are actually using 480 volts three phase, then the total VA of the plug is 480 x 30 x square root of 3. Roughly 24,940 VA, not 14,400
The 10 HP rating is quite a bit lower than you would get from dividing the VA by 736 (=34), for all of the reasons stated earlier.
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