DOUGLAS
lighting controls
Calculating Wire Lengths for Short Circuit Current Ratings
While calculating the voltage drop involves a simple equation (Ohm's Law: V=IR) where the voltage drop is directly affected by the
conductor's resistance, calculating the number of wire-feet required to impede a short circuit current is much more complex:
For 120/277VAC Loads:
(Breaker AIC Rating - Relay SCCR Rating) x (# of Conductors per Phase) x (Conductor Constant) x Voltage (I -n)
Conductor length =
2 x (Relay SCCR Rating) x (Breaker AIC Rating)
For 208/480VAC Loads:
(Breaker AIC Rating - Relay SCCR Rating) x (# of Conductors per Phase) x (Conductor Constant) x Voltage (I -l)
Conductor length =
1.73 x (Relay SCCR Rating) x (Breaker AIC Rating)
Wire
Breaker
Gauge Voltage AIC Rating
(AWG) (VAC)
(A)
10
120
(l-n)
12
120
(l-n)
14
120
(l-n)
10
277
(l-n)
12
277
(l-n)
14
277
(l-n)
10
208
(l-l)
12
208
(l-l)
14
208
(l-l)
10
480
(l-l)
12
480
(l-l)
14
480
(l-l)
Short Ciruit Current Rating (SCCR) of Relay
1,500
10,000
5,000
18,000
14,000
Required Conductor Length (ft.) between Breaker & Relay
14,000
22,000
65,000
14,000
22,000
65,000
14,000
22,000
65,000
14,000
22,000
65,000
14,000
22,000
65,000
14,000
22,000
65,000
35
37
38
22
23
24
14
14
15
81
84
88
51
53
56
32
33
35
8
9
11
5
6
7
3
4
4
17
21
25
11
13
16
7
8
10
2
3
5
1
2
3
1
1
2
4
7
11
2
5
7
2
3
5
0
2
3
0
1
2
0
1
1
0
4
8
0
2
5
0
1
3
N/A
1
2
N/A
0
1
0
0
1
N/A
1
5
N/A
1
3
N/A
1
2
14,000
22,000
65,000
14,000
22,000
65,000
14,000
22,000
65,000
14,000
22,000
65,000
14,000
22,000
65,000
14,000
22,000
65,000
70
73
77
44
46
48
28
29
30
162
169
177
102
106
110
64
67
70
15
18
22
10
11
14
6
7
9
35
42
50
22
26
32
14
17
20
3
6
10
2
4
6
1
3
4
8
15
23
5
9
14
3
6
9
0
3
7
0
2
4
0
1
3
0
7
15
0
4
10
0
3
6
N/A
1
5
N/A
1
3
N/A
0
2
N/A
3
11
N/A
2
7
N/A
1
4
Even with the relay panel mounted right next to the breaker panel, approximately 5-6 feet of wire is needed to connect the devices.
The above table shows that Douglas Lighting Controls relays rated at 14,000 SCCR need virtually no additional length of wire to impede
the short circuit current. The table also shows there is no advantage in rating a relay to exceed a 14,000 amp SCCR.
Douglas WR-6161 Relays are Fully Compliant with NEC 2005 Article 409
DOUGLAS
lighting controls