Current to use for design when only given max OCPD size

[Pitt123]

I'm sizing cables for a particular piece of equipment however the datasheets only give me the maximum upstream OCPD for the piece of equipment and not the actual input current. It gives the max OCPD upstream as 40A.

The piece of equipment is a battery charger and gives an output continuous current of 30A.

As far as far as the current to use for sizing cables what would be the best method to select the current. I initially based it off 30A input current and said that 30A X 1.25 = 37.5A and the next size up allowed for the 40A breaker as listed. Therefore I would base my design off of an input current of 30A and a design current of 37.5A . Does this seem right or should I use 40A as the base input current?

 

[iwire]

I would not use the output current for anything unless you had information about the units efficiency and of course you would need to convert the output to watts.

 

[Pitt123]

I would not use the output current for anything unless you had information about the units efficiency and of course you would need to convert the output to watts.

I wasn't necessarily using the output current but rather back calculating the current based off of the 40A OCPD on the input. I said that 40A/1.25 =32A. So I based this off of a 30A input. Mabye I should use 32A although I dont think it makes much difference.

 

[Jraef]

I wasn't necessarily using the output current but rather back calculating the current based off of the 40A OCPD on the input. I said that 40A/1.25 =32A. So I based this off of a 30A input. Mabye I should use 32A although I dont think it makes much difference.

Maybe I'm just being thick, but I guess I don't see why you are splitting the hairs here. You can't get a 37.5A breaker anyway. Put in the 40A CB and size the conductors for 40A. What's the issue? Is it because you have a VD issue to contend with and want to determine the conductor size based on the FLC? If so, I would assume the higher value; safer bet.

 

[mbeatty]

Maybe I'm just being thick, but I guess I don't see why you are splitting the hairs here. You can't get a 37.5A breaker anyway. Put in the 40A CB and size the conductors for 40A. What's the issue? Is it because you have a VD issue to contend with and want to determine the conductor size based on the FLC? If so, I would assume the higher value; safer bet.

I agree with Jraef. Go with 40A for both.:thumbsup:

 

[Pitt123]

I agree with Jraef. Go with 40A for both.:thumbsup:

If you assume the input current is 40A then a 40A breaker would be too small because 40*1.25 = 50A. If you are saying to just size the wire for 40A and use a 40A breaker, then this 40A rated wire does not account for any de-rating for temperature and conduit fill. My typical procedure is to choose a wire size, then apply the de-rating factors then verify that de-rated wire ampacity is still greater than load and works with given OCPD.

 

[Dennis Alwon]

If you assume the input current is 40A then a 40A breaker would be too small because 40*1.25 = 50A. If you are saying to just size the wire for 40A and use a 40A breaker, then this 40A rated wire does not account for any de-rating for temperature and conduit fill. My typical procedure is to choose a wire size, then apply the de-rating factors then verify that de-rated wire ampacity is still greater than load and works with given OCPD.

If the max overcurrent protection is 40 why do you think a 50 may be needed. One has to assume the manufacturer and UL got this correct.

 

[topgone]

If you assume the input current is 40A then a 40A breaker would be too small because 40*1.25 = 50A. If you are saying to just size the wire for 40A and use a 40A breaker, then this 40A rated wire does not account for any de-rating for temperature and conduit fill. My typical procedure is to choose a wire size, then apply the de-rating factors then verify that de-rated wire ampacity is still greater than load and works with given OCPD.

I have to agree with those above posts. It is better to err on the safe side, that is the idea. Using a 40A breaker on a 40A wire means you are overprotecting the wire because a 40A breaker is good for protecting a 32A-rated wire! The usual way is to know the load and choose a wire that will be able to carry the load and deliver the appropriate voltage at the load point. Then you will choose a breaker to protect your wire and your load. Hope that helps.