Re: Fuse Size

Posted by John T on July 16, 2022 at 05:23:59 from (45.16.200.171):

In Reply to: Fuse Size posted by nh8260 on July 15, 2022 at 18:27:19:

Good morning nh, regarding your good question:

but what size glass fuse do i need? I'm thinking 10-20 amp but want to do it right.

SHORT ANSWER: Max 10 amp fuse if the wire is 10 amp rated...........Max 20 amp fuse if the wire is 20 amp rated AND USE WIRE WHICH HAS A MINIMJM AMPACITY OF 125% OF WHAT THE CAMERA DRAWS then size the fuse accordingly, see below:

LONG ANSWER

Here's how I was taught under the NEC to size the conductor ampacity as well as overcurrent protection. Even though this was used for AC Power Distribution, the theory can suffice for your low voltage DC circuit:

1) Compute the Maximum Continuous Current MCC of the circuit. The labeling on the camera should specify what that is, Id GUESS it to be relatively LOW current (perhaps less than 1 to a few amps max) in which case a 10 Amp circuit may well suffice, BUT CHECK THE LABEL NOT WHAT ANYONE HERE ME INCLUDED MAY GUESS

2) Choose the conductors to have a MINIMJM AMPACITY of 125% of the MCC IE if the camera drew 4 max amps, the conductors must be rated for at least 1.25 x 4 = 5 amps.......If it drew 10 Amps the conductors need to be rated for at least 10 x 1.25 = 12.5 Amps......... Get the idea ???????

3) Size the Overcurrent Protection Device to PROTECT THE WIRE IE if 10 amp rated wire were used, a 10 amp max fuse or breaker.

OTHER CONSIDERATIONS

A) Some of those 12 VDC cigarette lighter type of power outlets may be rated for example 100 or 200 or possibly more Watts, so DO NOT use one rated less than the camera will draw.

B) While installing one anyway and to allow for other 12 VDC to possibly also be used, I would use at least a 200 Watt rated 12 VDC Power Outlet,,,,,,,,,Fed by at least 15 Amp rated wire,,,,,,,,,,Protected by the proper Overcurrent Protection Device (Example 15 amp for 15 amp wire) ALL STILL SUBJECT TO THE WIRE BEING RATED FOR AT LEAST 125% OF THE CAMERAS MCC

C) Many non electricians/engineers may not realize, the ampacity of single conductors in free air IS MUCH GREATER THAN IF THEY WERE ENCLOSED OR IN A JACKET OR RACEWAY IE While whats referred to as Romex jacketed wire with 12 Gauge Coper conductors is rated for 20 amps, a single 12 gauge wire in free air IS RATED HIGHER HOWEVER no harm is using bigger than necessary conductors, so if its a 20 amp circuit one can still use 12 gauge single conductor copper wire AND I TYPICALLY DO THE SAME !!!!!!!!!

There ya go hope this helps post back any questions

Best wishes to all here

John T

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