Instruction Manual

5
Transformers
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199
Secondary Fuse Recommendations
Secondary Voltage
V
OUT
24 110 115 120 220 230 240
VA Secondary Time Delay Dual Element Slow-Blow Fuse
50 3.2 0.75 0.6 0.6 0.3 0.3 0.3
75 5 1.125 1 1 0.5 0.5 0.5
100 6.25 1.5 1.4 1.25 0.75 0.6 0.6
150 10 2.25 2 2 1.13 1 1
200 12 3 2.8 2.5 1.5 1.4 1.25
250 15 3.5 3.5 3.2 1.8 1.8 1.6
300 20 4.5 4 4 2.25 2 2
350 20 5 5 4.5 2.5 2.5 2.25
500 30 7.5 7 6.25 3.5 3.5 3.2
750 40 10 10 10 5.6 5 5
1000 12 12 12 7 7 6.25
1500 17.5 17.5 17.5 10 10 10
2000 25 25 25 12 12 12
3000 35 35 35 17.5 17.5 17.5
5000 60 60 60 30 30 30
7500 90 90 80 45 45 40
10K 125 110 110 60 60 60
15K 175 175 175 90 90 80
25K 300 300 300 150 150 150
37.5K 400 200
50K 600 300
75K 800 400
100K 1200 600
167K 1800 900
Fuse = I times 167% next size smaller if secondary
current is less than 9 amp.
Fuse = I times 125% next size smaller if secondary
current is 9 amp. or higher.
A transformer has all the same component parts as a motor,
and like a motor, exhibits an inrush when energized. This
inrush current is dependent upon where in the sine wave
the transformer was last turned off in relation to the point of
the sinewave you are when you energize the transformer.
Although transformer inrush could run up to 30 to 35 times
full load current under no load, it typically is the same as a
motor, about 6 to 8 times normal running current. For this
reason it is important to use a dual element slow blow type
fuse, the same type of fuse you would use with a motor. If
using a circuit breaker, select a breaker with a time delay,
again the same type you would use with a motor. If the
timedelayisnotsufcient,youmayexperience“nuisance
tripping” – a condition where the breaker trips when
energizing the transformer but it functions properly after it is
re-started.
Secondary Overcurrent Protection
Overcurrent devices are used between the output terminals
of the transformer and the load for three reasons:
1. Protect the transformer from load electrical anomalies.
2. Since short circuit current is minimized, a smaller gauge
wire may be used between the transformer and the load.
3. Per NEC, a larger primary fuse may be used to
reduce nuisance tripping.
This is one of the most common transformer application
questions. If the transformer is a SolaHD T5H series the
tapisfullcapacity,butwemustdenewhatfullcapacity
means on one phase of a three phase transformer. A three
phase transformer built by SolaHD in a ventilated enclosure
(standard construction on 15 kVA and above) has a per
phase capacity equal to 1/3 of the nameplate rating.
Therefore, the tapped phase of a ET5H30S has a total
capacity of 10 kVA (1/3 of 30 kVA). The 120 volt tap is at the
center of this 240 volt winding so the capacity is 5 kVA on
either side of the tap (X1 to X6 and X3 to X6).
To determine the available capacity of the center tap, you
must know the three phase load applied to the 240 delta.
Each phase will supply 1/3 of the kVA to the three phase
load. If the ET5H30 has a 21 kVA, 3 phase load connected
to it, each phase is loaded at 7 kVA. Therefore, the tapped
phase has 3 kVA available (10 kVA - 7 kVA = 3 kVA). The
center tap can be loaded to 3 kVA without over loading the
transformer, but the load must be split so that no more than
1.5 kVA (1/2 the available capacity) is connected to either
side of the tap (X1 to X6 and X3 to X6).
Primary Overcurrent Protection
Capacity of Center Tap in Center Tap Delta Transformers
The general formula is:
Note: All 480 delta to 240 delta transformers stocked by SolaHD are
equipped with a center tap.
Transformer kVA - 3Ø Load kVA
6
=
kVA of each
Center Tap Circuit