480V chiller supplied at 208V

Status
Not open for further replies.
J

Junior_EE

Member
Location
New York City
Because of supply chain issues, a client is looking to select a 480V chiller despite our building having only 208V. Obviously we need a step-up transformer to accommodate the load.

Here is the info on the chiller @ 480V from the manufacturer:
359A MCA
600A MOPD

In order to size the breaker and wires for this on the 208V side of the step-up transformer, is it simply
MCA @ 208V = 359A * 480V / 208V = 829A?
MOPD @ 208V = 600A * 480V / 208V = 1384A? --> 1400A

Further, on the secondary of the step-up transformer, do I just supply 359A worth of wire to the chiller with 600A circuit breaker? I feel a little leery about having wire ampacity lower than the breaker on a transformer secondary.
 
Doesn't answer the question, but does the building have the capacity to add that size load at 208? Could be a long shot, but it is possible to add a 480v service to the building? That might be faster than getting a good-sized transformer and the related switch-gear.
 
zbang said:
Doesn't answer the question, but does the building have the capacity to add that size load at 208? Could be a long shot, but it is possible to add a 480v service to the building? That might be faster than getting a good-sized transformer and the related switch-gear.
Click to expand...
Yes, it has the capacity. We had planned for it already, but at 208V.
 
I would think your 1t step would be to determine the size transformer you need and size OCP & conductors from that.
With motor(s) involved it's not going to be a direct ratio.
(This thread makes note of transf. sizing with motors involved)

40 amp 480 Volt Three Phase "Baler"

Hello all, We are relocating an Three Phase 480 volt Baler on an 3 pole 60 amp breaker used continuously for recycling on an existing 75 kva 600 volt to 480 volt Transformer to another Building with only 120/208 volts. We have decided to purchase an 30 kva 208 volt to 480 volt Transformer for...
forums.mikeholt.com
 
With a new chiller, everything is probably freq drives, with a baler they are normally across the line starting, so that may not be a good comparison. I would let the engineers at the manufacturer figure that one out. Way too much liability if you miss on your calculations.
 
Junior_EE said:
Because of supply chain issues, a client is looking to select a 480V chiller despite our building having only 208V. Obviously we need a step-up transformer to accommodate the load.

Here is the info on the chiller @ 480V from the manufacturer:
359A MCA
600A MOPD

In order to size the breaker and wires for this on the 208V side of the step-up transformer, is it simply
MCA @ 208V = 359A * 480V / 208V = 829A?
MOPD @ 208V = 600A * 480V / 208V = 1384A? --> 1400A

Further, on the secondary of the step-up transformer, do I just supply 359A worth of wire to the chiller with 600A circuit breaker? I feel a little leery about having wire ampacity lower than the breaker on a transformer secondary.
Click to expand...
The MCA and MOPD is for the feeder to the chiller. What is upstream of that, including the transformer and the transformer secondary and primary wiring and OCPD, is up to you as long as it meets code.

Typically, the transformer primary would be protected at not more than 250% of the FLA primary Amps, and the secondary at 125% of the secondary FLA. The secondary OCPD is allowed to go to the next size up if it is under 800 A.

If the chiller is the only load on the secondary, you could use the feeder OCPD to the chiller to also serve as the transformer secondary OCPD.
 
petersonra said:
. The secondary OCPD is allowed to go to the next size up if it is under 800 A.
Click to expand...
Don't believe so... 240.21(C):
The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.
 
Junior_EE said:
Because of supply chain issues, a client is looking to select a 480V chiller despite our building having only 208V. Obviously we need a step-up transformer to accommodate the load.

