Why 400A in primary side of 75kVA, 3Ph, 480V?

M

marcosgue

Senior Member
Location
Tampa
Occupation
Electrician
Hello,The scenario is this:
Existent MDP with 350A MCCB feeding panel in remodel area with 500kcmil CU.
The PM wants to remove the panel and install 400A disco to feed step down Xfmr 75kVA,3Ph,480V and secondary is going to feed 200A disco and temp panelboard to feed power tools during construction.
Is my inderstanding since the primary current rating for the dry type Xfmr is 90A I can install j-box and taped the 500kcmil with 3awg and install 100A.
I told the PM the 400A is not necessary here. Can someone give me some opinion in this matter, any NEC reference.
I appreciate any help. Thanks in advance
 
A 100 amp primary breaker would most likely trip due to inrush current.
You would need to study breaker trip curves compared to transformer inrush data to be sure but in all probability it would not work well.
 
augie47 said:
A 100 amp primary breaker would most likely trip due to inrush current.
You would need to study breaker trip curves compared to transformer inrush data to be sure but in all probability it would not work well.
Click to expand...
Back in the day, we would feed 75kVA 480:208Y/120 at 100A because that fit in lighting panels as a branch breaker. Likewise we would only use a 200A on the secondary size as it was commonly stocked in our area. Losing a couple of amps of capacity did not concern us.
 
augie47 said:
A 100 amp primary breaker would most likely trip due to inrush current.
You would need to study breaker trip curves compared to transformer inrush data to be sure but in all probability it would not work well.
Click to expand...

You will also need to consider the impedance of the supply circuit.

100A primary protection on a transformer with 90A primary current rating is a real risk. It will work just fine if the supply impedance limits inrush current, but can be a real problem if the supply system is stiff.

My lab has a 75 kVA 480:480V delta:eye transformer with 100A primary protection. This transformer gets switched frequently but has never tripped on inrush. But the feeder from the main distribution equipment is quite long and limits the inrush.

To the OP: if this is in the planning stages, plan for > 100A, but if you already have 100A equipment in place you should just try using it.
 
winnie said:
You will also need to consider the impedance of the supply circuit.

100A primary protection on a transformer with 90A primary current rating is a real risk. It will work just fine if the supply impedance limits inrush current, but can be a real problem if the supply system is stiff.

My lab has a 75 kVA 480:480V delta:eye transformer with 100A primary protection. This transformer gets switched frequently but has never tripped on inrush. But the feeder from the main distribution equipment is quite long and limits the inrush.

To the OP: if this is in the planning stages, plan for > 100A, but if you already have 100A equipment in place you should just try using it.
Click to expand...
After your comments I was to consider 125A instead 100A, but never 400A that the PM wants, is ridiculous
 
marcosgue said:
After your comments I was to consider 125A instead 100A, but never 400A that the PM wants, is ridiculous
Click to expand...
Agreed. As @augie47 posted way up there, the _maximum allowed_ is 225A.

The range from 0A up to 225A is a 'design choice' that you get to make; lower numbers mean a greater chance of tripping on inrush, higher numbers mean you have to increase the size of the primary circuit conductors. Less than 90A and you are not using the transformer to full capacity, but perhaps you don't need the full 75 kVA. 125A is probably a good choice unless you have reason to think this installation will have particularly high inrush.

-Jonathan
 
marcosgue said:
Hello,The scenario is this:
Existent MDP with 350A MCCB feeding panel in remodel area with 500kcmil CU.
The PM wants to remove the panel and install 400A disco to feed step down Xfmr 75kVA,3Ph,480V and secondary is going to feed 200A disco and temp panelboard to feed power tools during construction.
Is my inderstanding since the primary current rating for the dry type Xfmr is 90A I can install j-box and taped the 500kcmil with 3awg and install 100A.
I told the PM the 400A is not necessary here. Can someone give me some opinion in this matter, any NEC reference.
I appreciate any help. Thanks in advance
Click to expand...
75 kVA at 480 V is about 90 Amps. The largest primary OCPD rating that would allow for is 250% if using both primary and secondary OCPD, or just a hair over 225 Amps. Table 450.3(B) does not allow next size up ratings on OCPD where primary and secondary protection are both supplied so the largest standard size CB or fuse rating that can be used is 225 Amp. But you would need to use 4/0 Cu for that. I would not be selecting a 100 A OCPD on the primary of this transformer. Too much risk of nuisance trips.

