Derating 3-Phase Receptacles

[fifty60]

I have a 3 phase receptacle rated for 480V and 60A. I am using the receptacle for only a single phase 480V load. I know I need to derate the HP ratings by the square root of 3. Do I also need to derate the amp ratings?

I do not think I would need to derate the amp ratings in this case.

 

[Smart $]

I have a 3 phase receptacle rated for 480V and 60A. I am using the receptacle for only a single phase 480V load. I know I need to derate the HP ratings by the square root of 3. Do I also need to derate the amp ratings?

I do not think I would need to derate the amp ratings in this case.

For compliance with the NEC, as long as the receptacle ampere rating is not less than the branch-circuit rating, you are good to go. Uncertain where you get the HP derating from...???

 

[fifty60]

For example, if I have a 3 phase receptacle that is rated for 20HP, can I connect a 20HP single phase load to it or does it have to be a 20HP 3 phase load?

For single phase I would need to derate, right?

 

[Smart $]

For example, if I have a 3 phase receptacle that is rated for 20HP, can I connect a 20HP single phase load to it or does it have to be a 20HP 3 phase load?

For single phase I would need to derate, right?

Okay, I see what you're saying. Yes you would have to "derate" to connect a single phase motor on a 3? HP-rated device.... but I feel that is not the proper way to do it...!!!

 

[fifty60]

What do you think is the proper way Smart $? Do you mean a better way to derate than the square root of 3?

 

[Smart $]

What do you think is the proper way?

Use a properly-rated, single-phase receptacle.

 

[fifty60]

The problem is the particular company that makes the specific receptacle (meltric) does not have HP ratings for their 460V single phase receptacles. Makes me angry, but they do not. They said they would be able to publish HP ratings by the end of the year for the single phase.

There is no way I can responsibly use that single phase receptacle, so I have to use the 3 phase receptacle and de-rate. I don't think I have much of a choice....

These meltric receptacles are unique. They break the circuit before the plug can be removed. They are unique because they are so compact.

 

[Smart $]

What do you think is the proper way Smart $? Do you mean a better way to derate than the square root of 3?

PS: There's more to "derating" a 3? HP-rated receptacle for use with a 1? motor than sqrt of 3.

 

[fifty60]

Please help me derate it properly. My main intention is that the equipment is safe. We normally do not provide plugs and certainly not the receptacle this is a special situation....

 

[Smart $]

The problem is the particular company that makes the specific receptacle (meltric) does not have HP ratings for their 460V single phase receptacles. Makes me angry, but they do not. They said they would be able to publish HP ratings by the end of the year for the single phase.

There is no way I can responsibly use that single phase receptacle, so I have to use the 3 phase receptacle and de-rate.

What's the 1? motor HP?

 

[fifty60]

It is a single phase 7.5KVA transformer....the equipment is 230V, only 460V is availaible..the equipment (powered from the secondary of the 7.5KVA transformer) has two 1HP compressors, small fan, 2KW of heat, and a .5KVA control transformer.

 

[Smart $]

It is a single phase 7.5KVA transformer....the equipment is 230V, only 460V is availaible..

Okay... but the xfmr power the motor and only the motor?

Still need load data, motor HP...

 

[fifty60]

the transformer powers the entire equipment. Equipment FLA is 28A. I have a 250V 2P+ G 60A 3HP plug from the equipment connected to a 60A receptacle 250V 2P + G 60A 3HP receptacle on the transformer.

The transformer is 7.5KVA. The primary is single phase 460V. I have the primary plug (460V 3P + G 20HP 60A with only 2Poles and ground being used) plugging into a 3P + G 460V 20HP 60A receptacle. Only 2Poles and a ground used on the receptacle as well.

 

[Smart $]

It is a single phase 7.5KVA transformer....the equipment is 230V, only 460V is availaible..the equipment (powered from the secondary of the 7.5KVA transformer) has two 1HP compressors, small fan, 2KW of heat, and a .5KVA control transformer.

Being that you have a combination load, I don't believe you are stuck with using only an HP-rated receptacle... at least not under the NEC. I realize your other posts indicate you manufacture machinery and may be under other requirements. TTBOMK, an HP-rated receptacle has the intent that only one motor is supplied with the receptacle.

I'd prefer to know what compliance requirements are in force before getting into NEC compliance issues...

 

[Smart $]

the transformer powers the entire equipment. Equipment FLA is 28A. I have a 250V 2P+ G 60A 3HP plug from the equipment connected to a 60A receptacle 250V 2P + G 60A 3HP receptacle on the transformer.

The transformer is 7.5KVA. The primary is single phase 460V. I have the primary plug (460V 3P + G 20HP 60A with only 2Poles and ground being used) plugging into a 3P + G 460V 20HP 60A receptacle. Only 2Poles and a ground used on the receptacle as well.

As noted in my other reply, I don;t believe you are stuck with using an HP-rated receptacle....

Seems you are reading my mind and replying before I post... :thumbsup:

 

[fifty60]

Most precise, but not completely accurate, would be Lab Equipment. UL would hold us to UL-61010. The equipment is not UL.

 

[fifty60]

THe major concern here for me is the plug coming from the transformer and the receptacle it will be plugged into. Here are the switch rated plug/receptacles:

http://www.meltric.com/digital-catalog-2013/index.html#29/z

No HP ratings for the 480V single phase, though the 480V 3 phase is rated for 20HP. I would really like to use the 480V single phase but am uncertain if I can or not on the 7.5KVA transformer primary....

The switch rated plugs/receptacles break the circuit before the plug can be removed. The equipment takes about 10 seconds to initialize before any motors/heaters can activate...

 

[Smart $]

Most precise, but not completely accurate, would be Lab Equipment. UL would hold us to UL-61010. The equipment is not UL.

Not familiar with that standard. Does it have any bearing on the matter? Are you satisfied with evaluation under NEC requirements? ...

THe major concern here for me is the plug coming from the transformer and the receptacle it will be plug into. Here are the switch rated plug/receptacles:

http://www.meltric.com/digital-catalog-2013/index.html#29/z

No HP ratings for the 480V single phase, though the 480V 3 phase is rated for 20HP. I would really like to use the 480V single phase but am uncertain if I can or not on the 7.5KVA transformer primary....

Well here we have the issue of whether or not UL requirements have any bearing on the matter. IMO, if the xfmr is cord connected and its load is cord connected, then you do not have to use an HP-rated plug or receptacle.

 

[fifty60]

I think the NEC requirements would be the most relevant in this instance.

 

[GoldDigger]

PS: There's more to "derating" a 3? HP-rated receptacle for use with a 1? motor than sqrt of 3.

What you basically need to do is calculate what size single phase motor would result in the same line (not phase) current as the stated/rated three phase motor.
Unlike a generator loading calculation, I think you only need to look at the current per pole, independent of other considerations. If not, please give us your analysis.

PS: Are you considering that the multiplier from FLA to starting current will be substantially different between a single phase and a three phase motor? Or will there just be a difference in starting torque?

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