Here is the info on the chiller @ 480V from the manufacturer:
359A MCA
600A MOPD

In order to size the breaker and wires for this on the 208V side of the step-up transformer, is it simply
MCA @ 208V = 359A * 480V / 208V = 829A?
MOPD @ 208V = 600A * 480V / 208V = 1384A? --> 1400A

Further, on the secondary of the step-up transformer, do I just supply 359A worth of wire to the chiller with 600A circuit breaker? I feel a little leery about having wire ampacity lower than the breaker on a transformer secondary.
Click to expand...
MCA and MOPD, as well as 240.21(C) would be used on the 480V secondary side,
of the transformer only.
Once you get to the 208V primary side,you would be following 450.3 and the rest of 240. Most probably you will be sizing the primary side components at a minimum of 125% of the chosen transformer primary current.

My suggestion is to get a true stepup transformer 208 delta to 480V/277.
 
hillbilly1 said:
With a new chiller, everything is probably freq drives, with a baler they are normally across the line starting, so that may not be a good comparison. I would let the engineers at the manufacturer figure that one out. Way too much liability if you miss on your calculations.
Click to expand...
They usually aren't able to provide me MCA/MOPD information when something is fed at a different voltage. But the owner is pushing this solution hard. Tough situation.
 
petersonra said:
only applies if you don't have secondary ocpd
Click to expand...
It always applies to secondary conductors and tap conductors. Those conductors must have an ampacity at least equal to the rating of the OCPD at the load end of the conductors.
The rule has always been that the ampacity of these conductors must be equal to or greater than the load end OCPD rating, but the rule was often violated so they added specific language too say that the provisions of 240.4(B) do not apply. Those provisions do not increase the ampacity of the conductors, as they only permit them to be protected with a device that has a rating greater than the conductor ampacity.
 
don_resqcapt19 said:
It always applies to secondary conductors and tap conductors. Those conductors must have an ampacity at least equal to the rating of the OCPD at the load end of the conductors.
The rule has always been that the ampacity of these conductors must be equal to or greater than the load end OCPD rating, but the rule was often violated so they added specific language too say that the provisions of 240.4(B) do not apply. Those provisions do not increase the ampacity of the conductors, as they only permit them to be protected with a device that has a rating greater than the conductor ampacity.
Click to expand...
I did not say you could skate on those requirements.

Read what I wrote, not what you think I might have written.

The MCA and MOPD is for the feeder to the chiller. What is upstream of that, including the transformer and the transformer secondary and primary wiring and OCPD, is up to you as long as it meets code.
Click to expand...

The feeder to the chiller is what the nameplate is about. Everything upstream has different requirements. Those requirements could be met by the feeder OCPD that could also serve as secondary protection for the xfmr and secondary conductor protection.
 
1) figure out what size transformer you need. It needs to be able to both carry the continuous load of the chiller and be able to handle the starting without excessive voltage drop. The 600A MOPD number gives you a hit at what the short term current requirements of the chiller are. You probably want to contact the manufacturer of the chiller to determine the _minimum_ OCPD which can be reliably used.

2) As petersonra describes, the MOPD and MCA apply only to the last OCPD and conductors feeding the chiller. For everything upstream of this, follow the rules for _transformers_. The MOPD and MCA numbers will inform the transformer selection, but don't dictate the transformer OCPD.

3) If you use a 600A breaker and 359A (or larger conductors) for the chiller, the conductors between the transformer and breaker will need to be rated for 600A.

4) If the 600A breaker is also serving as secondary protection for the transformer, then the secondary rated current will need to be at least 480A.

5) Upstream of the transformer you select OCPD on the basis of properly starting and protecting the transformer. The 1400A 208V number that you calculated? I expect that the transformer primary OCPD will be greater than this.

-Jon
 
It would be good to find out the SCCR of the 480V chiller. When selecting a transformer, its impedance should be considered since it might be helpful in achieving an appropriate SCCA on the feeder to the chiller.
 
I also think it is critical to know the circuit details of the chiller. If it is using a VFD or not will affect the decision on what type of transformer you need and how large it must be. Without that information, you can only guess and mistakes will be very expensive.
 
Status
Not open for further replies.
Top