ST1300P Need assistance identifying fork mounted fog lights

[ST11Nut]

I picked up a police bike recently and I'd like to identify these lights for future reference. I'm also trying to determine the fuse needed when I run new wiring to these lights.

There's no external stampings or marks on the lights that I can find.

 

[skidgillen]

Those are motolights. Www.motolight.com. I have them on my ST1300P. And we have them on our police Harleys. Do they not work currently?

 

[ST11Nut]

Thanks for identifying them.

The police wiring harness on this bike was in place and the wiring for all the accessories was done really poorly (no surprises there). I removed the police harness and extraneous wiring and I'm in the process of installing a fuse block and new wiring for all the accessories.

The motolight website doesn't identify the fuse size on there installation instructions. Does anyone know the correct fuse size for these lights?

 

[skidgillen]

20amp

 

[Andrew Shadow]

The motolight website doesn't identify the fuse size on there installation instructions. Does anyone know the correct fuse size for these lights?

From their website;
The 50 watts draw 8.3 amps
The 35 watts draw 5.8 amps
The 20 watts draw 3.3 amps
This is for both lights.

Their site also states that their wiring harness is designed to handle a maximum of 75 watts per side. 75 watts per light equates to about 12.5 amps for their harness. In theory, since the fuse is designed to protect the wiring, you should not have a fuse greater than 12.5 amps assuming that you are powering both lamps through the same fuse. If you are using any of the bulbs supplied by MotoLights as listed above you could go with a 10 amp fuse.

 

[ST11Nut]

Awesome, thanks very much for the info

 

[Larry Fine]

In theory, since the fuse is designed to protect the wiring, you should not have a fuse greater than 12.5 amps assuming that you are powering both lamps through the same fuse.

Sorry, but that's not quite correct. A fuse and its wiring should always be greater than the actual connected load, or you might have nuisance fuse-blowing and/or warm wire and/or over-heated terminations.

A fuse that is to protect wiring should not be larger than the wire's capacity, but both should be greater than the connected load. For a 12.5 amp load, at least 14 gauge and a 15 amp fuse; 12 gauge and 20 amp would be better.

 

[Andrew Shadow]

Sorry, but that's not quite correct. A fuse and its wiring should always be greater than the actual connected load, or you might have nuisance fuse-blowing and/or warm wire and/or over-heated terminations.

A fuse that is to protect wiring should not be larger than the wire's capacity, but both should be greater than the connected load. For a 12.5 amp load, at least 14 gauge and a 15 amp fuse; 12 gauge and 20 amp would be better.

I did not write that he should use a 12.5 amp fuse. I wrote that he should not use a fuse greater than 12.5 amps as anything greater would exceed the current carrying capacity of the MotoLights wiring harness.

He did not provide details regarding whether he is changing to a different size wire than what MotoLights supplies and if so what size. He was looking for what size fuse MotoLights recommends. MotoLights fuse selection can only be based on the wiring harness that they deliver these lights with- not what rewiring a customer might do.

Based on the information that he did provide and the way that he framed his question here is my logic trail.
MotoLights fuse selection is based on the wiring harness that they supply these lights with. They do not provide the wire size but they do state that their wiring harness has a maximum current carrying capacity of 75 watts per light. Based on a two light installation that is using this wiring harness I wrote that the maximum size fuse that should be used is 12.5 amps as to exceed this is unsafe to the wiring harness. If he rewires these lights with heavier guage wire than what MotoLights has supplied than that is a different ball game.

Based on the maximum current carrying capacity of the MotoLights wiring harness and the maximum wattage light bulbs that MotoLights delivers these lights with I suggested a 10 amp fuse. This still leaves 1.7 amps of unused capacity which should be plenty to prevent any nuisance fuse failures. It also is well below the current carrying capacity of the MotoLights wiring harnesses by 2.5 amps so there would not be any warm wires or terminals either. If he were to use higher wattage bulbs than what MotoLights supplies then again, a different ball game.

Assuming a set up as delivered by MotoLights, as he offered no information to the contrary, is this logic flawed?

 

[Larry Fine]

I did not write that he should use a 12.5 amp fuse. I wrote that he should not use a fuse greater than 12.5 amps as anything greater would exceed the current carrying capacity of the MotoLights wiring harness.

In that case, each harness should be separately fused to use the maximum capacity of each light position.

 

[Andrew Shadow]

In that case, each harness should be separately fused to use the maximum capacity of each light position.

For what reason?
As far as the configuration that is delivered by MotoLights I don't see why.
The maximum wattage light bulbs that are available from MotoLights for theses lights is 50 watts. Two bulbs equals 100 watts draw or 8.3 amps. This is well below the wiring harness's current carrying capacity of 12.5 amps.

 

[Andrew Shadow]

@Larry Fine

I have a question for you regarding the amperage draws that are listed for things like this as I never quite understood how they list them. As an example, these two 50 watt bulbs are listed as an 8.3 amp draw but that is calculated at 12 V. (100 watts divided by 12 V = 8.3 A)

Since most automotive/motorcycle electrical systems operate closer to 14 V while running this would equate to a 7.1 amp draw. Would it not be more realistic to rate them based on the running voltage? Or does it not matter and that variance is factored in to the specifications?

