35 A Power Supply on 30 A Branch Circuit

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jtinge

jtinge

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Location
Hampton, VA
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Sr. Elec. Engr
I have a cord connected power supply that has a 35 FLA input rating but will have a connected load such that the maximum continuous load will be less than 24 A. Can I power this from a dedicated 30 A branch circuit based on continuous load being less than 80% or do I have to power it from a 50 A dedicated branch circuit based on nameplate rating (35 A x 1.25 = 43.75 A, next high OCPD = 50 A)?
 
I don't think a cord connected load of this type has to have the branch circuit sized to it

Some might argue that it might somehow annoy the UL listing to replace the plug with one for a 30A circuit.

It is not going to get inspected anyway, so why worry about it all that much.
 
Things are not sized on the basis of rated capacity, but on the load they will be called upon to carry. Indeed, the design process starts by determining the load, and then we size everything upstream on the basis of that load. With a load of 24 amps, a 30 amp circuit will do fine.
 
charlie b said:
Things are not sized on the basis of rated capacity, but on the load they will be called upon to carry.
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I have to say that the actual load may not be the load we can use for circuit sizing, we may well have to use the units rated capacity.

220.14 Other Loads ? All Occupancies. In all occupancies,
the minimum load for each outlet for general-use receptacles
and outlets not used for general illumination shall
not be less than that calculated in 220.14(A) through (L),
the loads shown being based on nominal branch-circuit
voltages.

(A) Specific Appliances or Loads. An outlet for a specific
appliance or other load not covered in 220.14(B) through
(L) shall be calculated based on the ampere rating of the
appliance or load served.
Click to expand...

In my opinion if the unit says 'Input current 35 amps the circuit will have to be sized for 35 amps or more regardless if it only runs a load of 1 amp.
 
iwire said:
I have to say that the actual load may not be the load we can use for circuit sizing, we may well have to use the units rated capacity.



In my opinion if the unit says 'Input current 35 amps the circuit will have to be sized for 35 amps or more regardless if it only runs a load of 1 amp.
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I have to agree. If the label said "Maximum input current 35 amps", or similar, one might have a valid argument for a smaller circuit.
 
iwire said:
I have to say that the actual load may not be the load we can use for circuit sizing, we may well have to use the units rated capacity.



In my opinion if the unit says 'Input current 35 amps the circuit will have to be sized for 35 amps or more regardless if it only runs a load of 1 amp.
Click to expand...

I think if you are going to argue the outlet is only used for a specific piece of gear you are right. If it is just a GP utility outlet, how would you know what load is going to be hooked to it to even make a calculation.
 
jtinge said:
I have a cord connected power supply that has a 35 FLA input rating but will have a connected load such that the maximum continuous load will be less than 24 A. Can I power this from a dedicated 30 A branch circuit based on continuous load being less than 80% or do I have to power it from a 50 A dedicated branch circuit based on nameplate rating (35 A x 1.25 = 43.75 A, next high OCPD = 50 A)?
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My only question is: what kind of power supply is this, and if this power supply came with a cord and plug connection, what is the rating of this cord and plug?, there are some power supplys that are allowed to work from a range of circuits such as vehicle charging stations, where the output can be limited to limit the amount of input load.

Ok that was two questions:ashamed:
 
petersonra said:
I think if you are going to argue the outlet is only used for a specific piece of gear you are right. If it is just a GP utility outlet, how would you know what load is going to be hooked to it to even make a calculation.
Click to expand...

The circuit does not seem to be completely installed yet.

jtinge said:
I have a cord connected power supply that has a 35 FLA input rating but will have a connected load such that the maximum continuous load will be less than 24 A. Can I power this from a dedicated 30 A branch circuit based on continuous load being less than 80% or do I have to power it from a 50 A dedicated branch circuit based on nameplate rating (35 A x 1.25 = 43.75 A, next high OCPD = 50 A)?
Click to expand...

petersonra said:
I don't think a cord connected load of this type has to have the branch circuit sized to it

Some might argue that it might somehow annoy the UL listing to replace the plug with one for a 30A circuit.

It is not going to get inspected anyway, so why worry about it all that much.
Click to expand...

The circuit is being designed. As the designer, the OP needs to use the information available, '35 FLA' input rating, and supply the current the equipment may draw.

Suppose it is a UPS. The end user is likely to consider the output as fully available. We have no information to the contrary, all we know is that the load on the output will not exceed 24 amps.

We don't know the efficiency of this equipment, but it is not 100%.

So we don't know anything conclusive about the input current, except that the nameplate says 35 FLA amps.
 
If unit is marked with minimum supply values you may have an argument of what is required.

Otherwise what does it hurt to supply pretty much any circuit or device with less than the rating if the load is going to be less than the rating?

What is wrong with feeding a 125 amp panel with a 60 amp feeder, or supplying a 25 KVa transformer with circuit sized for 10 Kva because that is all the load requires, or putting a 3 hp motor on a 5 hp drive?

I had a customer once that had a 250 HP vfd that they no longer had a use for but needed a 50 HP motor connected to a vfd. So we connected it with overcurrent protection and conductors sized for 50 hp. Has been running fine for about 10 years.

I have also done the mentioned 25 Kva transformer connected to smaller supply, as well as smaller output because temporary power was needed and the larger than necessary transformer is what was readily available.