So 400 Amp is a viable rating for the primary disconnect if you put 225 Amp fuses in it. But, it is not required.

Presumably the secondary is 208V. That would give you about 208 Amps. You can go up to the next highest rating over 125% here so you could use up to 300 A.

If the costs of conductors is what is driving things, consider the use of Al instead of Cu.

These are design decisions. I don't know if the PM is responsible for these kind of design decisions in your company or not. I would probably select 200 A OCPD for the primary because it is cheaper than the 400 A rated disconnect but still gives me a lot of protection against nuisance tripping.
 
This transfomer just going to handle regular loads during the fase of construction, that's I was thinking use 125A in primary side and #1awg CU tapped from 500kcmil which is protected by 350A MCCB in the MDP.
In the secondary I've 4/0awg feeding 200A disco
 
marcosgue said:
This transfomer just going to handle regular loads during the fase of construction, that's I was thinking use 125A in primary side and #1awg CU tapped from 500kcmil which is protected by 350A MCCB in the MDP.
In the secondary I've 4/0awg feeding 200A disco
Click to expand...
The #1 AWG would be a tap conductor. Be aware of the requirements for both maximum length and minimum sizing of tap conductors.
 
jim dungar said:
Back in the day, we would feed 75kVA 480:208Y/120 at 100A because that fit in lighting panels as a branch breaker. Likewise we would only use a 200A on the secondary size as it was commonly stocked in our area. Losing a couple of amps of capacity did not concern us.
Click to expand...
I have seen quite a few of those transformers on 90 amp fuses (I assume just lack of knowledge, they didn't know you could exceed the transformer rating). I don't believe they ever didn't hold. I don't think a 100 amp RK5 fuse will ever blow on inrush. If you need to feed it from a smaller panel board, one trick is to use a sub feed terminal block that mounts like a breaker and then come off that to your fused disconnect using the tap rules - or just try the 100 amp breaker and see how it works.
 
electrofelon said:
I have seen quite a few of those transformers on 90 amp fuses (I assume just lack of knowledge, they didn't know you could exceed the transformer rating). I don't believe they ever didn't hold. I don't think a 100 amp RK5 fuse will ever blow on inrush. If you need to feed it from a smaller panel board, one trick is to use a sub feed terminal block that mounts like a breaker and then come off that to your fused disconnect using the tap rules - or just try the 100 amp breaker and see how it works.
Click to expand...
A 125A fuse requires a more expensive and larger 200A switch. Keeping it under 100A kept the job cheaper, with a similar savings by staying at 200A on the secondary. Also in those days it was hard to exceed 100A in the 480V panels if those provisions were not put in at the factory.

Of course now transformers have higher inrush currents than they did 30 years ago, and it is easier to get 125A breakers on the primary.
 
marcosgue said:
This transfomer just going to handle regular loads during the fase of construction, that's I was thinking use 125A in primary side and #1awg CU tapped from 500kcmil which is protected by 350A MCCB in the MDP.
In the secondary I've 4/0awg feeding 200A disco
Click to expand...
A standard 75 Kva installation that we see is 125 amp primary and a 200 or 225 amp secondary. Size the conductors accordingly.
 
You have 500 kCMIL for supply conductors. 400 amp disconnect was probably suggested to give you a place to transition those conductors as they won't typically work out so well in a 100 or a 200 amp disconnect enclosure. 225 amp fuses as mentioned would be fine if you have proper secondary protection. The secondary protection catches overload conditions. If the primary blows but there is no physical damage to the primary conductors - your transformer is bad and a 100 or 125 amp fuse wouldn't have saved it either. After you put in a tap box, the taps and the work to assemble all this you probably not really any further ahead cost wise than just using the 400 amp disconnect.

There also is fuse reducers if you really want to put it on a 125 amp fuse.
 
Is it truly a 3 phase 480 or a single phase 480. If you use Single phase 480 at 75 KVA you have 156.3 amps x 250% if your secondary is protected by 125% and you get 390.75 amps so next size up is a 400 amp breaker is it possible it is single phase.? Other wise 225 amps is the max you could size it to.
 
You must log in or register to reply here.
Top