Sizing Fuses for DC Power Supply (Primary Side)

[tan_man]

Hello

I am having trouble finding how to accurately size a fuse for a 1200 Watt DC power supply.

The power supply has a nominal input voltage of 115 VDC.

Can someone explain or show me where in the code I can find out how to accurately size a fuse for the primary side of the power supply?

Thank you very much!

 

[Besoeker]

Hello

I am having trouble finding how to accurately size a fuse for a 1200 Watt DC power supply.

The power supply has a nominal input voltage of 115 VDC.

Can someone explain or show me where in the code I can find out how to accurately size a fuse for the primary side of the power supply?

Thank you very much!

The input is 115Vdc?

 

[tan_man]

The input is 115Vdc?

correct

thanks for your reply

 

[Besoeker]

correct

thanks for your reply

I'm a Brit and not familiar with your code requirements or whether they cover fusing for DC inputs.
All I can offer is a bit of technical advice.
Interrupting DC is a whole different ball game to interrupting AC.

In the nature of what we do, we contend with both.
Cooper Bussmann do a whole range. Including DC.
I suggest that you look at what they have to offer that will do what you want to do.
I have the whole huge jing bang as hard copy and probably in electronic format on my current computer.

 

[plumb bob]

Could it be as basic as simple ohms law? :? OP- have you sized the wire?

 

[Besoeker]

Could it be as basic as simple ohms law?

If DC, that would approximately determine the input current. Allowances would have to be made for conversion efficiency if the output voltage id different to the input voltage. If not, what is the power supply doing?

But fuse selection still has to take into account interrupting capability.

 

[GoldDigger]

As well as inrush current.
And the time constants to actually protect the components in the supply (if that is even possible.)
Often the best you can do is keep the equipment from actually catching fire, limiting it to giving up its magic smoke instead.

 

[iceworm]

Hello

I am having trouble finding how to accurately size a fuse for a 1200 Watt DC power supply.

The power supply has a nominal input voltage of 115 VDC.

Can someone explain or show me where in the code I can find out how to accurately size a fuse for the primary side of the power supply?

The simple amswer is DC power system conductors and OCP are sized no differnt than AC power systems. But I don't think you are asking that.

As you have phrased your question I don't know of any code section. The NEC is not particularly concerned with protecting equipment. The focus is about protecting conductors, or structures from catching fire when a piece of equipment fails. The NEC is not a design guide. Consider 90.1 Purpose, and 90.2, Scope. The nec is about practical safeguarding. Installations are essentially free fron hazard, not necessarily efficient, convenient, or adequate.

It is extremely rare that a stand alone power supply is connected per the NEC - like maybe never. Power supplies are always part of something - they are not utilization equipment of themselves. So, give us some context.

When you say "accurately size", what exactly are you trying to do?

Prevent damage to the PS if output is overloaded
Prevent a fire if the ps fails internally
Something else that I have no clue.​

Is this supply inside of a panel, as in an industrial control panel, UL-508A or NFPA 79? Is the PS listed? If so, the listing number will help.

With the limited information we have, I'd say this is a design issue, not an NEC issue:
Determine the inrush and steady state current draw. Lay out the values on a T-C log-log graph
Pick out a fuse that is above these values. Verify by laying out the fuse trip curve on the same log-log paper.
Select conductors that have an ampacity equal to or greater than the fuse selection.

Tell us some more. There are plenty of bright people on here.

ice

 

[Besoeker]

Determine the inrush and steady state current draw. Lay out the values on a T-C log-log graph
Pick out a fuse that is above these values. Verify by laying out the fuse trip curve on the same log-log paper.
ice

Good advice.
But perhaps not all of the answer in this case.
At the risk of belabouring this point, if the PSU is fed bt 115Vdc, a fuse has to be capable of interrupting DC.
That, I know from experience, can be something of a challenge.

A couple of other points occurred to me since I previously posted on this.

If the PSU has a DC input and a DC output, then it it is probably some kind of DC-DC converter technology. We have used such units and most VFDs use them to generate low voltage DC from the DC bus. My experience is that most have a fold back type of current limit so that output overloading is unlikely ever to be an issue. On that basis it would need catastrophic failure of the unit to result in fuse operation. As you correctly pointed out, to stop things going up in flames.

The other thing that needs consideration is the source of that 110Vdc. Is it actually capable of supplying enough current under fault conditions to blow fuses?

