TL;DR
Commercial backup diesel generators in the Greater Philadelphia region almost universally run on ULSD (Ultra Low Sulfur Diesel) meeting ASTM D975, grade 2-D S15. That's the spec almost every manufacturer requires for warranty coverage. Anything else you hear (cetane minimums, cloud point, biodiesel blends) is a refinement of that baseline, not a replacement.
This guide explains each spec so you can verify you're getting what your generator needs.
The baseline spec: ASTM D975
ASTM D975 is the standard specification for diesel fuel oils published by ASTM International. It defines the categories of diesel sold commercially in the US. For backup generators, the two you'll see are:
- 2-D S15: No. 2 diesel, 15 ppm sulfur max. This is ULSD - Ultra Low Sulfur Diesel. What virtually every modern generator manufacturer requires.
- 2-D S500: No. 2 diesel, 500 ppm sulfur max. Low sulfur. Legal for off-road / stationary use in many applications, but NOT what most generator warranties require.
If your generator was installed in the last 15-20 years, the correct fuel is 2-D S15 ULSD. Full stop.
What "ULSD" means practically
- 15 ppm maximum sulfur
- Standard at every regional fuel terminal
- Required for road use and for most stationary generator warranty compliance
- Sometimes called "on-road diesel" even though it's used in stationary applications
What "off-road dyed diesel" is
Off-road (dyed) diesel is also ULSD in most cases - same 15 ppm sulfur spec - but it's dyed red to indicate it's exempt from federal highway taxes. It can be used in:
- Stationary generators (non-road)
- Agricultural equipment
- Off-road construction equipment
- Some marine applications
It cannot be used on public roads. For a backup generator, the fuel chemistry is the same as on-road ULSD. The difference is tax treatment.
Cetane number
Cetane is a measure of how readily diesel ignites under compression. Higher cetane = faster ignition = smoother combustion and better cold-start behavior.
- ASTM D975 minimum: 40
- What you'll typically get from regional terminals: 42-48
- What some generator manufacturers recommend: 45+
In the Greater Philadelphia area, standard terminal ULSD generally lands in the 42-45 range, which is fine for commercial backup generator use. If your generator spec or your facility AHJ requires a specific cetane minimum, ask your fuel supplier to confirm per-delivery.
Cloud point and cold-flow
Cloud point is the temperature at which wax crystals start forming in the fuel. Below cloud point, fuel flow slows and can eventually stop entirely.
Standard #2 ULSD has a cloud point around 20-25°F. For outdoor above-ground tanks in the Mid-Atlantic, that's marginal during a cold snap.
Options for cold weather:
- Winter blend #2 ULSD - standard at regional terminals during winter months, cloud point lowered to around 0-10°F
- Kerosene blend (No. 1 / No. 2 mix) - further reduces cloud point, reduces BTU slightly
- Cold-flow additive - chemical treatment that prevents wax crystal formation without changing fuel density
- Tank heaters - for critical facilities in very cold micro-climates
Fox Fuel stocks winter-blend ULSD for the cold months and can add kerosene or cold-flow additive on request.
Water and microbial contamination
Diesel stored in standby generators is vulnerable to two things that will cause a generator to fail during an outage:
- Water - condenses in the tank from temperature cycling, settles at the bottom
- Microbial growth - bacteria and fungi that feed on the fuel-water interface
Both are prevented by:
- Keeping the tank mostly full (reduces air space, reduces condensation)
- Running the generator regularly (turns over fuel)
- Adding biocide for critical tanks that sit idle between monthly tests
- Periodic fuel polishing (filter and clean in place, or pump out, filter, and return)
If your tank has been idle for over a year, or if you've had water issues, request fuel polishing before your next load test.
Biodiesel blends
Many regional terminals dispense B5 (5% biodiesel) or B20 (20% biodiesel) ULSD year-round or seasonally. Some generator manufacturers specify a maximum biodiesel content for warranty coverage (typically B5 or B20). If your generator warranty has a biodiesel limit, tell your fuel supplier - most can confirm the biodiesel content of their current terminal pull.
What to ask your fuel supplier
When you're setting up commercial generator fueling, ask:
- What ASTM grade do you deliver? (Answer should be ASTM D975 2-D S15 ULSD.)
- What's the typical cetane number? (40 minimum; 42-48 is normal.)
- Is the fuel winter-blended from November through March?
- What's the biodiesel content?
- Can you provide a delivery ticket with fuel type and volume for AHJ records?
What Fox Fuel delivers
- ASTM D975 2-D S15 ULSD year-round
- Winter-blend ULSD November through March
- Kerosene blend or cold-flow additive on request
- Fuel polishing available
- NFPA 110 documentation on request
- Off-road dyed diesel for stationary applications
Ready to set up your generator fueling
Call (215) 659-1616 or use the contact form. Share your tank spec, fuel type needed, and AHJ documentation requirements, and we'll set up a delivery plan.
Related resources
- /services/generator-fueling/
- /services/fuel-monitoring/
- /services/emergency-delivery/
Need a quote for your facility? Call (215) 659-1616 for commercial fuel programs across Montgomery, Bucks, Philadelphia, Delaware, and Chester counties.