No secondary over current protection?

[augie47]

Don, If it went that route wouldn't ground fault indicators be needed?

Could I supply a 60 amp receptacle without OCP?

I still say "no". 210.21(B) says a single receptacle shall have a rating not less than the branch circuit. 210.3 tells us the branch circuit rating is determined by the over current device.
In addition 210.19(C) tells us equipment must be protected by an overcurrent device.
The latitude in 240.21(C(1) for delta-delta or two wire transformers only applies to protection of conductors, not equipment. All of the transformer tap rules require termination at some type overcurrent device(s).

 

[Cold Fusion]

...In addition 210.19(C) tells us equipment must be protected by an overcurrent device.
The latitude in 240.21(C(1) for delta-delta or two wire transformers only applies to protection of conductors, not equipment. All of the transformer tap rules require termination at some type overcurrent device(s).

Aug -
Just asking - not telling.

How about if one dropped the primary OCP to 30A?

Or, if one did not want to change the secondary conductors and the load was 50A or less, then change the primary OCP to 25A?

Provided the transformer didn't trip on inrush, of course.

cf

 

[erickench]

I'll get back to you on this

 

[erickench]

Okay the primary and secondary protection is sized properly according to NEC table 450.3(B).

30,000/3=10,000 VA per phase. Primary current is 10,000/480=20.83 Amps

Applying Table 450.3(B) the protection must be a maximum of 250%.

2.5X20.83=52 Amps.

The 45 Amp circuit breaker is sufficient in this case. But the secondary protection is determined by applying the turns ratio factor.

480/240x45=90 Amps.

The ampacity of 8 AWG at 75' C is 50 Amps so they're are not protected.
But if this is a single outlet then the receptacle rating must not be less than the rating of the circuit which in this case would be 90 Amps. But the secondary current is never gonna go higher than 41.67 Amps by my calculation:
480/240x20.83=41.67

So if there is a violation it's because only one outlet is being used. Had there
been two or more there would not be a problem. But there really is no dangerous condition because the transformer is gonna saturate and the secondary current will reach a maximum level. If you want you can reduce the size of the circuit breaker since 250% is just a maximum.

 

[augie47]

curious, where does the 41.67 come from? with the 45 amp primary what prevents a 90 amp secondary load ?

 

[erickench]

curious, where does the 41.67 come from? with the 45 amp primary what prevents a 90 amp secondary load ?

A transformer saturates when it reaches full load. If my calculation is correct it would be:

10000/240=41.67 Amps

The secondary current will go no higher than this value. You've heard of the short circuit test?

 

[erickench]

Okay if there is a short circuit the secondary current will be very high. But at full load the secondary current is 41.67 amps.

 

[Cold Fusion]

Okay if there is a short circuit the secondary current will be very high. But at full load the secondary current is 41.67 amps.

eric -
It's a 30kva 480D/240D transformer with a 45A primary OCP. I'm pa2b and I can tell with out a calculator that doesn't fit.

Try this one again. I'm curious what you are thinking.

cf

 

[ohmhead]

Well this is from mike http://www.mindconnection.com/Merchant2/graphics/00000001/HoltMasterCalcLibrarySample.pdf

 

[ohmhead]

Well also read 450.3 A your over 6% your at 6.2 impedance that pri brk can be higher 400 % and secondary at 250 % but protect that load with its own circuit protection i agree if you had a fuse disconnect to put on that .

 

[iwire]

But if this is a single outlet then the receptacle rating must not be less than the rating of the circuit which in this case would be 90 Amps. But the secondary current is never gonna go higher than 41.67 Amps by my calculation:
480/240x20.83=41.67

I get 72 amps all day long from the secondary.

30 Kva 480 delta x 240 delta transformer.

30*1000/240/1.73=72.254 amps rated current and I imagine it would be capable of more than that for quite a while.

 

[erickench]

41.67x1.73=72.25 Amps in the secondary. But this would be at full load. As for saturation I was under the impression that the magnetic field reaches a maximum and the secondary voltage loses it's linear relationship with the primary i.e. turns ratio.

