Fuse Size

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horsegoer said:
Ok, so how does one determine or calculate that a 112.5 xfrmr would be sufficient to run a 150 FLA 480v hoist without any problems?
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You could hire an engineer.

At some point judgement would come into play in this decision and as often as not, judgment is based on experience with a specific situation.
 
horsegoer said:
Ok, so how does one determine or calculate that a 112.5 xfrmr would be sufficient to run a 150 FLA 480v hoist without any problems?
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What KVA is that 150 FLA?
Now * 125%
That is what the transformer size should be for a motor that runs continuously at FLA.

You have a hoist. Most likely it will not see 150 other than at start. The transformer can handle it. Your AHJ may have an issue.
 
ptonsparky said:
What KVA is that 150 FLA?
Now * 125%
That is what the transformer size should be for a motor that runs continuously at FLA.

You have a hoist. Most likely it will not see 150 other than at start. The transformer can handle it. Your AHJ may have an issue.
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Why would the AHJ care? Transformer sizing is not a NEC issue.

Potentially it could see a thousand or more amps at startup. He is going to have to look at the impedance of the transformer and figure out what the voltage drop is going to be during the startup of the motor and decide whether the voltage drop is acceptable or not.
 
petersonra said:
Why would the AHJ care? Transformer sizing is not a NEC issue.

Potentially it could see a thousand or more amps at startup. He is going to have to look at the impedance of the transformer and figure out what the voltage drop is going to be during the startup of the motor and decide whether the voltage drop is acceptable or not.
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I guess I always thought that if my customer hired me to put in a step down transformer it became an issue. POCO supplied, nope.
Yes, my suggestion was simplified.
The only larger hoists I worked with were wound rotor, been awhile, and the last of any size was on a VFD. They never saw FLA.
 
ptonsparky said:
I guess I always thought that if my customer hired me to put in a step down transformer it became an issue. POCO supplied, nope.
Yes, my suggestion was simplified.
The only larger hoists I worked with were wound rotor, been awhile, and the last of any size was on a VFD. They never saw FLA.
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Just goes to show there's a lot more to it then 150 amps - what size transformer?

Probably not something that you want to decide based on advice from random people on the internet who do not know all the facts.
 
horsegoer said:
Ok, so how does one determine or calculate that a 112.5 xfrmr would be sufficient to run a 150 FLA 480v hoist without any problems?
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If the published max output of the xfmr is 135A, and if the startup current of your hoist is only 15A more than that, and if the 150A draw would only be for a second or two, and if the running current of the hoist is less than 135A, and if it would not be run long enough to be considered a continuous load, and if there are no other significant loads on the xfmr secondary, then my best guess, not knowing any more than that, is that it would be fine, but if it makes your BH pucker, get a bigger xfmr.
 
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ggunn said:
If the published max output of the xfmr is 135A, and if the startup current of your hoist is only 15A more than that, and if the 150A draw would only be for a second or two, and if the running current of the hoist is less than 135A, and if it would not be run long enough to be considered a continuous load, and if there are no other significant loads on the xfmr secondary, then my best guess, not knowing any more than that, is that it would be fine, but if it makes your BH pucker, get a bigger xfmr.
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lol. Thanks. Problem is we're trying to find existing 120/208 breaker in the electrical room to feed the xfrmr/hoist. So have to be sure as that would affect 120/208 OCPD choices in exiting gear. My calcs say we'd need a 400A OCPD on the 120/208v side.
 
horsegoer said:
lol. Thanks. Problem is we're trying to find existing 120/208 breaker in the electrical room to feed the xfrmr/hoist. So have to be sure as that would affect 120/208 OCPD choices in exiting gear. My calcs say we'd need a 400A OCPD on the 120/208v side.
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Are you now saying the xfmr is 208 to 480?

If so, I would want an ocpd rating of something on the order of 600 amps on the primary side.
 
petersonra said:
Just goes to show there's a lot more to it then 150 amps - what size transformer?

Probably not something that you want to decide based on advice from random people on the internet who do not know all the facts.
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Well there is the random people of you-tube, or other similar sites and there is a smaller but more qualified group of people on a site like this one. But yes even then people on this site won't know all the facts just reading the content that gets posted, someone will likely know what questions to ask though to get closer to what needs to be known.
 
Goo
kwired said:
Well there is the random people of you-tube, or other similar sites and there is a smaller but more qualified group of people on a site like this one. But yes even then people on this site won't know all the facts just reading the content that gets posted, someone will likely know what questions to ask though to get closer to what needs to be known
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kwired said:
Well there is the random people of you-tube, or other similar sites and there is a smaller but more qualified group of people on a site like this one. But yes even then people on this site won't know all the facts just reading the content that gets posted, someone will likely know what questions to ask though to get closer to what needs to be known.
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Good point. I do believe there are people on here qualified to determine xfmr size, conductor sizes and breaker sizes for a 150a FLA load. Pretty basic calca, no?? Yes some of the calc details im not sure of.
 
horsegoer said:
I do believe there are people on here qualified to determine xfmr size, conductor sizes and breaker sizes for a 150a FLA load. Pretty basic calca, no?? Yes some of the calc details im not sure of.
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It is pretty basic to determine the amount of kVA required for the load.
It can be pretty complicated to determine the maximum kVA output of a transformer, which is why most people simply match the nameplate to the load for transformers 300kVA and smaller
For more information you can look at NEMA and IEEE publications, which are based on the loading profile and the resultant heating and cooling of the transformer.
 
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