I have also powered my welder with 50 amp plug from my portable generator with only a 30 amp breaker and receptacle - if you don't weld at a high amperage you can use the thing all day if you want, if you use it too high for too long you will be resetting the breaker. I am not much of a welder but am not going to buy a bigger generator for no more than I do use it in this way. I often weld steel racks to mount panels or control boxes on farms or grain storage sites, if I have someone else do it I am always waiting for them to get it done and they end up holding up my work.
 
kwired said:
I had a customer once that had a 250 HP vfd that they no longer had a use for but needed a 50 HP motor connected to a vfd. So we connected it with overcurrent protection and conductors sized for 50 hp. Has been running fine for about 10 years.
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I have no doubt it works fine, is also a direct NEC violation.


430.122 Conductors ? Minimum Size and Ampacity.
(A) Branch/Feeder Circuit Conductors. Circuit conductors
supplying power conversion equipment included as
part of an adjustable-speed drive system shall have an ampacity
not less than 125 percent of the rated input current to
the power conversion equipment.
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The 35 A rated input power supply in question has a 50 A Nema plug and #8 SO cord. There is an existing 30 A circuit with a fused disconnect the customer wants to use to feed the power supply. The original question was posed to determine if we can install a 50 A receptacle on the load side of the disconnect to feed the power supply or is there a specific code requirement to install a new 50 A circuit for the power supply, even if the load will not exceed 80% of the rating of the 30 A existing circuit.

I don't see how 430.122 applies since the power supply is not part of an adjustable-speed drive system.
 
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jtinge said:
The 35 A rated input power supply in question has a 50 A Nema plug and #8 SO cord. There is an existing 30 A circuit with a fused disconnect the customer wants to use to feed the power supply. The original question was posed to determine if we can install a 50 A receptacle on the load side of the disconnect to feed the power supply or is there a specific code requirement to install a new 50 A circuit for the power supply, even if the load will not exceed 80% of the rating of the 30 A existing circuit.

I don't see how 430.122 applies since the power supply is not part of an adjustable-speed drive system.
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Someone brought up an ASD application that worked fine, made sense for what he was doing, but did not meet code. Was not related to your problem.

The bottom line is this. What you have can be made to work. It will be safe, and adequate. It maybe cannot be made to meet code, at least I cannot think of any way that is sure to meet code.

Do I personally have a big issue with just putting a 50A receptacle in place of the 30A one? Nope. It is perfectly safe, and will serve the purpose. It just probably cannot be made to meet code IF (note the IF) you decide that this outlet is dedicated to this particular power supply. If it is just a general purpose outlet, that is another story.

Personally, I would probably just add a 50A outlet in parallel with the existing 30A outlet, and put a sign on it that says "30A max".
 
iwire said:
I have no doubt it works fine, is also a direct NEC violation.
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Have not looked yet but does it require overcurrent protection sized for the capacity of the unit also. Otherwise I am code compliant to install 500 KCMIL conductors and put them on a 100 amp breaker??

Any ideas for the basis of this requirement? Seems like it assumes the 250 Hp drive in my case will have a 250 Hp motor connected to it. I can understand requiring full sized conductors if the overcurrent protection is not reduced.
 
kwired said:
Have not looked yet but does it require overcurrent protection sized for the capacity of the unit also. Otherwise I am code compliant to install 500 KCMIL conductors and put them on a 100 amp breaker??

Any ideas for the basis of this requirement? Seems like it assumes the 250 Hp drive in my case will have a 250 Hp motor connected to it. I can understand requiring full sized conductors if the overcurrent protection is not reduced.
Click to expand...
I think only the conductors have to be sized that way. The OCPD can be any size that protects the conductors.

My guess is that someone decided that the "correct" way to size the incoming conductors on a VFD was 125% of the maxiumum current the VFD would draw and never considered someone might ever want to draw less than that.

Personally, I have never quite understood the 125% thing on any circuit. Either the wires are big enough or they aren't. What difference does it make if it is a motor circuit or if it never runs more than 239 minutes, or sometimes runs 241 minutes?
 
The power supply is used to power a calibration weight basket for a cryogenic wind tunnel model preparation assembly.
 
Volta said:
Wouldn't that take this farther away from compliance? 210.21(B)(2).
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compliance with what?

stop thinking this outlet is dedicated to some particular load. it isn't. it is a general purpose outlet. people can plug whatever the heck they want to into it. just like any other outlet.

now if you decide the outlet is dedicated to a single piece of equipment, you have a different issue.

so, the answer is, the outlet is NOT dedicated to this piece of equipment. it is just an outlet.
 
petersonra said:
compliance with what?

stop thinking this outlet is dedicated to some particular load. it isn't. it is a general purpose outlet. people can plug whatever the heck they want to into it. just like any other outlet.

now if you decide the outlet is dedicated to a single piece of equipment, you have a different issue.

so, the answer is, the outlet is NOT dedicated to this piece of equipment. it is just an outlet.
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Bob, I think you have it backwards. If the existing 30A circuit had a 30A receptacle on it, and a 50A receptacle was installed on the same circuit, the 50A receptacle would be in violation of 210.21(B)(3).

However, if the existing 30A circuit was going to be dedicated to this power supply, such that the 50A receptacle installed was a single receptacle on an individual branch circuit, then the 50A receptacle would be OK per 210.21(B)(1).

Being a dedicated 50A outlet on the 30A circuit would make the installation acceptable.
 
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