 

[tan_man]

The simple amswer is DC power system conductors and OCP are sized no differnt than AC power systems. But I don't think you are asking that.

As you have phrased your question I don't know of any code section. The NEC is not particularly concerned with protecting equipment. The focus is about protecting conductors, or structures from catching fire when a piece of equipment fails. The NEC is not a design guide. Consider 90.1 Purpose, and 90.2, Scope. The nec is about practical safeguarding. Installations are essentially free fron hazard, not necessarily efficient, convenient, or adequate.

It is extremely rare that a stand alone power supply is connected per the NEC - like maybe never. Power supplies are always part of something - they are not utilization equipment of themselves. So, give us some context.

When you say "accurately size", what exactly are you trying to do?

Prevent damage to the PS if output is overloaded
Prevent a fire if the ps fails internally
Something else that I have no clue.​

Is this supply inside of a panel, as in an industrial control panel, UL-508A or NFPA 79? Is the PS listed? If so, the listing number will help.

With the limited information we have, I'd say this is a design issue, not an NEC issue:
Determine the inrush and steady state current draw. Lay out the values on a T-C log-log graph
Pick out a fuse that is above these values. Verify by laying out the fuse trip curve on the same log-log paper.
Select conductors that have an ampacity equal to or greater than the fuse selection.

Tell us some more. There are plenty of bright people on here.

ice

Thank you for your reply!
Sorry for being too vague.

It's a Class 1 Div II Power Supply
I am trying to prevent a fire or any hazardous condition resulting from failure of the power supply.
I have the output supply fused to protect a valve actuator.

I just need to size a fuse meeting these guidelines but will not nuisance trip.

1200 W / 115 VDC = 10.43 A (@ 100% efficiency)

Does this info help?

Thank you all!

 

[iceworm]

... It's a Class 1 Div II Power Supply
I am trying to prevent a fire or any hazardous condition resulting from failure of the power supply. ...

Okay, that's a good start. Here is the picture I have:

DC source feedeng a DC fused disconnect.
Conductors from the fused disconnect to the power supply.
Power supply is an unknown black box. Output internaly fused.
Conductors from PS to device known as actuator​

... I have the output supply fused to protect a valve actuator.

Design phisosophy (per the worm): The ps output fuse does not protect the actuator. If the actuator starts pulling too much current - it's broke. There is nothing to save. Well maybe it costs less to rebuild if the fire gets put out quickly.It protects the PS from overlaod, it protects the conductors.

The reason I mention this is to reinforce: The fuse feeding the PS is not protecting the PS. As you correctly said:

... I am trying to prevent a fire or any hazardous condition resulting from failure of the power supply. ...

Here is the information we have on the PS:

It's a Class 1 Div II Power Supply
1200 W / 115 VDC = 10.43 A (@ 100% efficiency)

And the design spec is:

... size a fuse meeting these guidelines but will not nuisance trip.

As mentioned earier:
Somebody made it. There is to be a manufacturer/assembler. They have some application data. Does the PS have a nameplate? It is to be installed in a CI, D2. I don't think I have ever seen a piece of equipment rated for installation in a classified area where the nameplate didn't say that on the nameplate.

If you don't have this data, I don't have any techniques available to pick out OCP that I know won't leave the PS glowing red hot if it internally fails.

Highly recommend to look for the mfg/assembler data.

Additional thought: Is this a one-off piece of equipment assembled in a shop?

ice

 

[ggunn]

Thank you for your reply!
Sorry for being too vague.

It's a Class 1 Div II Power Supply
I am trying to prevent a fire or any hazardous condition resulting from failure of the power supply.
I have the output supply fused to protect a valve actuator.

I just need to size a fuse meeting these guidelines but will not nuisance trip.

1200 W / 115 VDC = 10.43 A (@ 100% efficiency)

Does this info help?

Thank you all!

(10.43A)(1.25) = 13A. Glass automotive fuses are rated to 250VDC, and 15A is a common size. Could it be that simple?

 

[GoldDigger]

Caution when generalizing.
The most common glass tubular automotive fuse types are SFE and AGX.
SFE types are only good for interrupting 32V DC.
AGX types, on the other hand are generally rated for 250V, but only up to 10A size. Above 10A, they are only rated for 32V.
There are quite a few other types, with a variety of ratings, but usually with different AC and DC limits.
In addition, not all fuse holders will have a DC interrupting rating to match the fuse types you can fit into them.

Tapatalk!