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Electrical loads, part two, plus alternators & fuses

To: triumphs@Autox.Team.Net, spitfires@Autox.Team.Net, rdaniels@snet.net
Subject: Electrical loads, part two, plus alternators & fuses
From: DANMAS@aol.com
Date: Thu, 7 Jan 1999 21:36:25 EST

A question was raised on one of the lists a day or so ago as to the capability
of a Triumph/Lucas alternator to handle the addition of high powered lights.
This got me to thinking, so I came up with the data presented in part one of
this drivel (I broke it into two parts because lengthy posts don't make out of
Mark's server). Part two will interpret the data.

The data presented is based on a 1971 TR6, with no optional electrical loads,
but with minor changes would apply to most any Triumph.

The first set of data, "Total electrical loads, listed by "fuse" assignment:"
is merely a listing of every electrical device and/or circuit in the car,
subdivided by fuse assignment. For convenience, I have labeled the fuses Red,
Green, Purple, or Non-fused to correspond to the colors of the wires fed by
the fuse:

Red fuse:  This fuse receives power from the headlight switch, via a red/green
wire, and is hot any time the headlight switch is in either the "park" or
"main" position.

Green fuse:  This fuse is fed from the ignition switch via a white wire, and
is hot only when the key is in the "run" or "Start" position.

Purple fuse:  This fuse receives power directly from the battery/alternator
via brown or brown with a tracer wires, and is hot all the time.

Unfused circuits:  These circuits receive power either directly from the
battery via a brown or brown with a tracer  wire (headlights) or from the
ignition switch via a white wire (warning lamps and ignition coil).

The "minus duplicates" refers to the fact that the high beams can receive
power from two sources - from the unfused source if the headlight switch is on
and the dimmer switch is in the high beam position, and from the purple fuse
if they are turned on by the high beam flasher switch. The high beams have to
be listed twice because of the two power sources, but they will still add only
once to the total load, even if both the dimmer switch and the flasher
switches are on. 

As you can see, the total electrical load exceeds the capacity of the
alternators by quite a bit. Fortunately, all these loads will never be on at
any one time, so the alternator doesn't have to have that much capacity. For
example, the odds of your blowing the horn and flashing the high beams, with
the hazard flasher going and the trunk, doors, and glove box open are pretty
slim. Also, the turn signals and the hazard flasher are on-off circuits, "off"
about as much as they are "on" when they are operating correctly, so the
average load is half that listed.

Other items, such as the horn, brake lights, and reverse lights, don't really
have to be supplied by the alternator, as they are normally only operated
intermittantly; They can draw from the battery if need be, rather than the
alternator if the alternator is at its maximum capacity. As long as the
alternator has sufficient capacity to recharge the battery between operations
of these loads, there is no problem. The horn may take 5 amps while it is
operating, but the alternator can replace the battery charge at a low amperage
rate over a longer period of time that that used by the horns. The horn may
blow for 10 seconds (if you're really angry), but the alternator can then have
an hour or so to recharge. A 5 amp discharge for 10 seconds can be replaced in
one hour at a charge rate of only 0.014 amp.

Taking all of that into consideration gives the data in the second part,
titled ""Worst case" constant loads:" The maximum load that a car's alternator
is likely to see, on a continous basis, is the load drawn while driving on a
cold winter night in the rain. Under these conditions, the heater fan, high
beams, marker/parking/tail lights, gauges, and the WS wipers will all be
drawing power, as well as the ignition coil. The total "steady state" load
will only be 22.6 amps, within the capabilty of the 15ACR alternators used in
the earlier TR6s. If you are driving in town under these conditions, however,
in stop and go driving, the alternator may not be able to keep up as the
output is much reduced at idle or low engine speeds. Later TR6s, with the 45
amp 18ACR alternators, should have no problem even in town driving.

If you are planning to add electrical accessories, add them to the analysis as

I've been away from the list for a few days, so I'm behind, but I'll try to
catch up on one item here. Someone (Tony Rhoades?) asked about using a high
powered alternator, with a current limiter to keep the output from exceeding
the ammeter/wiring rating in his car, basically limiting it to the output of
his current generator. That's not a bad idea at all. I don't offhand know how
to do it, but the idea is sound. During engine operation above about 1500 rpm,
the output of the stock generator is adequate for the loads, but at low engine
speeds, the generator (or an alternator) may produce less than half its rated
output. This is not enough to supply the loads under some conditions, so the
battery is called on to make up the difference. During extended low speed, in
traffic, driving, the battery can soon run down. With an alternator of twice
the required capacity, half output at low speeds will keep the loads supplied
without running down the battery. Normally, the alternator will never be
called on to supply more than the normal car's electrical loads, but if the
battery is allowed to discharge deeply, the re-charging current can be quite
high if the alternator has the capacity to supply it, high enough to pose a
problem for the ammeter and the wiring.

The next item is fuses. There is absolutely no reason to use a fuse of a
higher rating than the maximum current it will see. Unlike a lot of items,
over capacity has no benefit with fuses. As long as the fuse doesn't blow in
normal operation, the smallest fuse you can get by with is the "correct" size.
Given that, the maximum size to use for the "red" fuse would be 10 amps, as it
will never see more than 5.5 amps under anything other than a short circuit

Similarly, the max normal load on the "green" fuse is 5.6 amps, but a 10 amp
fuse will be inadequate here, because it is quite possible to have all the
loads on at one time. For this circuit, a 20 amp fuse is required. 

The "purple" fuse circuit is a judgement call. Normally, none of these loads
are on, but you could conceivably have them all on. I seriously doubt that
you'll ever have the hazard flashers on while you flash the high beams and
honk the horn!  You might, though, just to get someone's attention. In my car,
I use a 15 amp fuse for this circuit, with the understanding that in the very
unlikely event I should ever have enough items on to blow the fuse, none of
them are required for operation of the car. Needless to say, I carry spares.
In addition to those I carry in a tool kit, I also have two twenty amps spares
in the fuse box. In an emergency, these can be used for either of the three
fuse positions.

Just FYI, hope it's useful.

Dan Masters,
Alcoa, TN

'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74

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