First,

Let me start by saying I am new to the forum and joined for others expertise on a rather urgent topic I have for work.

I have a scenario where I am DC powering a load (-48 VDC power supply). The load draws 250 Watts at nominal operation. In order to get the power draw on the line under 100 Watts (NEC code) the OEM said to tie 3 lines (-48 VDC each) to the "single" load terminal. Is the voltage/current/power draw able to differentiate independent lines? i.e. would each line then draw 83.3 watts per line?

It seems to me like all it has done is created redundancy and not evenly split the power across all three feeds.

I have attached an illustration for example. Any feedback is greatly appreciated.



thanks,
J

 

I am not clear on what you are trying to do. Are you trying to use 3 separate power supplies to supply power to one device? Or do you have one power supply and 3 sets of wires going to the device?

Bob

 

sorry for confusion.

Yes, 3 "separate" power supplies. They are technically 3 separate fused positions off of a DC power plant to feed 1 device with a single power input/connector...

 

The power supplies will not share the load equally because of the small, unavoidable differences in output voltage among the supplies. Typically to insure that the power supplies more equally share the load a small resistor in places in series with each output. A resistor value that will drop a few tenths of a volt at the operating current for each supply is a good value to start with, depending upon the unloaded initial difference in output voltages.

If the output voltages are adjustable, then adjust them initially to as close as possible to the same voltage. An easy way to accurately check that that is to use a voltmeter to measure the difference voltage between the outputs, not the output voltage to ground.

 

The power supplies will not share the load equally because of the small, unavoidable differences in output voltage among the supplies. Typically to insure that the power supplies more equally share the load a small resistor in places in series with each output. A resistor value that will drop a few tenths of a volt at the operating current for each supply is a good value to start with, depending upon the unloaded initial difference in output voltages.

If the output voltages are adjustable, then adjust them initially to as close as possible to the same voltage. An easy way to accurately check that that is to use a voltmeter to measure the difference voltage between the outputs, not the output voltage to ground.

So with that said. Will all 3 lines draw power although they may not be equal in draw?

 

So with that said. Will all 3 lines draw power although they may not be equal in draw?

possibly.
Typically the one with the highest voltage will deliver the most power up until it hits its current limit then its output voltage will drop and others will take up the slack. Some systems will increase the output impedance of the power supplies or use other methods to attempt to balance the load..
Its very common in the telecom world (-48VDC) to have power supply systems with tons of rectifiers in parallel but many also feature fairly advances controllers.

Since the OEM recommended this and assuming you didn't just talk to a "sales person" over there.. I'd assume they have the required protection built in to safely be paralleled.

Other times you might use Diode OR'ing.. which can be accomplished with low vf diodes (schottky) or mosfets and ORing controllers.

 

possibly.
Typically the one with the highest voltage will deliver the most power up until it hits its current limit then its output voltage will drop and others will take up the slack. Some systems will increase the output impedance of the power supplies or use other methods to attempt to balance the load..
Its very common in the telecom world (-48VDC) to have power supply systems with tons of rectifiers in parallel but many also feature fairly advances controllers.

Since the OEM recommended this and assuming you didn't just talk to a "sales person" over there.. I'd assume they have the required protection built in to safely be paralleled.

Other times you might use Diode OR'ing.. which can be accomplished with low vf diodes (schottky) or mosfets and ORing controllers.

Thanks mcgyvr.

We have kicked it back to the OEM to verify that this is in fact their "recommended" solution in this scenario

 

What equipment is the OEM delivering? The supply or the load?

Redundant supplies such as this seems a way to back into a code class for low voltage/low power wiring, which is fairly open to applications. Things like outdoor lighting systems that observe that limit need not even be grounded.

Once you get over that limit you need three wire grounded systems, enclosed cable runs, rated cables, DC rated switched and breakers... can be a much more of a mess then using 3 supplies.

 

There are power supplies/Battery chargers which have a load sharing connection via a RJ45 cable. They allow up to 6 units to be paralleled and loadshare.