Maximum load on x-former

[inspector141]

What is the maximum load allowed on a 75 kva transformer, 480 volt delta primary and 120/208 Y secondary? The primary is protected at 125 amp OD. Can the 120/208 volt load on the sec be 260 amps? 208 amps x 1.25= 260 sec amps.
I know the size of the sec OD is not relevant as long as the primary protection does not exceed 125% and note 1, but what is the maximum load allowed? 208 amps or 260 amps?

My understanding is that a transformer can carry a load continuously which means it can be loaded to 260 amps.

 

[kc8dxx]

You can only load that transformer at 208 amps per phase. The values of the overcurrent protection devices is a completely separate issue.

 

[petersonra]

You can only load that transformer at 208 amps per phase. The values of the overcurrent protection devices is a completely separate issue.

Not completely true. if the ambient temperature is lower then what it is spec'ed for, you can put more load on it without harming the transformer. In fact, you can sometimes add cooling fan(s) and load it up even more. The manufacturer will have specific details.

 

[inspector141]

transformer loading

transformer loading

You can only load that transformer at 208 amps per phase. The values of the overcurrent protection devices is a completely separate issue.

I understand the rating and or setting of the OD is a separate matter.
Is the allowance of the 125% for the OD for the in-rush current or is it that the xformer can supply 125% of the rating continuously? If it can supply the load continuously, then why can't the xformer be loaded continuously to 260 amps?

 

[haskindm]

75KVA/208X1.732 = 208 amps. That is the maximum load of the transformer. If the transformer is rated to carry its maximum load continuously, that means you can load it to 208 amps continuously, not that you can overload it. Otherwise the maximum CONTINUOUS LOAD would be 166 amps (80% of 208 amps).

 

[bob]

I do not believe the code requires you to apply the 80% factor to a transformer.

 

[steve66]

I think the 125% is both to allow inrush current, and for the fact that breakers are not made to carry 100% of their rating continously.

So if you want to pull full load from a transfomer continuously, then you need a breaker that is about 125% above the rated current.

Steve

 

[iwire]

75KVA/208X1.732 = 208 amps. That is the maximum load of the transformer. If the transformer is rated to carry its maximum load continuously, that means you can load it to 208 amps continuously, not that you can overload it. Otherwise the maximum CONTINUOUS LOAD would be 166 amps (80% of 208 amps).

Well put and I agree. 100% :smile:

 

[Mike01]

208fla

208fla

Since the xfmr fla is equal to 208A and if the ocpd is sized @125% than couldent you then apply 200A continuniously with a 250A ocpd? or what if you supplied two panels with 150A mcb's each could be loaded to 120A thus making the xfmr overloaded to 240A? you could supply two 150A panels due to the 6 disconnect rule and not exceeding the next largest ocpd of 300A. so the xfmr could operate at over 100% of it's rating. not saying this is a good engineering practice, but it's possible. and what if you provided a 200A 100% rated breaker than you could load the xfmr. more. With that being said I believe it's really a function of your load and what situtation fits your application and if you are running that close to put in a larger xfmr.

 

[jim dungar]

Since the xfmr fla is equal to 208A and if the ocpd is sized @125% than couldent you then apply 200A continuniously with a 250A ocpd? or what if you supplied two panels with 150A mcb's each could be loaded to 120A thus making the xfmr overloaded to 240A? you could supply two 150A panels due to the 6 disconnect rule and not exceeding the next largest ocpd of 300A. so the xfmr could operate at over 100% of it's rating. not saying this is a good engineering practice, but it's possible. and what if you provided a 200A 100% rated breaker than you could load the xfmr. more. With that being said I believe it's really a function of your load and what situtation fits your application and if you are running that close to put in a larger xfmr.

Simply put: a transformer is rated for 100% loading, just like a motor or a resistant heater.

In reality, a transformer is rated for FLA continuously, based on a specific ambient temperature (40?C) + a hot spot allowance (30?C) + the insulation temperature rating (normally 150?C for a dry type unit) for a total temperature of 220?C for a standard ventilated dry type. If you decrease the ambient it can supply more current, if you use force air cooling it can supply more current. But a transformer is a dumb animal, it will try to supply any amount of current you want until it fails.

The NEC only requires the 125% (or 80%) factor for conductors and protective devices, but people get so used to the factor they want to use it whenever they see any ampacity given.

 

[jameselectric]

75KVA/208X1.732 = 208 amps. That is the maximum load of the transformer. If the transformer is rated to carry its maximum load continuously, that means you can load it to 208 amps continuously, not that you can overload it. Otherwise the maximum CONTINUOUS LOAD would be 166 amps (80% of 208 amps).

Either my math is terrible, or something is off. Please explain

75kva (75,000) /208 =360.57

360.57 X 1.732 = 624.519

help me see what you guys are doing here, how you get 208 out of that??

 

[frenchelectrican]

Either my math is terrible, or something is off. Please explain

75kva (75,000) /208 =360.57

360.57 X 1.732 = 624.519

help me see what you guys are doing here, how you get 208 out of that??

James.,,

here is the way you do this

take 75,000 / 208 / 1.73 =208.4 amp [ / = divided ]

the reason you have to do is get wattage [ KVA ]/ [ voltage] / [ 1.73 for 3? loads only if single phase don't use this one ]

this formala will work with other size and voltage as well

Merci, Marc

 

[jghrist]

Either my math is terrible, or something is off. Please explain

75kva (75,000) /208 =360.57

360.57 X 1.732 = 624.519

help me see what you guys are doing here, how you get 208 out of that??

Another way to look at it is to divide the single phase kVA by the single phase (phase-to-neutral) voltage to get the current in each phase.

75 kVA?3=25 kVA=25000 VA
208?sqrt(3)=120 V
25000?120=208 A

It's the same as Marc's calculation when you realize that 3?sqrt(3)=sqrt(3)

 

[inspector141]

transformer loading

transformer loading

OK, just for clarification: assume primary fused at 250%

A. 75000/360= 208 sec amps x 125% OCPD= 260 amp. next size is 300

B. Load is Fluorescent lights at 200 amps continously (ballast load)

C. 200 amp x 125%(cont load) = 250 amps(xformer and conds sized to this)

D. sec OCPD set at 300

E. sec conductors sized at 350 kcmil(310 amps)

F. sec load is 250 amps

I have an engineer who says this is code. Is it?

Are the conductors required to be sized at 125% based on the sec OCPD? Because if they are, then the xformer should be allowed to carry 260 amps. Or are the conductors not required to be sized to the OCPD on the secondary?

 

[inspector141]

transformer loading

transformer loading

OK, just for clarification: assume primary fused at 250%

A. 75000/360= 208 sec amps x 125% OCPD= 260 amp. next size is 300

B. Load is Fluorescent lights at 200 amps continously (ballast load)

C. 200 amp x 125%(cont load) = 250 amps(xformer and conds sized to this)

D. sec OCPD set at 300

E. sec conductors sized at 350 kcmil(310 amps)

F. sec load is 250 amps

I have an engineer who says this is code. Is it?

Are the conductors required to be sized at 125% based on the sec OCPD? Because if they are, then the xformer should be allowed to carry 260 amps. Or are the conductors not required to be sized to the OCPD on the secondary?

 

[bob]

Either my math is terrible, or something is off. Please explain

75kva (75,000) /208 =360.57

360.57 X 1.732 = 624.519

help me see what you guys are doing here, how you get 208 out of that??

The formula should read 75000/(208 x 1.73) = 208 amps.