 

[iwire]

41.67x1.73=72.25 Amps in the secondary

OK.


But what how does that relate to this?

But at full load the secondary current is 41.67 amps

At full load the secondary current would be 72 and change.

 

[erickench]

The full load secondary amps is higher than the ampacity of the 8 AWG conductors so you may have a problem there. Also, the primary is 36 Amps. 45/36=1.25. It looks like the OCPD is sized only for primary protection.

 

[don_resqcapt19]

Don, If it went that route wouldn't ground fault indicators be needed?

Could I supply a 60 amp receptacle without OCP?

Bob,
Yes, a ground fault alarm would be required. There is also a possible issue with 250.20(B)(1), but my take on that is since one of the ungrounded conductors will exceed 150 volts to ground if you bond X4, that this system is not a system that is required to be grounded as long as there are no line to neutral loads.

The secondary wire and receptacle would have to be rated for 90 amps if you keep the 45 amp primary OCPD.

You would be legal with 60 amp secondary conductors and the 60 amp receptacle if you changed the primary OCPD to 30 amps. If you do that there may be an issue with the transformer inrush tripping the primary OCPD.

 

[pfalcon]

240,21(C)1 says for single voltages only. The tag on the transformer states (2) voltages. If this was a single voltage out i would agree.

Rick

Transformers are multi-use devices. With X4 not connected this is a single voltage transformer by usage. The tag covers all configurations that may be made. Circuit design determines whether 240.21(C)(1) applies.

 

[pfalcon]

Don, If it went that route wouldn't ground fault indicators be needed?

Could I supply a 60 amp receptacle without OCP?

240.21(C)(2) and 240.21(C)(6)
Primary OCPD protects the secondary conductors up to 10' or 25' but must then hit an OCPD.

 

[pfalcon]

Aug -
Just asking - not telling.

How about if one dropped the primary OCP to 30A?

Or, if one did not want to change the secondary conductors and the load was 50A or less, then change the primary OCP to 25A?

Provided the transformer didn't trip on inrush, of course.

cf

Calculated is 45A; Next size up is 50A. Installation of a 30A will guarantee trips. Transformers typically have a 50:1 to 85:1 inrush and require special (dual element) fuses to withstand inrush.

 

[pfalcon]

Okay the primary and secondary protection is sized properly according to NEC table 450.3(B).

30,000/3=10,000 VA per phase. Primary current is 10,000/480=20.83 Amps

Applying Table 450.3(B) the protection must be a maximum of 250%.

2.5X20.83=52 Amps.

The 45 Amp circuit breaker is sufficient in this case. But the secondary protection is determined by applying the turns ratio factor.

480/240x45=90 Amps.

The ampacity of 8 AWG at 75' C is 50 Amps so they're are not protected.
But if this is a single outlet then the receptacle rating must not be less than the rating of the circuit which in this case would be 90 Amps. But the secondary current is never gonna go higher than 41.67 Amps by my calculation:
480/240x20.83=41.67

So if there is a violation it's because only one outlet is being used. Had there
been two or more there would not be a problem. But there really is no dangerous condition because the transformer is gonna saturate and the secondary current will reach a maximum level. If you want you can reduce the size of the circuit breaker since 250% is just a maximum.

20.83A is phase current not line current. Fusing is on line current of 36.1A. Without secondary is limited to 125% or 45A (50A next size up).
Secondary conductors are protected as long as primary OCPD is equal or less than 480/240 * 50A (8AWG) = 100A.

 

[pfalcon]

I get 72 amps all day long from the secondary.

30 Kva 480 delta x 240 delta transformer.

30*1000/240/1.73=72.254 amps rated current and I imagine it would be capable of more than that for quite a while.

Correct on line current.
72A * 125% = 90A (100A next size up)
You should fuse down to required current draw on the secondary.

8AWG max current = 50A
50A * 240/480 = 25A for max primary OCPD to protect secondary conductors

45A minimum on primary OCPD or you will be blowing fuses on the transformers. Size 8awg not sufficient.