 

[AV8R]

The primary purpose of the in line fuse is to protect the wire harness from causing a fire at worst, and damaging the wire harness at least. Thus, the fuse should pop before the wire‘s max current capacity is exceeded. Typically the fuse should be rated at 50 to 80% of the wire‘s max loading. The other thing to be aware of is that are slow and fast blow fuses. Slow blow fuses are not the best choice to protect a wire harness and may also be damaged but not blow when near the limit.

 

[Larry Fine]

For what reason?
As far as the configuration that is delivered by MotoLights I don't see why.
The maximum wattage light bulbs that are available from MotoLights for theses lights is 50 watts. Two bulbs equals 100 watts draw or 8.3 amps. This is well below the wiring harness's current carrying capacity of 12.5 amps.

I thought you were talking about a pair of 75's.

 

[AV8R]

Andrew, that’s actually quite an astute observation.

The operating voltage and current on the alternator would be a better target for determining the quiescent current. Which in this case is lower than the 12v idealized steady state condition. This operating at a lower current is safer for the harness.

Typically, the rating on the bulbs or LEDs are a max rating, and not the normal power they see; so again this is good for the fuse and harness.

Another factor, is duty factor. If you are modulating the lights, the total output power being drawn will be using less than when not modulated or wig-wagged. This is yet another way the current and power usage is lower than the max ratings.

@Larry Fine

I have a question for you regarding the amperage draws that are listed for things like this as I never quite understood how they list them. As an example, these two 50 watt bulbs are listed as an 8.3 amp draw but that is calculated at 12 V. (100 watts divided by 12 V = 8.3 A)

Since most automotive/motorcycle electrical systems operate closer to 14 V while running this would equate to a 7.1 amp draw. Would it not be more realistic to rate them based on the running voltage? Or does it not matter and that variance is factored in to the specifications?

 

[Larry Fine]

@Larry Fine

I have a question for you regarding the amperage draws that are listed for things like this as I never quite understood how they list them. As an example, these two 50 watt bulbs are listed as an 8.3 amp draw but that is calculated at 12 V. (100 watts divided by 12 V = 8.3 A)

Since most automotive/motorcycle electrical systems operate closer to 14 V while running this would equate to a 7.1 amp draw. Would it not be more realistic to rate them based on the running voltage? Or does it not matter and that variance is factored in to the specifications?

"In theory, there's no difference between theory and practice; in practice, there is a great difference between theory and practice."

It really depends on the voltage used in the 50-watt rating. This should be stated in the bulb's specifications.

Ohm's Law states that one volt will force 1 amp through one ohm.

V = R x A
A = V / R
R = V / A

"Watt's Law" (not really called that) states that one volt at one amp will consume one watt.

W = V x A
A = W / V
V = W / A

The bulb's resistance is the constant, and everything else depends on the actual voltage. If the 50w rating is at 12v, you calculate the resistance, then calculate the current, and thus the actual wattage, at the actual applied voltage.

 

[AV8R]

Keep in mind, that not all electrical devices have linear impedance or I-V curves.
So, for the example of LED headlamp modules, there will be a non linear IV characteristic; which is voltage dependent.



Even incandescent light bulbs have non linear I-V behaviors



But generally speaking, as long as the voltage or current deviations are small compared to the IV curve, the behavior can be approximated as near linear. Knowing what kind of light is being used and how much current/voltage differentials are in view; then one can see if a linear approximation is an accurate assumption.

 

[Larry Fine]

Some devices are even constant-power users, like switching power supplies. The higher the applied voltage, the lower the current.

All of this is why I made the theory vs. practice comment.

 

[drtyrrel]

Motolights will sell you a complete or partial wiring harness. It comes with a relay and is a nice unit. Give them a call, I think they are currently having 20% off. Email or call for fuse size. The caliper mount instructions below show a 20 amp fuse. I'd call them to make sure because they make different length harnesses and I see one on the sight with a 15 amp fuse.

http://motolight.gingerquillclients.com/assets/uploads/2015/11/Motolight-Caliper-Mount-Installation-Instructions.pdf

 

[Andrew Shadow]

I thought you were talking about a pair of 75's.

The 75 watts they reference is the maximum MotoLights lists their wiring harness as being capable of handling per light.
Thanks for answering the question. I have seen "Watts Law" before but honestly never pay much attention to it. For kicks I am going to run the calculation just to see what it reveals.

Typically, the rating on the bulbs or LEDs are a max rating, and not the normal power they see; so again this is good for the fuse and harness.

Of course it is listed at the higher consumption level of 12 V because that is what the circuit sees when it is running on the battery. It wouldn't make much sense to list the minimum that it consumes. A case of sleep stupid on my part. I realized the answer to my question right after I posted it and turned off the computer and was to tired to go back and delete it.

 

[Andrew Shadow]

The caliper mount instructions below show a 20 amp fuse. I'd call them to make sure because they make different length harnesses and I see one on the sight with a 15 amp fuse.

I am confused again. Their website (quoted below) states that their harness is rated for a maximum of 75 watts per light. For two lights that is a total of 150 watts which is 12.5 amps. Why would they supply 15 and 20 amp fuses for a wiring harness that they claim is only rated for 12.5 amps?

Q: Can I upgrade the stock 35 watt lamp?

A: Yes. Our harness is designed to safely handle 75 watts per side.